8. Capacitor Discharge Bonder (2026)
I've decided to publish this design as I progress through it, so you can accompany me in my various triumphs and setbacks until it is complete, after which I'll revise the content to finished project state. Along the way you can watch me fall in the mud and emerge with my next smart idea...
New things I've discovered or issues I'm trying to solve are highlighted in RED.
As this is a true design log, these are genuine comments to myself.
Whilst designing a replacement IR energy emission element for a FTIR chemistry analyser, [Repairs: FT IR Source], I needed a bonder to weld its Kanthal A-1 [T18a] resistance wire to a solderable lead wire.
EBay is flooded with cheap Chinese capacitor discharge bonders, but they are far less powerful than sellers would have you believe and those that appear to work, do so more out of creating a spark than a bond: typically their energy is in the order of 100s of mJ.
NOTE: issues highlighted in red or fuschcia still need resolving.
AI
I brainstormed the initial design with DeepSeek AI, similar to Iron Man and JARVIS [G28] but minus the fancy virtual 3D. Having never owned an electric welder, I could not guess the physical and electrical parameters and had to start from scratch, and from first principles.
In 2026 towards the end of the design I noticed Google AI had become much more capable, and I used it to fine-tune DeepSeek's initial advice, particularly the low resistance mechanical path to the capacitor discharge star points.
I have included their advice below in different italicised shades of blue, see above. For more information on Deepseek, see [HOME: PC Builds Win 10 fixes & AI].
Initial assessment
I told DeepSeek I wanted to avoid high voltages and it provided an initial design assessment:
Ideal Circuit Parameters:
Voltage: 20V - 50V (Lower voltage promotes current flow over arcing).
Capacitance: 5,000µF - 20,000µF (Large capacitance for a longer pulse).
Current Limiting Resistor: Very low, just to control the peak, e.g., 10mΩ - 50mΩ.
Electrodes: Use a flat electrode on the Kanthal side and a pointed electrode on the TCW.
The flat electrode will protect the Kanthal from vaporising.
Step-by-Step Experimental Procedure: Start low at 1 Joule.
Example: With a 10,000µF cap, V = √(2 x 1 / 0.01) ≈ 14V.
Required energy
I got DeepSeek to do the hard work:
Mathematically determine using existing datasets and established theory, the minimum weld energy density in Joules to bond a 0.3mm Kanthal wire to a 0.6mm tinned copper wire (TCW).
DeepSeek:
A complete model must include the energy to raise the temperature of both the TCW volume and the Kanthal volume to the bonding temperature.
Let's define the bonding temperature as the point where the TCW is molten (~1085°C) and the Kanthal is sufficiently hot for the molten copper-tin alloy to wet it and form a intermetallic bond.
We will use 1100°C as the target interface temperature.
Define the Two Volumes to be Heated:
Volume of a cylinder = π × r² × l
Where:
r is the radius of the TCW.
l is the length of the TCW cylinder being heated.
TCW Volume (V_tcw):
For 0.6mm diameter TCW and a 0.3mm bond width (the Kanthal diameter):
TCW radius r = 0.6/2 = 0.3mm
TCW length l = 0.3mm
V_tcw = π x (0.3 mm)² x 0.3mm ≈ 0.0848mm³
Kanthal Volume (V_kan):
Diameter = 0.3mm
l = 0.3mm (symmetrical)
V_kan = π x 0.15mm)² x 0.3mm ≈ 0.0212mm³
Calculate mass of TCW and Kanthal:
m = V × ρ
Where:
m is the mass of the wire volume (in grams).
V is the volume of the wire segment being heated (in mm³).
ρ is the density of the material (in g/mm³).
Mass for the TCW:
ρcu = 8.96×10−3g/mm³
mtcw = Vtcw × ρcu
Mass (m_tcw) = V_tcw x ρ_cu = 0.0848 mm³ x 8.96e-3 g/mm³ ≈ 7.60e-4g
Mass for the Kanthal:
ρkan = 7.1×10−3 g/mm³
mkan = Vkan × ρkan
Mass (m_kan) = V_kan x ρ_kan = 0.0212mm³ x 7.1e-3 g/mm³ ≈ 1.51e-4g
Calculate Energy to Heat TCW to 1100°C:
The total energy equation used was the sum of the sensible heat and latent heat required to raise the material to the bonding temperature and melt it:
Qtotal = (m × c × ΔT) + (m × L)
Where:
Qtotal is the total energy required (in Joules).
m is the mass of the material (in grams).
c is the specific heat capacity of the material (in J/g·°C).
Tinitial is my lab temperature which is usually 27°C (I like it warm).
ΔT is the temperature change, Tfinal − Tinitial (in °C).
L is the latent heat of fusion (in J/g). This term is only applied to the TCW, as it is the material that melts.
This was calculated separately for the TCW and Kanthal volumes, then summed:
Qtotal = Qtcw + Qkan
Calculate Energy to Heat TCW to 1100°C:
ΔT = Tfinal − Tinitial = 1100°C - 27°C = 1073°C
Q_heat_tcw = m_tcw x c_cu x ΔT = (7.60e-4g) x (0.385 J/g·°C) x (1073°C) ≈ 0.314J
Q_melt_tcw = m_tcw x h_cu = (7.60e-4g) x (205J/g) ≈ 0.156J
Q_total_tcw = 0.316 J + 0.156 J = 0.472J
Calculate Energy to Heat Kanthal to 1100°C:
ΔT = Tfinal − Tinitial = 1100°C - 27°C = 1073°C
Q_heat_kan = m_kan x c_kan x ΔT = (1.51e-4 g) x (0.46 J/g·°C) x (1073°C) ≈ 0.074J
Kanthal does not melt, so no latent heat is added.
Calculate Total Theoretical Minimum Energy:
Q_total = Q_total_tcw + Q_heat_kan
Q_total = 0.314J + 0.074J = 0.544J
Conclusion:
The mathematically determined minimum energy require to heat the combined volumes of both wires to a bonding temperature of ~1100°C when starting from 27°C ambient, is approximately 0.54J.
In other words, the minimum required energy is 0.54J.
We also need to know the maximum energy because we don't want to destroy it.
The optimum energy for a good bond is between the two.
Maximum permitted energy
The upper limit is defined by the point of catastrophic failure: vaporizing the wires.
Let's calculate the energy required to vaporize the bonding volume.
1Energy to Vaporize the TCW Volume (0.0848mm³):
Mass (m_tcw) = 7.60e-4g (as before)
Energy to Heat from 27°C to Boiling Point (2562°C):
ΔT = 2562°C - 27°C = 2535°C
Q_heat_to_boil = m_tcw x c_cu x ΔT = (7.60e-4g) x (0.385 J/g·°C) x (2535°C) ≈ 0.742J
Latent Heat of Vaporization (L_v,Cu):
4790 J/gQ_vaporize = m_tcw x L_v,Cu = (7.60e-4g) x (4790J/g) ≈ 3.64J
Total to Vaporize TCW: 0.742J + 3.64J = ~4.38J
Energy to Heat Kanthal to its Boiling Point (~3000°C)
Mass (m_kan): 1.51e-4g (as before)
ΔT = 3000°C - 27°C = 2973°C
Q_heat_kanthal = m_kan x c_kan x ΔT = (1.51e-4g) x (0.46J/g·°C) x (2973°C) ≈ 0.207J
Conclusion: The Maximum Energy
The total energy to begin vaporizing the materials is dominated by the TCW.
