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Capacitor Discharge Welder Part 1 By Phil Prosser Make your own thermocouples or battery packs! If you're skilled enough, you might even be able to weld studs to sheet metal. This project lets you build a safe lowvoltage mini spot welder. safe and low voltage I t’s more expensive to buy thermocouples than to weld the tips of K-type thermocouple wire, available cheaply by the reel. And getting a custom-­made battery pack for repair or for your project is also pricey. But the availability of used battery packs and individual cells means building custom batteries is a real option – as long as you have a way of welding tabs onto them. Safely welding tabs to batteries is more challenging than you might think. You cannot use solder to make the joints as the metal does not ‘wet’ easily, and you need to get it dangerously hot to make the joint. This can damage the plastic insulators inside the battery, leading to catastrophic failure of the cell. Tabs on professionally made cells are welded on. This project allows you to do the same yourself. Professional battery welders are generally ultrasonic welders, capacitor discharge welders or high-current spot welders. Most are way out of the ability for hobbyists to build. Capacitor discharge welders are at the lower end of the professional spectrum. These use energy stored in a bank of capacitors to deliver the weld energy to the workpiece. A common characteristic of all battery tab welders is that they deliver an awful lot of energy (typically 100-200 joules, or more) to the connection in as short a period as possible. Safety warning Capacitor Discharge Welding works by generating extremely high current pulses, and consequently, strong magnetic fields. Do not build or use this project if you have a pacemaker or similar sensitive device. This device can generate sparks and heat. Users must wear appropriate personal protective equipment such as welding glasses that meets all relevant regulations and standards, and which provide mechanical and IR/UV protection. 18 Practical Electronics | March | 2023 Features and Specs Weld energy: adjustable, from a few joules up to 208-365J (depending on number and type of capacitors used) Weld pulse duration: 0.2-20ms with optional 0.1ms pre-pulse, 5ms before main pulse Safety features: trigger lockout during charging, foot switch triggering, kill switch Capacitor charging: 2A or 5A (selectable); switch-mode for high efficiency and fast charging Welding leads: 1m min length suggested, but can be customised Power supply: 24V DC, 2.5A minimum (6A+ recommended) Options for DIY One approach is to use a car battery or Li-ion cell with a beefy switching device. A very large SCR or FET is used to short the battery across the ‘weld spot’ for a short period. While this can work, it has a hidden problem. The current is high enough to create a weld but not high enough to do it quickly. As a result, there can be a large ‘heat-affected zone’ and the weld quality varies depending on the health of your battery. The other practical alternative is to roll your own Capacitor Discharge Welder. This is somewhat more expensive than using a big battery but provides more predictable results. Our design also gives you a lot of control of the weld energy and time. Capacitor Discharge (CD) Welders These do exactly what they say on the box. They comprise a capacitor bank that you charge up, and then electronically short it across the workpiece using one or more large FETs, SCRs or other very tough semiconductor switches. The weld is formed by resistive heating in the workpiece. All of the energy that goes into the weld is from the capacitors. This provides you with certainty and repeatability about how much energy is delivered. The energy is also delivered very quickly, in a few milliseconds, which means the weld is done before heat conducts far from the joint. The downside of this is that you need capacitor(s) that can take the abuse of massive discharge pulses, which can get expensive. The upside is that you can control the energy delivered to the weld in two dimensions, both by selecting the voltage the capacitor is charged to and by how long you turn the switches on. Practical Electronics | March | 2023 The front panel of the Capacitor Discharge (CD) Welder. Our approach We want to do better than simply paralleling as many capacitors as we can find and using a giant SCR to switch them. Our goal is a project that allows you to choose the overall scale of