​Whilst designing a replacement IR energy emission element for a FTIR chemistry analyser [See 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.​

​​​​​​

Issues highlighted in red or fuschcia still need resolving.

​

Deepseek AI

​​​

​​I brainstormed with DeepSeek AI, similar to Iron Man and JARVIS [G28] but minus the fancy virtual 3D.
I did manage to teach it how to draw an NMOSFET using text alone (it took 15 iterations!), but then it made a pig's ear of the rest of the initially trivial circuit and I abandoned this route. 

 

DeepSeek's text description is fine with the caveat you have to question everything it says because it is often wrong, even if you state at the beginning 'do not guess, only use datasheet values' however sometimes it encounters a dichotomy in its heuristic rulebase and chooses the wrong leg. This is understandable given it is only its fifth release; in 2026, AI-assisted design is still in its infancy:

Date      Version        Key Changes
28/05/25  R1-0528        Enhanced reasoning, optimized front-end, reduced hallucinations
21/08/25  V3.1           Hybrid reasoning architecture, improved agent capabilities, higher efficiency
22/09/25  V3.1-Terminus  Language consistency improvements, optimized Code Agent and Search Agent
29/09/25  V3.2-Exp       Experimental version with enhanced capabilities
01/12/25  V3.2           Official release with high computing efficiency and balanced performance

 

In addition, each session is allocated 128k resource units and the closer you are to using them up the more it overwrites what you told it to begin with, resulting in incorrect answers. A simple rule of thumb is if the vertical window scroll bar is about as high as this symbol '|' you need to start a new session because once the 128k is used up, you are no longer allowed to ask it questions. Fortunately DeepSeek will provide you with a summary that you can enter to continue where you left off. Not unlike the disposable clone Mickey 17 [G29], this design got through 7 Deepseek sessions: the answers you see presented are often the conclusion of multiple iterations correcting errors that ate up the resource.

 

DeepSeek is very capable but a novice would find it very hard to design circuitry. As an engineer, hopefully I've spotted the pitfalls. I've both learned from it as well as enjoyed the interaction. The only downside is after I end each session, it will remember nothing it has learned if I start a new one. Thankfully sessions can be saved because 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.

​

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.

 

Implementation

​

As this bonder is only required for the IR element, I'm not interested in making a full project of it that would ideally be PIC based. Instead and to save time, lab instruments provide all voltages:

​

Amrel PPS-1202 0-18V, 4A, 1mV rms ripple, programmable linear PSU with isolated outputs,

​Manson EP-613  12V, 0.5A, 5mV rms ripple, linear PSU with isolated outputs (fixed output),

​​

The Amrel PPS-1202 PSU charges the capacitors,

The Manson EP-613  PSU powers the drive electronics,

​​​

​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

​

Although the voltage isn't dangerous, the short circuit current is significant. However even though I proved empirically the Panasonic EEUFC1E222 has a long self discharge time, the voltage is critical to maintaining the optimum bond and I didn't want to add a permanent bleed resistor because it would begin to discharge the capacitors as soon as they were charged. Instead I added a manual resistive discharge circuit. I had a Welwyn W21 3W 4.7Ω resistor in stock.

​

The capacitor bank + is connected to the 4.7Ω W21 resistor which goes to the anode of a BT151 thyristor. The cathode of the BT151 goes to the capacitor bank -. The gate of the BT151 is connected to a small push button momentary-on switch, and its other end to the normally closed contact of a 5V reed relay.

 

I chose a reed relay that requires very little current to energise at 5V, which is the highest voltage I could expect to regulate from the minimum anticipated capacitor voltage. The 5V regulator shown on my schematic REV 4 is a 100mA 78L05 with a 2V dropout voltage. I will instead use a 50mA LP2950ACZ-5 with 200mV dropout, as it only has to power a 5mA reed relay.

​

The relay N.C. contact connects the capacitor bank + through a 1kΩ resistor and red LED into the SCR gate, satisfying both the SCR and LED current. The relay coil is energised by a +5V voltage regulator across the Amrel PPS-1202 where it enters, so the SCR cannot discharge the capacitor bank and the PSU: discharge can only take place when this PSU is off.

​​​

Deepseek ran the calcs:

​

Peak Current: Ipeak =  17V/4.7Ω  ≈ 3.62 A

Peak Power: Ppeak   =  17V²/4.7Ω ≈ 61.5 W

Discharge Time Constant: τ = R × C = 4.7Ω × 35,200µF ≈ 0.165s.