Q_max_theoretical ≈ 4.38J + 0.21J ≈ 4.6J
Safe Operating Range:
Theoretical Minimum: ~ 0.54JT
Theoretical Maximum (Vaporization): ~ 4.6J
This gives a theoretical range of ~ 0.54 to < 4.6J
'Sweet spot'
I asked DeepSeek where it thought the sweet spot for a bond might be within the range 0.54 to 4.6J:
Based on the calculated minimum (0.54J) and maximum (4.6J) energies, the sweet spot for a reliable bond will likely be in the range of 1.5J to 2.5J.
Reasoning:
Margin over Minimum (0.54J):
The theoretical minimum has no safety factor. It assumes perfect, lossless heat transfer. In reality, significant energy is lost to conduction into the cold ends of the wires, radiation, and heating the electrodes. A practical energy needs to be 2 to 3 times the minimum to ensure the bond volume actually reaches the required temperature.
This suggests an operating point above ~1.1 - 1.6J.
Safety Margin from Maximum (4.6J):
The maximum energy is a catastrophic failure point (vaporisation). A good design operates far from this cliff. A sweet spot should have a comfortable buffer, at least 40-50% below the maximum, to account for variations in hand placement, surface oxidation, and capacitor voltage tolerance.
This suggests operating below ~2.3 - 2.8J.
The overlap of these two derived ranges—'well above the minimum' and 'safely below the maximum' points directly to a sweet spot of 1.5J to 2.5J.
Required capacitance
The next step was to determine the required capacitance to bond the wires. I asked DeepSeek to calculate the capacitance for 1.5J and 2.5J at several low voltages:
C = 2E / V²
1.5J 5V 120,000µF 2.5J 5V 3200,000µF
1.5J 10V 30,000µF 2.5J 10V 50,000µF
1.5J 15V 13,300µF 2.5J 15V 22,200µF
1.5J 20V 7,500µF 2.5J 20V 12,500µF
A low ESR capacitor is essential to produce the required high peak pulse current and I would be wiring several capacitors in parallel to drop that. The best choice on eBay seemed to be Panasonic EEUFC1E222 2,200µF 25v 105C 12.5m x 35mm with ESR 22mΩ.
I then asked it to calculate for 12V: 27,777µF
27,777µ / 2,200µF = 12.6 capacitors.
The capacitors were on offer in packs of 4, so I bought 4 packs, total 35,200µF; ESR 22mΩ/16 = 1.4mΩ.
Energy at 12V: E = 0.5 x 0.0352 x (12)² = 2.53J.
To get exactly 2.5J from 35,200µF, charge it to 11.92V.
For 1.5J, charge to 9.23V.
I set the maximum voltage at 12V.
Destruction voltage
I asked DeepSeek to calculate the 35,200µF capacitor voltage at 4.6J:
E=0.5 x CV²
_______
V = / E
√ 0.5 x C
Where:
E = Energy in Joules (4.6J)
C = Capacitance in Farads (0.0352F)
V = Voltage in Volts
_____________
V = / 4.6 = 16.17V
√ 0.5 x 0.0352F
Conclusion:
To store 4.6J of energy in 35,200µF you would need to charge it to ~16.2V.
The maximum voltage of 12V gives a 26% margin of safety.
Capacitor bank safety discharge
Initially I designed this in but it isn't necessary, as simply initiating another weld pulse into the anvil will discharge any residual voltage.
Capacitor bank charging current
Based on the calculation below, the Amrel current limit will be set to 1A.
Capacitor bank energy formula: t = (C × V) / I
C = 35,200µF
V = 12V (maximum)
I = 1A
t = (0.0352 × 12) / 1 = 0.4224 seconds
Capacitor bank discharge mechanism
The bonder is essentially a large switch that discharges a capacitor bank across the materials to be welded. Due to the large currents involved and the necessity to control pulse width, a physical switch is out of the question. Transistors are wildy inefficient and the best cost-effecitve solution is multiple N-channel MOSFETS switched in parallel to share the current.
Capacitor bank wiring
The best way to wire a capacitor bank is to ensure wires from every capacitor are of equal length in order to minimise inductance and resistance. Star points are required but the + star point takes priority over the 0V starpoint. Deepseek puts it succinctly:
For a capacitor discharge welder, the capacitor bank positive lead star-pointed to a short cable to the tungsten electrode is significantly more critical than the ground lead star point.
Explanation:
The weld energy is delivered in a high-current pulse. The dominant parasitic element in this path is inductance (L), not resistance. Inductance opposes the instantaneous rise of current (di/dt). The voltage drop across an inductance is V = L × di/dt.
Summary:
Positive Lead (Priority): This must be the absolute shortest, most direct path possible from the capacitor bank positive terminal to the tungsten electrode. This minimises loop inductance, ensuring the fastest possible current rise time into the weld joint .
Ground Star Point (Secondary): Implement a star ground for the control and return paths (TC4422 ground, negative supply) at the capacitor bank negative terminal. This isolates the sensitive gate drive from high-frequency noise created by the main discharge loop.
Mosfet selection
DeepSeek recommended using a fast 30V 240A (1050Apk) IRLB3813 NMOSFET: RDSon 1.6 mΩ max @ 10V Vgs. Its high switching current, low RDSon and relatively low 650pF typ reverse transfer capacitance, Crss, were key reasons for its selection: low Crrs allows for very fast switching with a high-current driver. For improved reliability I wired two in parallel with separate drivers.
Google AI later recommended increasing this to four, as peak current could reach 1600A and it said the 1050A figure is the silicon failure point; 400A each is a good safety margin.
Thermal dissipation
Absolute Maximum Continuous Current (Id) of 240A per MOSFET from the IRLB3813 datasheet, giving a worst-case combined pulse current of 480A.
Conduction Loss (P_cond):
Formula: Pcond = I² × Rds(on) × D
I_pulse: 480A
R_{ds(on)} (total for 2x MOSFETs): 0.0008Ω
Duty Cycle (D): 0.001 (10ms pulse every 10s) [10ms worst case; in reality <1ms should suffice]
Calculation: Pcond=(480)² × 0.0008 × 0.001
Pcond=230,400 × 0.0008 × 0.001=0.184W (per MOSFET)
Switching Loss (P_sw)
Formula: Esw ≈ 1² × Vds × Id × (tr+tf)
V_ds: 12V
I_d: 480A
t_r + t_f: 200ns (estimated total switching time)
Calculation:
Esw = 0.5 × 1² × 480 × (200×10−9)
Esw = 0.000576J (Joules per switching cycle)
Power: Psw = Esw × Switching Frequency
Psw = 0.0000576 W
Total Power Dissipation (P_total):
Formula: Ptotal = Pcond + Psw
Calculation:
Ptotal = 0.184 + 0.0000576
Ptotal ≈ 0.1846W (per MOSFET)
Heatsink Requirement Analysis:
Junction-to-Ambient Thermal Resistance (RθJA): 62°C/W
Temperature Rise: ΔT = Ptotal × RθJA = 0.1846W × 62 ≈ 11.4 °C
Conclusion:
A heatsink is completely unnecessary. The MOSFETs will run only slightly above ambient temperature, with a worst-case temperature rise of ~11°C.
Gate driver
The NMOSFETs are each separately driven by a Microchip (formerly Telcom) TC4422 9Apk fast MOSFET driver. DeepSeek recommended the drivers be decoupled with ceramic X7R 100nF and 10µF:
The TC4422s are powered from the Manson EP-613 12V supply. There is a MF-R050 30Vmax Ihold 0.50A Imax 40A polyswitch where the supply enters and a 1.5KE15CA TVS across the +12V physically close to the TC4422s. In the event of it triggering, the green +12V PSU LED will extinguish.