the Capacitor Discharge Welder, allowing you to select the most cost-effective capacitors for your application. While researching this, we came across Ian Hooper’s work, which prompted the modular and scalable approach that is presented here, see: https://bit.ly/pe-mar23-sw1 Our design uses multiple Energy Storage boards which stack, allowing you to build a welder with the capacity you need. A separate Power Supply Module allows you to control the voltage and provides a constant charge current to the capacitor bank. A Controller Module enables you to program the weld pulse width you want. These features are typically found on professional kit. Our charger is based on a switch-mode regulator, which means that we can control the current charging the capacitors without using a resistor or linear regulator – both of which would otherwise get stinking hot! With the recommended 10 Energy Storage Modules (ESMs), we have 1.2 farads of storage, which we can charge to about 2-25V DC. The pulse width can be varied from under one millisecond through to 20ms. Hold up there, Dr Evil! Are we seriously talking about shorting a 1.2F capacitor across the weld joint? At just 25V, this is 375J! Let’s think this through; there are safety issues to be considered here! We have intentionally used a maximum charge voltage of 25V, which is well below the Extra Low Voltage threshold and reduces voltage-related safety hazards to operators. We use a 24V DC 6A plug pack to charge it up, so no mains wiring is involved. But the Capacitor Discharge Welder stores an awful lot of energy. This warrants great caution in use, with the risk of burns and arcing. Safety must be at the front of your thinking when using it. From a design perspective, we seek to minimise the risk of inadvertent firing, ie, ‘uncontrolled output’, including by using: n A fire button that only enables the output for a few milliseconds, minimising the risk of creating an arc when placing the weld probes on the workpiece. n An interlock stopping firing during charging, avoiding multiple shots. n An enable/kill switch. n A footswitch to fire the Welder while keeping both hands free. Operating principle The basic idea behind the Capacitor Discharge Welder is shown in Fig.1. This simple welder model consists of the capacitors, connections and MOSFETs. Note that the MOSFETs pull the negative lead down to ground potential but are ‘flipped’ in this figure for clarity. This seems simple enough, but the question at the forefront of our minds is: will the capacitors and MOSFETs survive the very high currents involved, especially on a repetitive basis? To do this, we need to determine what the peak current is likely to be and how it decays over time. To assess this we need a model of all the parts involved, starting with the capacitors and the boards on which they mount. Most of the recommended capacitors have an ESR (equivalent series resistance) specification close to 20mW, so we’ll start with that figure. For the capacitor closest to the ‘output’ end of the board, we calculate a trace resistance (both positive and negative) of 0.5mW, 19 The finished Power Supply Module used in the Capacitor Discharge Welder. Its main job is to charge the capacitor bank, but it also provides power to the rest of the circuitry. giving 20.5mW. The other capacitors are a bit further away, so we calculate figures of 21.27mW and 22.05mW. These three capacitors are in parallel, so we can calculate their combined source resistance as 20.5mW ∥ 21.27mW ∥ 22.05mW = 7.08mW. Then we add the MOSFET on-resistance (1.7mW ∥ 1.7mW = 0.85mW), the PCB track resistance from the MOSFETs to the bus bar and the resistance of the connections to the bus bars, giving us a total of 8.33mW per module. We’ve paralleled ten of these modules, giving an overall source impedance of 0.83mW (10% of the figure above). To this, we must add the resistance of the bus bars (around 0.1mW each), the welding tips (a total of about 0.5mW) and then the welding cables. We’re using 1m-long cables with 7.1mm2 cross-sectional area for a figure of 2.6mW each, dominating the final source resistance value, which is 7.53mW. Given this, what is the maximum current we can deliver? Will the FETs even survive? Of course, the workpiece will never be 0W. With reasonably pointy probes welding a 0.15mm-thick nickel strip, this will be more like 5mW. But we will conservatively use a value of zero for our calculations. This tells us that it would be a terrible idea to fire the welder with the bus bars