​

Discharge Profile:

Capacitor discharge time =  V(t) =  V0 x e^−T/τ

Time to 10% (~1.7V):  t ≈2.3 × τ ≈ 0.38s

Time to  5% (~0.85V): t ≈3.0 × τ ≈ 0.50s

​

The W21 complies with EN 140401-002 which mandates the resistor must withstand 5x 3W = 15W for 5s.

We have a ~61.5W pulse for ~0.4s:

​​

An alternative is a significantly larger 10W cermet resistor rated to 2.5x = 25W for 2.5S.

​

Capacitor Exponential Discharge Pulse Energy through 4.7Ω:

Edischarge = 0.5 x C x V² = 0.5 × 35,200µF ×17² ≈ 5.09J

CECC 40201-002 Qualification Constant Width Pulse Energy:

CECC discharge = P × t = 15W × 5s = 75J.

Discharging the capacitor produces <7% of this.

​

The SCR gate series resistor (R_G) value is calculated using the source voltage (V_source) and the required gate trigger current (I_GT).

 

Formula:

R_G = (V_source - V_GM) / I_GT

V_GM is the gate-cathode voltage during triggering, typically ~0.7-1.5V.

EFor a BT151 with I_GT = 10mA and a 12V source:

R_G ≈ (12V - 1V) / 0.01A = 1100Ω.

​

The BT151 TO-220 metal tab is big enough not to need a heatsink for the 12V/4J discharge:

​

Justification:

Single-Pulse, Low-Energy Event: The discharge is a single, non-repetitive event with a total energy of at most ~4J (from 17V). The average power over the discharge period is irrelevant; the critical metric is the peak surge current and the I²t rating.

 

Peak Current & I²t Calculation:

Peak Current (I_peak): V_max / R = 17V / 4.7Ω ≈ 3.6A

Discharge Time Constant (τ): R × C = 4.7Ω × 0.0352F ≈ 0.165s

 

I²t Value:

For an exponential decay, I²t ≈ (I_peak² × τ) / 2 = (3.6A² × 0.165s) / 2 ≈ 1.07A²s

 

Comparison with SCR Ratings (e.g., BT151-500R):

Peak Non-Repetitive Surge Current (I_TSM): 40A (typical).

I²t Rating: 6.4 A²s (typical).

The calculated values (3.6A peak and 1.07A²s) are substantially lower than the SCR's robust ratings.
The thermal mass of the SCR's silicon die alone is sufficient to absorb the single pulse of heat without a significant temperature rise necessitating a heatsink.

​​​​

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 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, PPS-1202 negative, Manson negative) at the capacitor bank negative terminal. This isolates the sensitive gate drive from high-frequency noise created by the main discharge loop.
 

The capacitors are arranged as 4 columns of 4 rows: -||+ -||+ -||+ -||+, wired as quadrant corner blocks of 4 x + starpointed and & 4 x - starpointed. A 6AWG lead is split into four, and each portion connected to a 4 cap starpoint, making sure all leads are the same overall length.

​​

Star Points

​

There is one 0V star point origin, which is the centralised point of the combined capacitor bank - terminals; all 0V signals are referenced to this point. The capacitor high current discharge path is thick 6AWG wire. The same is true of the + terminals which have their own Vcap star point and high current discharge path 6AWG cable.

​

The capacitor - is the capacitor bank - star point and central 0V star point reference.
The capacitor + is the capacitor bank + star point.

 

Power supplies

​

Thin 0V wires are separately routed from the capacitor - to the places where the incoming Amrel 0-12V and Manson fixed +12V supplies appear on the pcbs. From those points, twisted pairs of the separate 0V & 0-12V and 0V & +12V signals are carried to their respective electronics. The thinner the wire from the capacitor -, the higher the inductance and the less noise is carried to the pcbs; the psu wires are only as thick as they need to be to carry their required currents: 

​

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

​

Depending upon my supplies: 24AWG for the Amrel, 28AWG for the Manson and 30AWG for the sense.

However 24AWG is a convenient gauge for all.

Amrel sense wires
 

The Amrel PSU sense leads are twisted together and connected to capacitor+ and capacitor -. This ensures the supply protection diodes and wiring are compensated for by the PSU, and its indicated programmed voltage is present on the capacitor bank. ADD SENSE PICKOFFS TO SCHEMATIC REV 5

Charge warning LED

A 2k resistor from the capacitor + goes to the anode of a red led and its cathode to the SCR anode at the point where it joins its 4.7Ω resistor. When the button is pressed the scr turns on and the LED will illuminate indicating the capacitors have charge, and will go off when they are discharged. Although this may appear unnecessary given the fast rate of discharge, it serves to warn they are charged, without loading the bank when it is needed.