After the fuse the 12V supply has 2x 10µF MLCs in parallel for low ESR fast current pulse decoupling.
The TC4422 datasheet recommends decoupling with 1.0µF for each 1000pF of load capacitance.
IRLB3813 input capacitance = 8420pF. Required Bulk Capacitance = (8420pF / 1000pF) x 1.0µF = 8.42µF x2.
Gate Driver ICs: 4 x TC4422, 9A peak, fast low side MOSFET drivers.
Local Decoupling: 2x 10µF + 100nF MLCs 50V (for high current switching) between driver Vdd and GND.
Gate Resistors: 2.2Ω to 4.7Ω, non-inductive (carbon composition or metal film) per MOSFET gate.
Datasheet advice:
Layout: star point for gate drive connections. Keep driver IC and capacitors 1-2cm near MOSFET gates.
Pulse width
A dual retriggerable HEF4538 one-shot determned the pulse width which is only necessary to ensure the MOSFETs are on long enough to discharge the capacitor bank.
Discharge time constant: τ = R × C
For the 4.7Ω discharge resistor, τ = 165ms but for the weld itself, the resistance is the extremely low resistance of the MOSFETs, cables, and workpiece:
Assume total loop resistance (R_loop) ~5mΩ
τ_weld = 5mΩ × 35,200µF = 176µs
DeepSeek general information:
The minimum usable pulse width is approximately 0.2ms (200µs) for capacitor discharge welding of fine materials, though practical equipment often starts around 0.45-0.65ms depending on the application.
I chose 2ms as this satisfies both my needs and DeepSeek's advice.
Since +12V is the only constant supply, a CMOS HEF4538 dual retriggerable one-shot produces this pulse. The first one-shot Q- is wired to its positive input to force it to a *non-retriggerable one-shot. The negative input to both one-shots is from a small push switch to 0V in parallel with 1µF and with a 10kΩ pullup to +12V. The second one-shot Q- drives a white LED giving a visual indication of the weld taking place; *as it's only an LED, switch chatter extending Q2 is actually an advantage.
Component Values:
One-shot #1 (pulse): R = 20kΩ, C = 100nF time = 2ms on Q+ to TC4422.
One-shot #2 (LED): R = 10kΩ, C = 1µF time = 10ms for HE WE LED on Q- to LED cathode.
HEF4538 Iout ±3.6mA max; I'll drive the LED at 2mA.
HE WE LED 3.2Vf means 12V-3.2V/2mA = 4.4 kΩ so I'll limit its current with 5.1 kΩ to +12V.
Power up reset to clear both one-shots CLR- pin: 10kΩ to +12V & 100nF to 0V.
100nF decoupling across supply pins.
This chip is located as far away from the electrodes as possible to minimise inductive triggering, which is why there is a second pcb near the power input terminals called 'timing pcb'. Its 2ms pulse is routed to the TC4422s via miniature screened audio coax with its shield originating at the HEF4538 0V which originates at the Manson 0V star point ADD TO SCHEMATIC REV 5]; the TC4422 can accommodate large ground bounce, so is not affected. Although the TC4422 has good noise immunity, a 100pF X7R MLC filter has been added to each TC4422 input at the end of its coax.
Gate resistor
The TC4422 is capable of a 9A peak output current. The gate resistor sits between its output and the NMOSFET gate input.
Minimum Resistor for 9A with Vdd set to 12V [Manson EP-613 PSU]:
Rg = 12V = 1.33Ω
9A
Target Range (2.2Ω to 4.7Ω):
With 2.2Ω, the initial peak current is I = 12V/2.2Ω ≈ 5.45A
This is well within the driver's capability but provides strong control.
With 4.7Ω, the initial current is I = 12V/4.7Ω ≈ 2.55A
This is ideal for damping and still provides very fast switching.
The 2.2Ω to 4.7Ω range is the ideal starting point, ensuring robust performance and stability.
Resistor wattage
I asked Deepseek to calculate the maximum wattage of 2.2Ω and 4.7Ω gate resistors:
Peak Instantaneous Power (W_peak)
This is the power the instant the TC4422 switches on, when the full supply voltage is dropped across the resistor.
V_drive = 12V (from Manson PSU)
Formula: Ppeak = Vdrive²
R
Peak Instantaneous Power:
For 2.2Ω, P = 12²/2.2 = 144²/2.2 ≈ 65.5 W
for 4.7Ω, P = 12²/4.7 = 144²/4.7 ≈ 30.6 W
Average Power (W_avg) per Pulse:
This is the total energy dissipated in the resistor during one gate charge/discharge cycle, divided by the pulse width. The energy dissipated in the resistor is approximately half of the total gate energy.
Q_g (Gate Charge) = 110nC
V_drive = 12V
Energy in Gate (E_gate):
Egate ≈ 0.5 × Qg × Vdrive = 0.5 × 110×10⁻⁹ × 12 ≈ 0.66 μJ
Energy in Resistor (E_res):
Eres ≈ Egate ≈ 0.66 μJ (It shares roughly half the energy).
Pulse Width (t_pulse) (Assume a conservative 1ms):
Formula: Pavg = Eres/tpulse
Pavg = 0.66×10⁻⁶/1×10⁻³ = 0.66×10⁻³W = 0.66 mW
Conclusion:
The average power is negligible (~0.66mW) for both resistors.
The critical specification is the peak instantaneous power and the pulse handling capability.
2.2Ω Resistor: Must handle ~66W pulses.
4.7Ω Resistor: Must handle ~31W pulses.
A wirewound resistor is not suitable because it will:
Slow down the switching speed by resisting the rapid current change (dI/dt).
Cause ringing and voltage overshoot on the gate signal, which can stress or damage the gate oxide.
You must use non-inductive, pulse-rated resistors (e.g., metal oxide film or carbon composition) rated for these high peak pulses, even though the average power is tiny; a standard 1/4W carbon film resistor would be destroyed. Requirement: Must withstand a ~65W peak pulse for ~100ns.
I asked Deepseek to recommend a suitable resistor.
It suggested non-inductive Vishay PR02 2W metal film:
Analysis:
The resistor is subjected to a high-power pulse when switching the MOSFET gate. Suitability is determined not by its continuous power rating, but by its ability to absorb the pulse's energy without damage.
Calculate Pulse Energy:
Peak Power, Ppeak: 65W
Pulse Duration, tpulse: 100ns
Pulse Energy, Epulse: Ppeak × tpulse = 65 × 1×10⁻⁷ = 6.5 μJ
Compare to Resistor Thermal Capacity:
A 2W metal film resistor has a larger mass. Let's estimate 0.5 grams (5 x 10⁻⁴ kg) and a specific heat capacity similar to ceramic/glass/plastic (~800J/kg·°C).
The energy required to raise its temperature by just 1°C is:
E = m × c × ΔT = (5 x 10⁻⁴ kg) × (800J/kg·°C) × (1°C) = 0.4J
Conclusion:
The pulse energy is 6.5µJ/400mJ = 0.00001625 (or 0.001625%) of the energy needed for a 1°C rise.
This means the 6.5µJ pulse is over 61,500 times maller than the energy needed to raise the resistor's temperature by a single degree Celsius. Therefore, the Vishay PR02 0.5W resistor is perfectly suitable and will not be damaged or degraded by the 65W, 100ns gate drive pulse.
The component's thermal mass renders the brief power spike harmless.