shorted. If we omit the lead resistance, the load will be 1.5mW plus whatever shorts the bars. This gives a worst-case current of 16,000A or 800A per MOSFET, which is right up against their 1ms safe operating area (SOA) curve. The MOSFETs might survive this, but whatever shorts the bus bars might not! Under ‘normal’ operation, the worstcase current will be 3300A with the 1m leads perfectly shorted. This is 166A peak per MOSFET (two per module) for a few milliseconds. The specified devices are rated to handle 192A continuously, and their SOA is 600A for 10ms, giving us a reasonable safety margin. Under more realistic conditions, and with a 5mW workpiece, the maximum current will be 25V ÷ (7.53mW + 5mW) = approximately 2000A. This can be controlled by reducing the operating voltage and pulse width. 20 This analysis might seem over the top – but a CD Welder is quite a device! I was just a little intimidated the first time I fired it in anger! Major parts The resulting CD Welder block diagram is shown in Fig.2. We will discuss each part and explain some of the challenges they present. 1) Power Supply Module The problem with charging a 1.2F capacitor is that to any regular power supply, it looks like a short circuit. Also, when fired, the CD Welder power supply is shorted out. It must be able to tolerate this on a repetitive, long-term basis. A linear regulator might do the job, but it would face several problems. For a start, it would get hot! Also, if we use a 5W resistor to limit the charging current, the initial current will be 5A, but it will not fully charge the capacitor for close to 20 seconds. We determine this by solving the equation Vcap = Vin × (1 − e-t ÷ (RC)) for t, with a value of Vcap close to Vin. This convinced us to instead use a switch-mode regulator with a 5A (or 2A) constant current output. This only dissipates a few watts even when running flat out. An equation for calculating the charge time is C = Q ÷ V, where C is in farads, Q in coulombs and V in volts. Differentiating and rearranging this equation gives us dV/dt = I ÷ C. With I = 5A and C = 1.17F, dV/ dt is 4.3V per second. Note that you can also determine your actual capacitor bank capacity using this equation by measuring its charge rate and then solving for C. 2) Control Module We need a way to trigger all the capacitors to dump their charge into the welding probes simultaneously, for a defined period. We have used the venerable NE555 timer IC to do this. The Controller needs to work in a tough electrical environment, so using a ‘bulletproof’ chip in a simple configuration is the way to go. We hope you are picking up on the attention we are paying to EMI/EMC and the currents involved here! Professional controllers offer a ‘twopulse weld’ mode. The initial pulse cleans the surface between the parts and the second pulse makes the weld. This feature is easy to provide, so we did. Three timer ICs generate the initial pulse, then a delay, then the second pulse. Fig.1: the basic concept of the Capacitor Discharge Welder is a capacitor bank of around 30 capacitors in parallel that are charged up and then connected across the heavy welding leads when the MOSFETs are switched on. The trick is making sure everything survives this process as over 1000A can flow! Practical Electronics | March | 2023 The Control Module uses four 555 timer ICs. Energy Storage Module (ESM) The Storage Module takes inspiration from Ian Hooper’s work (mentioned above), then extends this to provide us more control over the switching and increases robustness to back-EMF. This ESM accepts 10mm lead pitch (spacing) caps with a diameter up to 35mm. This provides you with many options for sourcing these expensive parts. We recommend you use caps of known provenance from the likes of Farnell, Mouser or Digi-Key. Online prices that seem too good to resist are usually a bad choice with capacitors. The ESMs bolt to bus bars, allowing paralleling of an arbitrary number of modules. They provide fast switching using two onboard high-current MOSFETs and a dedicated FET driver. They also have an inbuilt flyback diode to protect against the backEMF and are easy to build, wire up and service. Switching really high currents is not a simple thing to do. By switching each module rather than the whole bank, we can ‘divide and conquer’. The recommended bank of 30 capacitors on 10 ESMs will each see currents in the region of 50A per capacitor every time a weld is made. The RMS ripple current rating of the recommended capacitors is about 