​​

PSU supply indicators

 

Green LED (3.2Vf) & 2kΩ 1/4W resistor across the 12V supply from the Manson EF-613.

Green LED (3.2Vf) & 1kΩ 1/4W resistor across the 5V regulator powered by the Amrel PPS-1202.

​

​Mosfet switch

​​​​

A low side mosfet switch source connects via another 6AWG lead to the star-pointed 0V that is also the negative terminal of the capacitor bank. The NMOSFET drain leads to the negative electrode 'anvil'. A positive voltage pulse on its gate turns the mosfet on and when the positive electrode (the capacitor bank +) is placed on the wires to be welded, current flows through the weld joint and returns through the mosfet drain to the capacitor negative terminal. 

 

ESD protection

​

However this configuration means the drain of the mosfet is exposed to the world, and it could be damaged by static electricity. In addition, 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, I placed a P6KE15CA Transient Voltage Suppressor (TVS) close to the mosfets and across the drain to 0V.
I also placed one across the capacitor bank and a fuse on its supply inlet in case the charging PSU develops a fault (don't want the capacitor bank to blow).  

​​

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.

 

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.

 

​​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.

​

Self-discharge protection

​

The Amrel PPS-1202 PSU charges the capacitors at 1A. This PSU has isolated outputs and sense inputs. I added a forward conducting diode between the PSU + and the capacitor bank + feeding back to PSU sense+. Similarly a reverse biased diode with its cathode to the PSU - and its anode to capacitor - which goes to the star pointed 0V on the NMOSFET sources and back to PSU sense- the diodes protect the PSU from damage when 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, suggesting 6A.

​

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.

 

The R-6 is a huge 9mm dia package but it's difficult to find a smaller one at 6A, even at low voltages; Schottky diodes are unsuitable due to their high reverse leakage current.

 

Vcap fuse
 

The second channel of this PSU has a fault that drives its output to 29V [Repairs: 10 Power supplies]. To counter the channel I'm using developing the same fault, I added a P6KE15CA, 15V transzorb​ across the capacitor bank and a 4A polyswitch fuse where the supply comes in: MF-R400 30Vmax Ihold 4.00A Imax 40A. In the event of it triggering, its (+5V supply) green PSU LED will extinguish.

​

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:

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The TC4422s are powered from the Manson EP-613 12V supply. I placed 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.

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After the fuse I added two 10µF MLCs in parallel, ensuring 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.
 

​Gate Driver ICs: 2 x TC4422, 9A peak, fast low side MOSFET drivers.
Local Decoupling: 10µF + 100nF MLCs (ideal 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.

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​Pulse width

I was going to use an external variable pulse generator but the pulse width 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:

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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.

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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.

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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.

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Gate resistor

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The TC4422 is capable of a 9A peak output current. The gate resistor sits between its output and the NMOSFET gate input.

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Minimum Resistor for 9A with Vdd set to 12V [Manson EP-613 PSU]:

Rg = 12V = 1.33Ω
     9A

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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.

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The 2.2Ω to 4.7Ω range is the ideal starting point, ensuring robust performance and stability.

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Resistor wattage

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I asked Deepseek to calculate the maximum wattage of 2.2Ω and 4.7Ω gate resistors:

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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.

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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

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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.

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Q_g (Gate Charge) = 110nC

V_drive = 12V

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Energy in Gate (E_gate):

Egate ≈ 0.5 × Qg × Vdrive = 0.5 × 110×10⁻⁹ × 12 ≈ 0.66 μJ

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Energy in Resistor (E_res):

 

Eres ≈ Egate ≈ 0.66 μJ (It shares roughly half the energy).

 

Pulse Width (t_pulse) (Assume a conservative 1ms):

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Formula: Pavg =  Eres/tpulse

​Pavg = 0.66×10⁻⁶/1×10⁻³ = 0.66×10⁻³W = 0.66 mW

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Conclusion:

The average power is negligible (~0.66mW) for both resistors.

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The critical specification is the peak instantaneous power and the pulse handling capability.

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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.

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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.

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I asked Deepseek to recommend a suitable resistor.

It suggested non-inductive Vishay PR02 2W metal film:

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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.

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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.

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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.

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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.

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