I couldn't find reasonably priced 2.2Ω and 4.7Ω PR02s but I did find 50x 10R 5% PR02s on eBay for £4.60.
Deepseek approved of my idea to parallel them by soldering them on top of each, minimising inductance.
The large number is also beneficial in finding matching resistors out of the 5% tolerance although when I came to measure them, I found most were close to spot-on.
Deepseek now recommended starting with 5Ω:
5.0Ω (2x 10Ω in parallel): Slower, more damped, most stable.
3.3Ω (3x 10Ω in parallel): A balanced middle ground.
2.0Ω (5x 10Ω in parallel): Fastest switching, highest current, highest risk of ringing.
Start with the 5Ω configuration.
Use an oscilloscope to check the gate waveform for ringing and overshoot.
If the waveform is clean and stable, try 3.3Ω to see if you can achieve faster switching without introducing instability. Only proceed to 2.0Ω if the 3.3Ω waveform remains clean.
Capacitor bank wiring
The best way to wire a capacitor bank is to ensure wires from every capacitor are of equal length in order to minimise inductance and resistance. Star points are required but the + star point takes priority over the 0V starpoint. Deepseek puts it succinctly:
For a capacitor discharge welder, the capacitor bank positive lead star-pointed to a short cable to the tungsten electrode is significantly more critical than the ground lead star point.
Explanation:
The weld energy is delivered in a high-current pulse. The dominant parasitic element in this path is inductance (L), not resistance. Inductance opposes the instantaneous rise of current (di/dt). The voltage drop across an inductance is V = L × di/dt.
Summary:
Positive Lead (Priority): This must be the absolute shortest, most direct path possible from the capacitor bank positive terminal to the tungsten electrode. This minimises loop inductance, ensuring the fastest possible current rise time into the weld joint .
Ground Star Point (Secondary): Implement a star ground for the control and return paths (TC4422 ground, negative supply) at the capacitor bank negative terminal. This isolates the sensitive gate drive from high-frequency noise created by the main discharge loop.
ESD protection
A capacitive discharge welder acts like a large fast inductive pulse generator and induces noise spikes on power rails. To counter this and potential ESD, P6KE15CA Transient Voltage Suppressors (TVS) are across all MOSFET D & S and across drain to 0V. I also placed one across the capacitor bank.
The P6KE15CA is a Bi-directional TVS: it clamps both positive and negative voltage spikes equally, and its 15V standoff breakdown range is 14.3V - 15.8V (capacitor voltage will be 12V max), clamp 27.2V maximum (the mosfet is rated at 30V max). It has a peak pulse power dissipation of 600W for a 10/1000µs transient waveform with a 10A peak, defined by two parameters:
t₁ (10µs): pulse rise time, measured from 10% to 90% of the peak current (I_PP).
t₂ (1ms): pulse duration, from 50% on the rising edge to 50% on the falling edge.
Essentialy this is the equivalent of a long, high-energy surge of the type you might expect with a capacitive welder. The energy from a fast ESD spike is less, but still needs to be clamped.
Normal Operation: The TVS diode is invisible. It has a very high impedance at voltages below ~14.3V.
ESD/Overvoltage Event: If a voltage spike tries to push Vgs beyond ±16V, the TVS diode instantly 'clamps' the voltage, shunting the destructive current away from fragile semiconductors.
Placement: Solder it as physically close as possible between the MOSFET source and 0V.
Ideal capacitor bank
Given the capacitor bank sits between the anvil and electrode, the ideal implementation for the mechanical arrangement is an axial capacitor bank with + and - leads on opposite ends. However the trend nowadays is radial and I could not find an affordable axial capacitor matching the low ESR and high ripple current of the radial Panasonic EEUFC1E222.
Bank implementation
A hybrid axial bank can be built by splitting the radial leads. The capacitor +ve lead length is more critical than the -ve, so the +ve is kept short and the -ve lead is bent to run down the side of the insulated capacitor can with TCW crimped to it to extend it beyond the can bottom. Each can is rotated 45° so its -ve lead has internal space to run within the capacitor stack.
Google AI:
1. ALWAYS CLEAN ALL WIRES WITH IPA BEFORE CRIMPING
2. 1MM TCW HAS ROUGHLY 1µH OF INDUCTANCE PER MM
The capacitor lead is 0.8mm (~21 SWG), the additional TCW is 19 SWG (1.016mm dia).
GOOGLE AI:
1. 1mm of TCW has roughly 1µH of inductance
2. A 2.5mm² crimp ferrule is the best fit
3. Use two in series to lower inductance
4. leave a 2mm gap between them to stop adjacent crushed crimps impinging.
Self adhesive Kapton tape wrapped around the can secures the -ve lead to it.
The capacitors are arranged as two rows of 8, wired as 4 blocks of 4 with their +ve and -ve leads separately routed in an X shape toward the gap in the centre, ensuring every lead is the same length.
Brass/copper clamps
Instead of solder, the capacitor leads are clamped between 1mm thick soft, annealed copper sheets, themselves clamped by solid brass bars secured by torqued button head M6 Allen bolts.
This 'Cold Weld' assembly is superior to soldering and will provide the lowest possible resistance for the 176µs rise time and easily handle the 1600A peak without micro-arcing. This design maximizes surface contact while protecting the brittle MOSFET leads from the mechanical 'kick' of the discharge.
The bolt heads run upward from a 3mm thick fibreglass sheet suspended on the back of the box lid by TBD mm square ABS bars. Nordlock washers below M6 nuts on the top clamp bar counter vibration from the intense weld pulse. The ABS bars are secured to the lid by TBD mm screws at the box centre and corners.
There are three clamps, all with the the bottom brass bar measuring 1/2" square. on top of this ia a 1mm thick sheet of soft annealed copper. The height is based on the 1/2" diameter of the capacitors because the equidistant capacitor leads enter them at this level. This also means the MOSFET are retained at the level. Once the leds are present, a second 1mm cupper sheet is added, then the top brass clamp.
The top brass bar on the capacitor -Ve lead clamp (the 0V star point) is also 1/2" square.
The top brass bar on the capacitor +ve lead clamp (the _ve star point) is 1/8" high.
The top brass bar on the MOSFET Source lead clamp (lwading to the anvil) is 1/8" high.
Step-by-step assembly for the 1600A solderless power clamp.
Cleaning: Use a Scotch-Brite pad to buff the MOSFET leads, copper plates, and braids until they are bright and shiny.
Clean everything with IPA.
Apply a tiny smear of conductive contact grease if you have it.
Thermal Grizzly Conductonaut
21/05/26: 1g £10.99:
https://www.ebay.co.uk/itm/147237387303
Insert four M3 Grade 8.8 (or A4 Stainless) bolts into the 3mm fibreglass base and up through the brass.
Add Nord-Locks: Place an M3 Nord-Lock pair under each nut.
Critical: Ensure the 'cams' (wedge sides) of the Nord-Lock pairs are facing each other.
Initial Snug: tighten both nuts by hand until the sandwich is held together and aligned.
Final Torque: evenly tighten bolts to 0.3Nm.
Capacitor -ve copper/brass clamp
The capacitor -ve leads, MOSFET Drains and their individual TVS, as well as the capacitor block TVS, are all clamped within the 0V star point clamp.
Capacitor +ve copper/brass clamp
THe capacitor +ve leads, the capacitor block TVS and the braids that lead to the electrode areall clamped within the +ve star point clamp.