10A, but the limiting factor for aluminium electrolytic capacitors is heating. The average current is very low because of our low pulse rate, so the I2R losses are insignificant. Capacitor choice The capacitors for a CD welder are the main expense. During the development of this project, we spent much time investigating the trade-offs in the total energy stored, capacitor voltage rating and the safety and robustness of the switching system. The choice has also been complicated by parts availability. The 20212022 drought for electronic components (especially semiconductors) is making our life extremely difficult, as even seemingly ordinary parts are hard to get. Perhaps surprisingly, this includes capacitors, especially large electrolytics. Luckily, there is a range of choices you can make in selecting your capacitors. For 25V-rated capacitors, we recommend that you aim for a total capacitance of no less than 1F. Ideally, hit the 1.2F mark for some spare capacity. Table 1 shows some good choices here. If you choose to use 16V capacitors, you can probably save a few pounds. In this case, aim for a total capacitance of no less than 1.5F and ideally 2F if you want a bit of extra margin. All of the options shown in Table 2 will total around £100 or so. Remember that the welding process is about the energy delivered to the weld – the actual capacitance is a means to an end, and using a higher voltage makes this easier. You will find availability and price can be something of a ‘headscratcher’, and we are sure you will have hours of ‘fun’ working out your best value for money! Fig.2: a modular approach makes building the CD Welder easier. A mains power ‘brick’ is fed into the power supply, which provides a constant current to charge the capacitor bank. Said bank comprises eight or more Energy Storage Modules (ESMs – 10 in our case) connected in parallel using bus bars. The control circuit provides the timing and the ability to trigger all the ESMs to dump their charge into the welding probes simultaneously. Practical Electronics | March | 2023 Probably the only thing we would advise against is using much larger capacitor values than we recommend – our models show that for the values in the tables above, it should be OK, but much more capacitance on a module could lead to MOSFET failure. So how much energy do we need? We found about 130J was sufficient for the tabs we welded. We’re confident a welder with 200J total storage would suit our needs. The recommended design can deliver 370J, which would definitely provide margin throughout its life. Circuit details Fig.3 is the circuit diagram of the Power Supply module. The regulator used is an MC34167 device, a switch-mode regulator operating at 71kHz. It is operated in a buck (step-down) configuration, using a 220μH filter/energy storage coil and 15A schottky flyback diode with two 1000μF smoothing capacitors on the output. These will help reduce radiated EMI during charging, but the >1000A pulses will still play havoc with any sensitive electrical device nearby. To turn a voltage regulator into a current source, we need to sense the output current and convert this into a voltage as feedback. This is done by the INA282 shunt monitor IC, IC2, with a 10mW series shunt. The INA282 has a gain of 50 times, so its pin 5 output delivers 500mV/A. This is further amplified by a factor of about 6.5 by op amp IC3a, resulting in 2.8V/A to the feedback pin (pin 1) of IC1. An example weld of a 0.12mm-thick strip of nickel at 15V with a 20ms weld time onto an AA cell used for testing. The result was that the tab could not be pulled off with any reasonable amount of force applied. 21 Table 1 – suitable 25V-rated capacitors (M=Mouser, DK=Digi-Key) Capacitor value # ESMs Caps per ESM Total capacity Energy stored Suitable parts 56,000μF 8-10 2 0.9-1.1F 280-350J DK: 338-3866-ND 39,000μF 8-10 3 0.9-1.17F 300-365J M: B41231A5399M002 DK: 338-3743-ND 33,000μF 10 3 1F 310J M: SLPX333M025E9P3 | B41231A5339M000 | 380LX333M025K052 DK: 338-1613-ND 22,000μF 14 3 0.92F 288J M: SLP223M025H5P3 | 380LX223M025J052 DK: 495-6159-ND | 338-4172-ND | 338-2431-ND Table 2 – suitable 16V-rated capacitors (M=Mouser, DK=Digi-Key) Capacitor value # ESMs Caps per ESM Total capacity Energy stored Suitable parts 68,000μF 12-14 2 1.6-1.9F 208-243J M: B41231A4689M000 | 380LX683M016A052 DK: 495-6141-ND | 338-2273-ND 56,000μF 10-12 3 1.7-2.0F 220-256J M: B41231A4569M000 | SLPX563M016H4P3 47,000μF 14 3 2F 256J M: B41231B4479M000 DK: 338-2458-ND | 338-2318-ND 39,000μF 