The capacitor +ve leads are symmetrically arranged to emerge from the X point into their own brass / copper clamp. On the other sideof the clamp four 6mm² copper braids lead up through the box lid to the electrode, which is a 12mm dia solid copper rod, their M8 ring crimps secured to it by an M8x22mm button head allen bolt torqued to 22Nm.
For now, the braids pass through insulating 9mm wide heatshrink tubing purely as it was supplied in a flat form width that matches the braid.
The braid has a loose loop form to allow the outside electrode to be pivoted, with the slack taken up inside the box. Initially I was going to run the braid through a gland but instead settled on a slit as this maintains the best box EMC to protect the internal electronics from the weld impulse.
Capacitor -ve & MOSFET copper/brass clamps
On the other side of the capacitor -ve clamp, four equispaced IRLB3813 30V 240A (1050Apk), TO-220 N-channel MOSFETs Drains are screwed to their own block via M3 bolts with the upper clamping brass block secured by Norlocks below nuts torqued to 0.8Nm.
The MOSFET Drain block is aligned horizontally so the MOSFET Sources are clamped to the capacitor -ves, together with a 1.5KW TVS across each, with minimal space separating the blocks (essentially the length of the MOSFET bodies).
Google AI:
Because you are dealing with (fast current changes), the inductance of the TVS leads matters.
Bolt or clamp the TVS leads directly across the Drain and Source copper blocks of your MOSFET sandwich.
Short Leads: Keep the TVS legs as short as possible. Even 10mm of wire adds enough inductance to let a 'spike' bypass the TVS and hit the MOSFET silicon first.
GOOGLE AI:
1. USE NO MORE THAN 50MM OF EACH BRAID TO MOSFET SOURCE (ADDS 12µs DELAY; 25MM ADDS 7µs DELAY)
2. RUN BRAIDS CLOSE TO EACH OTHER. 4 IN PARALLEL IS BETTER THAN 1 FAT ONE.
Separately two pairs of 6mm² copper braids clamped to the other side of the Drain block lead a short distance 40mm? TBD to a 12mm dia copper tube where their M8 ring crimps are secured by an M8 nut and Norlock, on an M8 button head allen bolt secured to the outside top of the brass anvil. The bolt runs through the copper tube which is secured to the underside of the brass anvil whilst insulated from the aluminium box, minimising resistance and inductance.
0V star Point
There is one 0V star point origin, which is the centralised point of the combined capacitor bank -ve clamping block; all 0V signals are referenced to this point.
Power supplies
May 2026: initially I was going to power the bonder from lab supplies as I thought this would simplify things but the more I got into the design the more I realised this raised more issues associated with noise and conductance and I instead decided to replace them with internal supplies.
The box base has a standard 3-pin mains IEC C14 male connector followed by a DPDT switch. I'm relying on the mains plug fuse set at 3A as the mains feeds into a CE qualified Tiger TP1147, 20W 12V 2A brick with figure IEC C8 plug. A 90° degree C7 (socket) to C7 plug adaptor plugs into the brick, making it just short enough to fit sideways in the box.
Earth
The TP1147 brick is unearthed but the 3-pin C14 provides this connection which is wired to the pre-tapped earth point on the box base using Faston crimps, and from there, a second earth wire that mates with an earth spade on the lid.
The 12V output lead from the brick powers the electronics on the lid.
[T21] wire AWG & properties
Wire CSA Amps Resistance Vdrop over
gauge @30°C /km /150mm 0.15m @ 1A
30AWG ~0.05mm² 0.14A ~335Ω ~55.0mΩ ~55mV
28AWG ~0.08mm² 1.0A ~216Ω ~32.4mΩ ~32.4mV
24AWG ~0.21mm² 3.5A ~88Ω ~13.2mΩ ~13.2mV
22AWG 0.33mm² 5.0A ~53Ω ~8.0mΩ ~8.0mV
24AWG is a convenient gauge for all power & charging.
Twisted pairs of the separate 0V & 0-12V and 0V & +12V signals are carried to their respective electronics. Wires are only as thick as they need to be to carry their required currents.
PCB and 0V shield
A non-annealed 1mm thick rigid copper sheet measuring 100x100mm is secured to the top of the capacitor bank -ve clamping block by its Nordlocks and M6 nuts. The opposite end of the sheet is retained on the top of the capacitor +ve clamp block using self adhesive pcb mounts. This sheet acts as an 0V shield to minimise EMI from the capacitor bank.
6mm brass standoffs secured to each corner of the copper sheet retain the only pcb in the box on the lid. The copper sheet is essentially the 0V star point and the 0V supply to the components on the pcb.
Gate wiring
The pcb holds the TC4422 gate drivers which are arranged in a single row with each horizontally aligned to its MOSFET. The gate lead from each MOSFET is bent vertically at the point where it widens emerging from its TO-220, where an RF EMI suppression bead is placed over the lead. After this two 10R PRO2 gate resistors wired in parallel are soldered to the gate lead and run vertically upward to the pcb where they are soldered close to their respective TC4422 output pins. The vicinity of the 0V block and presence of the copper sheet minimises disturbance to the vertical gate lead which is as short as practicable.
Bias Ground: Connect the TC4422 0V (GND) return wires directly to the 0V star point to ensure the gate drive sees the true Source potential.
A HEF4538 one-shot central and inward from the TC4422s provides their 2ms driving pulse and its output is wired to them as an equidistant star point, with inner leads the same length as outer leads.
Dc-dc converter
The brick 12V enters the pcb via a 2-pin pcb screw terminal block and powers the pcb electronics. Separately, it passes through a DPDT relay and into an up/down dc-dc converter module based on the LTC1340.
Self-discharge protection
The dc-dc converter charges the capacitors at 0.5A (set by a trimpot) through protective diodes. A forward conducting diode on the positive lead to the capacitor bank +ve and on the return from the bank -ve to the converter, protect it from damage if it is turned off and there is still charge on the capacitors; it also stops the capacitors from discharging through it.
I asked DeepSeek to recommend suitable diodes. Initially I was considering charging the capacitors at 4A and asked for 6A diodes. Now it's unlikely to be charged at more than 1A; the initial design had no relays and avoided schottkies due to their high reverse current but this is also no longer the case so these will now be replaced by schottkies:
Recommendation: FR607 6A fast recovery rectifier
Datasheet (Diodes Incorporated FR607):
Average Forward Current (I_F): 6A (at Tc=75°C).
Peak Forward Surge Current (I_FSM): 150A (8.3ms).
Repetitive Peak Reverse Voltage (V_RRM): 1kV.
Reverse Leakage Current (I_R): < 10µA at 25°C.
Package: R-6.
Relays, dc-dc & DMM
A second DPDT relay on the dc-dc converter output after the protection diodes allows it to be disconnected from the capacitor bank ready for the welder to be fired.
The non-energised position of both relays connects the dc-dc converter via its discharge protection diodes to the centres of the capacitor bank +ve and -Ve clamping blocks so it charges the capacitors by default. Fail safe circuitry ensures a weld cannot be invoked when the capacitor bank is being charged.
When the relays are energised, the capacitor bank is isolated ready for a weld.
The two DPDT relays are located each side of the HEF4538 towards the sides of the pcb, with the dc-dc converter at the opposite end of the pcb, orientated with its drive MOSFETs furthest away.
A 4-digit DMM on the top of the lid indicates the capacitor bank charge voltage. The DMM is wired to the relay contacts fed by the dc-dc converter, with power scavenging from it so when the relays cut out the dc-dc, the DMM remains powered by its residual energy.
The DMM is directly on top of the lid with a small hole through it solely for its 3 leads, maximising EMI protection for the internal electronics. 1.5Kw TVS across the dc-dc and DMM add further protection.
A lockable dial on the lid top rotates a 10-turn 10k pot inside the box that is wired through coax in place of the dc-dc converter voltage control trimpot.
Switch controls
The relays switch out both the input and output of the converter when the CHARGE/FIRE switch on the top of the lid is in the FIRE position, removing EMI from the massive current pulse during the weld. LEDs are hardwired to the relays to indicate status: Charging: red, Fire: green.
ADD A LINK FROM THE CHARGING RELAY CONTACT TO THE HEF4538 TO PREVENT THE WELDER FROM BEING FIRED.
A push switch on the lid top drives the 1st HEF4538 one-shot to produce the 2ms pulse to the TC4422s. The 2nd one-shot provides a 10ms pulse to a high efficiency white LED indicating the f
Electrodes
I asked DeepSeek to recommend suitable electrodes:
Positive electrode
Look for blue 2% Lanthanated Tungsten Electrodes, often used for TIG welding, diameter 1mm: the small mass will provide a good point for focusing current and has low thermal inertia so will heat to emission temperature faster and cool faster after the discharge, which is beneficial for pulsed operation. These usually come with flat tips and you will need to grind a pointed tip on it.
Why Lanthanated Tungsten:
It has excellent high-temperature stability, good arc characteristics, and is non-radioactive (unlike the popular but radioactive Thoriated Tungsten).
1.6mm Lanthanated Tungsten; grind a small, flat tip on it ~0.5mm to 1.0mm diameter.
This small flat tip concentrates the current onto a precise spot on the wires, but the flat surface applies pressure without cutting into them.
For TIG welding electrodes, the color code for Lanthanated Tungsten is not universally standardized, but the most common conventions are:
Gold: Typically denotes 1.5% Lanthanated (EWLa-1.5)
Blue: Typically denotes 2.0% Lanthanated (EWLa-2.0)
For this micro-welding application, the Blue (2.0% Lanthanated) is the better choice.
Reasoning:
The higher lanthanum oxide content (2.0% vs 1.5%) provides slightly better electron emissivity and arc stability at lower currents. This translates to a more consistent and reliable performance for the precise, low-energy pulses in a capacitor discharge welder, giving a more forgiving and stable electrode.
Whilst searching for a 1.6mm electrode I found a 1.0mm one which Deepseek agreed was a better choice:
For the purpose of a discharge electrode or a robust, high-temperature point (not TIG welding), the 1.0mm blue lanthanated tungsten is a superior choice to the 1.6mm.
Sharper Tip:
A 1.0mm rod can be ground to a much finer, sharper point than a 1.6mm rod. This creates a higher electric field density, which promotes easier and more reliable arc initiation or electron emission at lower voltages.
Keeping the 1mm tip flat makes it easier for the weld arm to press down upon the wires and hold them in their place; the weight of the arm on a pointed tip on the relatively soft TCW leads could damage them.
Faster Heating/Cooling:
The smaller mass has lower thermal inertia. It will heat to emission temperature faster and cool faster after the discharge, which is beneficial for pulsed operation.
Material Suitability:
Lanthanated tungsten (blue) is an excellent general-purpose choice. It holds a sharp point well, has good electron emissivity, and is more forgiving than pure tungsten.
Durability:
The 1.0mm tip will erode slightly faster than a 1.6mm tip under identical arcing conditions however it will still last for thousands of cycles.
Negative electrode (anvil)
A large, flat-faced electrode. This acts as the anvil. It provides a stable, supportive backing for the
workpiece.
Initially DeepSeek was happy using the aluminium box as the negative anvil, but as the design progressed, later sessions questioned this:
Aluminium (235 W/m·K): Original baseline (1.5J-2.5J).
Copper (400 W/m·K): Sinks heat fastest, requiring the largest energy increase (~+0.5J est.).
Brass (115 W/m·K): Will sink heat slower than Aluminium.
The aluminium box presents a high and variable contact resistance. This is not the bulk resistance of the aluminium, but the high and non-reproducible surface contact resistance of the aluminium oxide layer at the microscopic points of contact with the Kanthal wire. This could lead to inconsistent weld energy delivery.
Copper does not have this issue but has a thermal conductivity 60% higher than aluminium that would likely increase the energy requirement by 0.5J, Shifting the target operational 'sweet spot' from 1.5J - 2.5J, to approximately 2J - 3J (10.7V - 13.1V), with little room for experimentation above this: if the 12V limit is now raised to 14V, that leaves only 13.6% below the 16.2V point of vaporisation.
Copper thermal conductivity is 40% greater than aluminium.
Brass thermal conductivity is 51% less than aluminium.
Brass is a better compromise as it requires less energy than aluminium and retains the safety margin.
The anvil is a 3mm thick 100mm x 100mm brass sheet secured to but insulated from the aluminium box lid.
Electrode wiring
The wiring from the capacitor bank + to the + electrode is four xxxmm long 9mm wide 6mm² copper braids terminating in 8mm ring crimps bolted to the end of a 12mm copper rod using a brass M8 x 10mm screw and three 1mm compressible copper washers.
The electrode arm pivots inside a Delrin block mounted on top of the brass plate. The -ve electrode (return) is a brass plate connected to the MOSFET Drains via four xxxmm 9mm wide 6mm² braids and a short length of 12mm dia copper tube bolted to the underside of the brass anvil.
Electrode assembly
The electrode assembly consists of two vertical blocks of low friction Delrin bolted to the box lid, with a small cube of Delrin pivoted on a M6 bolt between them, the 12mm copper rod running perpendicular to it, through the cube centre. The Delrin blocks were bought ready machined, ensuring tight tolerances for the 0.3mm wide Kanthal weld area.
At the opposite end, a central slit in the rod extends inward to a 1mm dia vertical hole within which the 1mm dia lanthanated tungsten electrode runs, clamped by a single horizontal bolt through the rod.
The rod was chosen to be 12mm because it perfectly matches the internal 0.5" diameter of my Pearson 410 current probe, which itself bears down upon and keeps the weld surfaces together. Rubber O-rings each side of the 410 secure it to the rod.
+Ve star Point
As observed at the outset, the +ve star point on the capacitor bank +ve terminal is more critical to the weld than the 0V star point and whilst its length to the electrode is longer, it is copper throughout vs. the 0V Star point at the capacitor -ve that traverses MOSFETS, copper braid & rod, and brass sheet.
EMC
The welder generates a powerful, fast-changing magnetic field with every discharge. The earthed metal box prevents the weld field from inducing stray voltages and currents in the sensitive timing (HEF4538) and gate drive (TC4422) circuits, which could cause misfiring or erratic behaviour.
Construction
The box internal height is 59mm. The box lid internal height is 16mm.
The anvil is a 100mm x 100mm x 3mm thick brass sheet.
A 100mm x 100mm x 2mm thick fibreglass sheet below the brass isolates it from the aluminium box lid.
A 100mm x 100mm x 0.3mm fibreglass sheet above the brass acts as a weld template with slits cut into it for the Kanthal wire to be pressed against the anvil at the weld point whilst isolating the Kovar lead.
Four Nylon screws from within the drilled aluminium box lid pass through each corner of the 2mm fibreglass sheet and are retained by matching threads tapped in the brass sheet above it. The screws then extend above the brass sheet, securing the 0.3mm fibreglass template sheet via washers and nuts.
The template is easily reversed to weld the other side of the IR source.
Alternative custom templates can be fitted to weld different components.