14 3 1.6F 210J M: B41231A4339M000 | 380LX393M016A032 | 16USG39000MEFCSN25X50 DK: 338-2261-ND All the SMD components are located on the underside of the Energy Storage Module (ESM). If pin 1 of IC1 is lower than 5.05V, the regulator increases its output. Similarly, if the input is higher than 5.05V, the output duty cycle and thus voltage/ current is reduced. So with 2.8V/A, we get an output current close to 1.8A (5.05V ÷ 2.8V/A). The 5A version of the circuit changes two resistors (values shown in green), setting the gain of IC3a to 2.2 times, so its output is 1.1V/A and therefore, the current limit is around 4.6A (5.05V ÷ 1.1V/A). So that the capacitor charging stops when it reaches the desired voltage, the output voltage is applied to potentiometer VR1 via a 27kW resistor and the reduced voltage at its wiper is buffered by op amp IC3b. This is fed into the ‘current sense’ input of IC3a (pin 3) via diode D3, which ‘ORs’ these voltages together. This means that when the output voltage is lower than the set limit, the circuit operates as a constant-current source. When the output voltage reaches the programmed limit, the voltage from VR1 exceeds the current sense voltage, and regulation is now voltage-controlled. When in current-limit mode, we switch on the CHARGE LED connected across CON3. At the same time pin 7 of CON4 is pulled low, which acts as an interlock in the controller circuit on the ‘fire’ switch. This is used to stop the user from making a weld before the capacitors are fully charged. Controller circuit The controller circuit is shown in Fig.4. Three NE555 devices, IC4-IC6, are set up as monostable (single-shot) pulse generators in series (output to trigger input), with a fourth (IC7) acting as a high-current buffer. This allows us to generate a first pulse, a delay and a second pulse. The main weld pulse is controllable using 100kW potentiometer VR2, variable from under 1ms to about 20ms. About 10 of these ESMs are joined together to form a capacitor bank for the CD Welder. 22 Practical Electronics | March | 2023 Capacitor Discharge Spot Welder Power Supply Module Fig.3: the Power Supply circuit derives a 15V rail to run the remainder of the circuit from the 24V DC input using a simple linear regulator. The rest of the components form the constant-current switch-mode step-down regulator. It’s based around switching regulator IC1 with shunt monitor IC2 and op amp IC3 used to make it deliver a fixed current until the capacitor bank reaches the fully charged voltage selected using potentiometer VR1. If the ‘two-pulse’ switch connected to CON8 is open, only the output trigger pulse from IC6 is fed (via diode D6) to timer IC7, so a single trigger pulse goes to pin 9 of CON7. If that switch is closed, the outputs pulse from both IC4 and IC6 result in a trigger pulse. Timer IC5 provides the delay between these pulses. We chose the NE555 as a driver because it can operate from 15V, can deliver 200mA, has a fast rise time (300ns) and can easily drive our TRIGGER bus. This switches all the energy storage modules simultaneously. The ‘fire’ input to the Controller, connected to CON5, is a switch to ground. We have included PNP transistor Q2 to inhibit the input while the capacitors are charging. When the INHIBIT line from pin 7 of CON7 is low, Q2 is on and it holds the trigger input feeding pin 2 of IC4 high. The 1μF capacitor between its base and the 15V rail avoids noise coupled into the INHIBIT line from causing problems. Similarly, if the pins of the ENABLE header (CON6) are shorted (eg, via a switch), this will prevent triggering by switching on Q2 via diode D8. The control interface PCB design uses tightly-packed surface-mounted components to increase its EMI robustness and avoid false triggering. Practical Electronics | March | 2023 ESM circuit This is shown in Fig.5. There isn’t much to it – mainly just the three (or two) storage capacitors, two MOSFETs and the dual MOSFET driver, IC8. We explained earlier why we are using the very high-current IRFB7430 FETs. These must be tightly controlled in terms of switching time, and switch on and off cleanly. The TC1427 MOSFET driver can deliver up to 1.2A into the FET gates, switching them in 25ns. It has input hysteresis, which will help our robustness to noise. The alternative, pin-compatible IX4340NE in the parts list can deliver an even higher current of 5A for very rapid switching indeed. IC8’s inputs are connected to the TRIGGER bus from the NE555 which has a 15V swing, again seeking to avoid false