The electrode arm is a 12mm diameter x 150mm long brass rod that rotates on a pivot at one end.
At the other end a central slit with a clamping bolt secures the 1mm tungsten electrode to the arm.
A copper braid 200mm x 9mm: 6mm² is folded over and its M8 end rings are bolted into the pivot end of the 12m rod using a brass M8 hex screw with interspaced M8x1 brass compression washers. The M8 bolt tightening torque is 8Nm (5.9ftLb); the shear limit is 12Nm.
Lazy loop
The braid is insulated by loose M8 flat heatshrink as it passes through a 10mm slit in the box lid top. A 'lazy loop' at the bottom compensates for arm rotation.
The electrode rod rotates within two Delrin vertical supports 32mm deep x 11mm thick x 65mm tall.
The rod pivots on an M6x60mm screw with 1 washer each side, secured by a Nyloc.
The rod bears down on the target weld leads when horizontal, pressing them into the anvil.
The pivot end of the 12mm electrode arm is held within a small block of Delrin, the lower section of which is drilled for the non-threaded portion of the M6 pivot bolt. The block pivots within the Delrin side supports providing low friction precision movement that could not be achieved if the rod alone was drilled.
The Delrin vertical supports are secured by ??mm thick Delrin spacers top? and bottom
2x M4?xTBDmm bolts each side of centre fix the Delrin sides to the Delrin spacers.
2x M5?xTBDmm countersunk screws in the aluminium box lid pass up into and secure the Delrin blocks.
Oscilloscope trigger
A 50Ω isolated BNC socket with its shield connected to 0V star point and its centre pin connected to the Q- o/p of the HEF4538 LED drive half so its leading -ve edge can act as a trigger for an oscilloscope.
Current monitoring
A [E3] Pearson 410 current monitor is a loose fit on the 12mm electrode rod and held in place by neoprene O-rings each side at the hinge end with sufficient space for the rod to be raised. The centre hole in the 410 is 0.5". The tungsten electrode is removed and the 410 and O-rings slid onto the rod.
With a 50Ω inline terminator into a 1MΩ oscilloscope channel, the 410 can support up to 5000A, producing an output of 50V (1A = 100mV). The anticipated peak current is estimated at 1600A.
Box 0V referenced current monitoring
A 50Ω isolated BNC socket with no connection to its centre pin but its screen connected to 0V star point
to be used to provide a box referenced earth. A short length of 50Ω coax connected the 410 to a BNC T adaptor on this, the remaining socket connected by 50Ω BNC cable to an isolated i/p oscillosocpe (e.g. Tek THS710A).
Spring
The weight of the arm and 410 should be sufficient to hold the Kovar lead over the Kanthal but a spring may be added to compensate for electrode bounce that could cause arcing due to rapid heating and electromagnetic forces from the near instantaneous 1600A surge, particularly if the 410 is removed.
Coil pPreparation on welder
The Kanthal wire leads on the ceramic former are bent outward 90° behind the Kovar leads.
The assembly is laid on the fibreglass template: 1 Kanthal end passing through 0.25mm wide slits onto the brass anvil. The Kovar is laid across the Kanthal.
The heavy copper welder arm's 1mm electrode is lowered onto the KOvar where it crosses the Kanthal.
Components
YET TO BE UPDATED FOR CCT REV 7
2 x FR607 6A fast recovery rectifier, R-6 [Vcap +,-] REPLACE WITH SCHOTTKIES
16x Panasonic EEUFC1E222 2,200µF 25V, ESR 22mΩ, 105°C
4 x NMOSFET IRLB3813 30V 240A (1050Apk), TO-220 [1600A max/4 = 400Apk each]
7 x P6KE15CA bi-directional 15V 600W Transzorb TVS, DO-201 [Vcap, TC4422s, Mosfets]
4 x TC4422 9A peak fast low side MOSFET driver, DIP-8
1 x HEF4538 dual retriggerable one-shot [2ms pulse & weld pulse LED]
[OPTION] 1x LP2950ACZ-5 TO-92 5V 50mA regulator 200mV dropout [OPTION: debug logic]
2 x turned pic IC sockets 16-pin [1x HEF4538, 1x spare: potential logic driver for debug]
1 x square pad pcb 80mm x 105mm
2 x 100pF X7R MLC [HF decouple: TC4422 inputs to increase noise immunity]
1 x 100nF X7R MLC [input: LP2950ACZ-5 LDO 5V regulator]
1 x 100nF X7R MLC [HF decouple: HEF4538]
1 x 100nF X7R MLC [timing: HEF4538 one-shot Q2 for 2ms to pulse white LED
3 x 100nF X7R MLC [HF decouple: TLC4422 12V supplies and LP2950ACZ-5 output]
1 x 1µF X7R MLC [delay: HEF4538 Q1, Q2 power-up reset]
1 x 1µF X7R MLC [timing: HEF4538 one-shot Q1 for 2ms weld pulse]
1 x 10µF X7R MLC [output: LP2950ACZ-5 LDO 5V regulator]
5 x 10µF X7R MLC [LF decouple: TLC4422 12V supplies (2x10µF each) & OPTION: LP2950ACZ-5 input]
2 x 10µF X7R MLC [in parallel where 12V enters the board]
1 x 3mm 2mA green LED [READY TO WELD]
1 x 3mm 2mA red LED [CHARGING]
1 x 3mm 2mA white LED [weld pulse indicator]
20x 10Ω 2W Vishay PR02 [non-inductive: for experimental gate drive [initially 5Ω each]
1 x 2k0Ω 1/4W resistor [12V red LED (3.0Vf)]
1 x 2k2Ω 1/4W resistor [12V green LED (3.0Vf)]
1 x 5k1Ω 1/4W resistor [12V white LED (3.2Vf)]
4 x 10kΩ 1/4W resistor [pull down: mosfet gates]
1 x 10kΩ 1/4W resistor [pull up: HEF4538 EN- to +12V]
1 x 10kΩ 1/4W resistor [pull up: HEF4538 CLR- to +12V]
1 x 20kΩ 1/4W resistor [HEF4538 one-shot Q2 for 2ms to pulse white LED from +12V]
1 x low current push switch [to activate HEF4538 weld pulse one-shots]
[OPTION] 1x LP2950ACZ-5 LDO 5V regulator
2 x Miniature screened audio coax for the 10k pot & DMM
4 x copper braid 200mm x 9mm 6mm² with M8 end rings [to be cut to length]
tinned copper wire 19SWG (1mm) [capacitor lead extension]
1 x blue 2% Lanthanated Tungsten Electrode 1mm dia x 150mm
2 x isolated BNC for debug
1 x Aluminium enclosure 160mm x 220mm x 80mm
1 x 100mm L x 100mm W x 2.0mm fibreglass sheet below brass sheet anvil
1 x 100mm L x 100mm W x 0.3mm fibreglass sheet above brass sheet [Kanthal weld aperture template]
4 x Delrin offcuts 65mm x 30mm x 20mm
1 x Brass sheet 100mm square x 100mm wide x 3mm [negative electrode anvil]
4 x Brass bar 1/2" x 1/2" x 4" [105mm / 4.1/8" long: lid inner width]
2 x Brass bar 1/2" x 1/8" x 4" [105mm / 4.1/8" long: lid inner width]
ABS 1/4" square to support pcb on lid underside [2x box length + 1 centre width]
Copper rod 12mm diameter x 150mm [weld arm]
4 x button head allen bolts M3 [MOSFET drains]
8 x button head allen bolts M6
4 x Norlock M3
8 x Norlock M6
1 x Brass M8x6 screw [braid connection point at electrode arm hinge end]
3 x copper M8x1 compression washers [braid connection point at electrode arm hinge end]
1 x M6 x 60mm hinge bolt / sleeve bolt
4 x M5 x 20mm Nylon screws to isolate the brass sheet from the aluminium box
General screws + spacers + washers + Nylocs
Element parts: MUST USE GOLD-PLATED *KOVAR* NOT TINNED COPPER
Kanthal A-1
1x hollow ceramic tube ID 0.7 OD 1.2mm x 100mm for element [TCW in tube ends to support coil]
OR 1x ceramic rod 1.2mm x 100mm for element [TCW wrapped around rod ends to support coil]
1x ceramic rod ID3.3mm x2 in OD10mm x 250mm long TC spacer [to be cut to length: Est. 10mm]
Initial crimp based element:
0.75mm² x 8mm bare crimp [0.3mm Kanthal wrapped around 0.6mm TCW]
508j01 P1150585 REV7 26-04-26 cct for bonder LIBS7 P.74
I MAY NEED A FORWARD DIODE + RESERVOIR CAP ON THE DC-DC I/P.
508j02 P1150582 REV7 25-04-26 box assembly LIBS7 P.75
508j03 P1140956 R1 TWC & PR02 combos LIBS7 P.20
508j04 P1150947 REV5 18-05-26 pcb over caps LIBS7 P.87
508j05 P1150945 REV5 18-05-25 assy side LIBS7 P.87
508j06 P1150920 REV2 16-05-26 caps in lid LIBS7 P.85
Placing the work
Method:
The pointed tungsten electrode contacts the larger lead TCW; the Kanthal is pressed against the flat brass anvil.
The weld forms at the interface of highest resistance and current density, which is the contact point between the two dissimilar wires. Placing the pointed electrode on the larger TCW and the flat electrode under the finer Kanthal is the correct method. Here's why:
Current Concentration:
The sharp point on the TCW forces the immense current to focus into a tiny area at the contact point with the Kanthal.
Controlled Heat Generation:
This creates an intense, localized hotspot precisely where you want the metals to fuse—at their junction.
Anvil Function:
The large, flat electrode under the Kanthal provides a stable, cooling backing that prevents the fragile Kanthal from being vaporized and contains the weld.
Sequence of events
The wires are placed and the electrode brought down on them.
If present The Pearson 410 BNC is connected to an oscilloscope: 50Ω terminator, 1MΩ input.
The CHARGE/FIRE switch is set to CHARGE.
The capacitor voltage set on the 10-turn lockable dial.
The box is plugged into the Mains and its switch is set to on.
The red LED illuminates to indicate the capacitor is charging.
When the DMM indicates the capacitor is charged, the switch is set to FIRE,
The green LEd illuminates to show the unit is ready to weld.
The weld button is pressed to make the weld and the White LED should flash.
Calibration first
There will be quite a bit of experimentation first with just the wires alone to determine the best bond voltage.
Making the bond (weld)
THIS WAS BASED ON THE ORIGINAL PLAN TO WELD 0.6mm TCW TO 0.3mm KANTHAL.
THIS NEEDS TO BE REVISED FOR KOVAR
Based on the wire's tiny cross-sectional area and the goal of a localized fusion weld without vaporization, the sweet spot is likely 1 - 2 Joules.
Reasoning:
Below 1J: Risk of a cold, brittle joint due to insufficient melting.
Above 2J: High risk of vaporizing the 0.3mm Kanthal wire at the weld point.
Test:
Start at 1J, inspect the weld (it should be a fused nugget, not a sintered powder), and increase energy in 0.2J increments until you get a strong bond.
Make the bond using a cross-wire configuration.
No Bond? Increase energy by 0.25J - 0.5J and repeat.
Weak Bond? Increase energy slightly.
Sputtering/Splash? Energy is too high. Decrease immediately
The TCW will require less energy to melt than Kanthal, so the Kanthal is the limiting factor.
Reasoning:
Melting Point:
Kanthal A-1 (FeCrAl): ~1500°C
Copper (Cu): 1083°C
Brass: 900°C to 940°C
Solder Tin (Sn): ~232°C
The tinned copper wire will soften and melt its surface coating at a much lower temperature than the Kanthal will even begin to soften. The brass anvil will remain well below its melting point because
the weld energy is highly concentrated at the wire interface and brass has excellent thermal conductivity (~115 W/m·K), spreading heat rapidly across the large anvil.
The Welding Process:
The goal is to raise the interface to a temperature where the metals fuse. The tinned copper, with its lower melting point, will become molten first. This molten alloy wets the surface of the hot Kanthal, forming a eutectic bond.
Why Kanthal is the Limiting Factor:
You must pump enough energy into the junction to get the Kanthal hot enough to be wetted by the molten copper/tin. If the Kanthal isn't hot enough, the molten solder will just bead up on it without bonding. The entire process is gated by the temperature of the higher-melting-point material.
So, the statement holds: the Kanthal's need to reach bonding temperature is the limiting factor, which is why we focus the current on it via the pointed electrode on the tinned copper side.
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Datasheets
Additional notes from Google AI:
Dry Run Check:
For the first test, charge the bank to only 1V or 2V.
If the voltage stays steady when CHARGE/FIRE is set to FIRE, the hybrid axial conversion is a success.
If the MOSFETs aren't 'snapping' fast enough (you'll hear a 'hiss' instead of a 'crack' during the weld), you can parallel a third or fourth 10R to drop the gate resistance further,
ASK GOOGLE AI IF ANY OF THESE ARE A PROBLEM WITH KOVAR:
To prevent the 1mm lanthanated tungsten from sticking to the 0.6mm TCW (Tinned Copper Wire), you need to manage the intermetallic bonding that happens when the tin on the wire melts and 'wets' the tungsten surface.
Follow these steps to pre-condition the flat tip:
The "Polished Flat" Finish
The Problem: Micro-scratches from grinding act as "teeth" that molten tin can grab onto.
The Fix: After grinding the tip flat, polish the face using 600 to 1000 grit emery paper. A mirror-like finish reduces the surface area available for the tin to "key" into.
Controlled 'Blueing' (Oxidation)
The Trick: Unlike most welding where you want a clean electrode, a very thin layer of tungsten oxide acts as a non-stick barrier against molten copper and tin.
The Method: Before your first real weld, heat the tip slightly with a torch until it turns a straw or deep blue colour. This microscopic oxide layer is conductive enough for the pulse but chemically 'slippery' enough to prevent the tin from wetting the tungsten.
The 'Carbon' Barrier (The Pencil Trick)
The Hack:
Rub a graphite pencil (HB or 2B) across the flat face of the tungsten tip before the first few welds.
Why: Graphite is a natural release agent and a decent conductor. It prevents the molten tin from making a direct molecular bond with the tungsten. After 2–3 'hot' pulses, the graphite will bake into the surface pores, providing a long-lasting non-stick coating.
Periodic 'Flick' Cleaning
Maintenance: Every 5–10 welds, check the tip for a 'silvery' buildup of tin. Do not file it off. Instead, use a brass wire brush or a piece of Scotch-Brite while the tip is still warm. This removes the tin without scratching the underlying tungsten.
⌠ ⌡ ∫ │ ─ √ φ θ Θ ∂ δ ζ ξ ς λ ψ ω τ µ Ω ∆ Δ ∑ ∏ π Ξ ○ ≠ ³ ² ±
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