switching due to noise. By driving all Energy Store Modules with the common Trigger signal, we aim to ensure that all Energy Store Modules are switched on and off at as close to the same time as possible. The Welder in action Scope 1 (overleaf) is a digital oscilloscope capture showing the voltage After building your CD Welder. It’s useful to make some test welds on scrap metal to get an idea of how much voltage and time is needed to form a decent weld. Too much energy will burn and distort the metal, and even blow holes in it, as shown on the left tab. On the right, you can see that we managed to weld the tab to the can without destroying it. 23 Parts List – Capacitor Discharge Welder 1 250 x 200 x 130mm ABS enclosure [Altronics H0364A] 1 Power Supply module (see below) 1 Controller module (see below) 8-14 Energy Storage modules (see below and Tables 1-2) 1 82W 5W 10% resistor (for testing) 1 0.27W 5W 10% resistor (for testing) 1 panel-mount digital voltmeter (optional; to display selected voltage) [eBay, AliExpress etc] Switches/connectors 3 two-way polarised header plugs with pins (foot switch, enable, charge) [3 x Altronics P5472 + 6 x P5470A or 3 x Jaycar HM3402] 12 10-way IDC line sockets [Altronics P5310 or Jaycar PS0984] 1 3-pin circular microphone inline socket (for footswitch cable) [Altronics P0949] 1 3-pin circular microphone chassis-mount connector (for footswitch) [Altronics P0954] 1 footswitch (trigger) [Altronics S2700 or Jaycar SP0760] 1 miniature chassis-mount SPDT toggle switch (two pulse select) [Altronics S1310 or Jaycar ST0555] Wire/cable/etc 1 1m length of 8AWG red power wire (welding lead) 1 1m length of 8AWG black power wire (welding lead) 1 200mm length of 17AWG red tinned extra-heavy-duty hookup wire [Altronics W2283] 1 200mm length of 17AWG green tinned extra-heavyduty hookup wire [Altronics W2285] 1 1m length of twin speaker cable, rated to handle at least 5A 1 2m length of two-core heavy-duty microphone cable (footswitch lead) [Altronics W3028] 1 1m length of 10-way ribbon cable 1 100mm length of 20mm diameter heatshrink tubing (for welding cables) 1 300mm length of 12.7mm diameter heatshrink tubing (for handles) 1 100mm length of 10mm diameter heatshrink tubing (for welding cable lugs) Hardware 2 260mm length of 10 x 10mm square aluminium bar (bus bars) 2 100mm length of 10 x 10mm square aluminium bar (handles) 6 M4 x 10mm panhead machine screws (for handles and welding connections) 2 M4 shakeproof washers (for welding connections) 10 M3 x 10mm tapped spacers (for joining modules together) 4 M3 x 16mm panhead machine screws (for Presspahn shield) 40 M3 x 6mm panhead machine screws (module connections) 44 M3 shakeproof washers 2 6mm heavy duty eyelet crimp lugs for 7/8AWG wire [Altronics H1757B] across the capacitor bank just after the Welder is triggered. In this test, only one ESM has been connected. You can see the sudden drop in voltage to around 5V over about 20ms when the weld is 24 1 60 x 40mm sheet of Presspahn or similar insulating material [Jaycar HG9985] Power Supply (one needed) 1 double-sided PCB coded 29103221, 150 x 42.5mm 1 220μH 5A toroidal inductor (L1) [Altronics L6625 or Mouser 542-2316-V-RC / 542-2200HT-151V-RC] 1 10kW 9mm linear right-angle potentiometer with plastic shaft (VR1) [Altronics R1906] 1 10A M205 slow-blow fuse (F1) 2 PCB-mount M205 fuse clips (F1) 2 2-way mini terminal blocks, 5/5.08mm pitch (CON1, CON2) 1 2-way polarised header, 2.54mm pitch (CON3) 1 2x5 pin header (CON4) 1 micro-U TO-220 heatsink (for REG1) [Altronics H0627] 1 mini-U TO-220 heatsink (for IC1) [Altronics H0625, Jaycar HH8504] 2 TO-220 insulating kits with silicone washers and plastic bushes (for REG1 and IC1) 2 M3 x 10-16mm panhead machine screws, shakeproof washers and nuts (for mounting heatsinks) 4 M3 tapped spacers 8 M3 x 6mm panhead machine screws and shakeproof washers 1 PCB pin (optional) Semiconductors 1 MC34167TV or MC33167TV 0-40V 5A integrated buck regulator, TO-220-5 (IC1) 1 INA282AIDR bidirectional current shunt monitor, SOIC-8 (IC2) 1 LM358 dual single-supply op amp, DIP-8 (IC3) 1 LM7815 15V 1A linear regulator, TO-220 (REG1) 1 BC546 65V 100mA NPN transistor, TO-92 (Q1) 1 6.2V 400mW zener diode (ZD1) [1N753, Altronics Z0318] 1 6TQ045-M3 45V 6A schottky diode, TO-220AC (D1) 1 1N4004 400V 1A diode (D2) 2 1N4148 75V 150mA signal diodes (D3, D4) Capacitors 2 1000μF 50V low-ESR electrolytic 2 220μF 50V low-ESR electrolytic 1 10μF 50V electrolytic 1 2.2μF 50V X7R multi-layer ceramic 6 100nF 50V X7R multi-layer ceramic 1 100nF 50V SMD M2012/0805 size multi-layer ceramic Resistors (all 0.25W 1% metal film unless stated) 1 27kW 1 12kW 6 10kW 1 8.2kW (for 5A version) 1 3.3kW (for 5A version) 1 2.2kW 3 1kW 1 0.01W (10mW) 1% 1W shunt [Mouser OAR1R010JLF] made, and the recharge, which takes a few hundred milliseconds. Measurements taken from this screen capture let us calculate the total capacitance and the weld current using the formula C = Q ÷ V introduced earlier, along with C = I ÷ (dV/dt). We know the charge current I is close to 2A. We measure a 10.5V increase in voltage over 616ms, so: Practical Electronics | March | 2023 Controller (one needed) 1 double-sided PCB coded 29103222, 150 x 42.5mm 1 100kW 9mm linear right-angle potentiometer with plastic shaft (VR2) [Altronics R1908] 3 2-way polarised headers, 2.54mm pitch (CON5, CON6, CON8) 1 2x5 pin header (CON7) 1 jumper shunt (optional) Semiconductors 4 LM555 timer ICs, DIP-8 (IC4-IC7) 1 BC556, BC557, BC558 or BC559 30V 100mA PNP transistor, TO-92 (Q2) 4 1N4148 75V 150mA signal diodes (D5-D8) Capacitors 2 10μF 50V electrolytic 1 1μF 63V MKT 1 1μF 50V multi-layer ceramic 1 220nF 63V MKT 1 220nF 50V multi-layer ceramic 7 100nF 63V MKT 4 10nF 63V MKT 2 1nF 63V MKT Resistors (all 0.25W 1% metal film) 1 220kW 2 33kW 3 10kW 1 4.7kW 4 1kW Energy Storage module (parts for one module) 1 double-sided PCB coded 29103223, 150 x 42.5mm 1 2x5 pin header (CON9) 1 2-way mini terminal blocks, 5/5.08mm pitch (CON10) 4 M3 tapped spacers 8 M3 x 6mm panhead machine screws and shakeproof washers Semiconductors 1 TC1427COA713 or IX4340NE dual low-side MOSFET driver, SOIC-8 (IC8) 2 IRFB7430PbF 40V 409A MOSFETs, TO-220 (Q3, Q4) 1 RFN20NS3SFHTL 20A 350V fast recovery SMD diode or similar, TO-263S-3/D2PAK (D9) 1 red LED (LED1) Capacitors 3 39mF 25V high ripple current snap-in capacitors, 10mm lead spacing, 35mm diameter [Mouser B41231A5399M002 or Digi-Key 338-3743-ND or alternatives as per Table 1 or 2] 1 1μF 16V X7R ceramic, SMD M2012/0805 size 2 100nF 50V X7R ceramic, SMD M2012/0805 size Resistors (all SMD 1% M2012/0805 size unless stated) 1 10kW 1 100W 2 10W 1 1.5kW 1W 5% axial (through-hole) Scope 1: the recharge voltage curve for a single Energy Storage module at 2A. The voltage increases by 10.5V in 616ms. Note also the discharge curve visible here, which we calculate as being 130A. Scope 2: the recharge voltage curve with all ten ESMs in parallel. This time the charge rate is 5A, and using the formula given in the text, we calculate the total capacitance as a hair under 1.2F. Scope 3: 200A pulse into a load. The yellow trace is the voltage on the negative output. The blue trace is for the capacitor voltage, which shows a dip for the initial pulse then exponential decay. The welding cables and copper-tipped probes. C = 2A ÷ (10.5V ÷ 0.616s) = 0.117F, which is pretty much spot on for three 39,000μF capacitors in parallel. Scope 2 shows a similar curve for all ten ESMs in parallel. The voltage increases by 8.03V in two seconds at 4.8A, which tells us the bank in total is just under 1.2F. Practical Electronics | March | 2023 Turning now to what happens when the CD Welder is used, Scope 3 shows the CD Welder set to 15V welding tabs in a typical application. More voltage than this starts to blow holes in the tabs. This scope grab shows the 1.17F capacitor bank voltage dropping by 4.416V in 25 Fig.4: the control circuit is based on four of the good old NE555. When triggered, IC4 generates the fuse discharge pulse (if the ‘two pulse’ switch is enabled), IC5 produces the inter-pulse delay, and IC6 delivers the second welding pulse. VR2 allows the second pulse duration to be varied between about 0.2ms and 20ms. Capacitor Discharge Spot Welder Control Module Capacitor Discharge Spot Welder Energy Storage Module (ESM) Fig.5: the capacitors that store all the energy for welding are mounted on these ESMs, two or three per board. Each ESM also has two MOSFETs to dump their energy into the welding leads, a dual MOSFET driver to ensure they switch on and off cleanly, and a back-EMF clamping diode to catch any reverse spikes due to lead and other stray inductances. 2.7ms, which we calculate is a discharge of just under 2000A. Next month Next month we’ll have the assembly 26 details of the three modules, then the whole unit, plus testing and use instructions. In the meantime, you can peruse the parts list and start gathering the components you will need to build it. Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au Practical Electronics | March | 2023