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Capacitor Discharge Welder This Capacitor Discharge Welder has been carefully designed to deliver just the right amount of weld energy each time. When completed, it makes a neat package that’s easy to build and safe to use, so long as you follow our advice. Having described how it works, let’s get into making it. Part 2: By Phil Prosser 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 AS/NZS 1337.1, DIN 169 Shade 3 welding glasses. These provide mechanical and IR/UV protection. 100 Silicon Chip Australia's electronics magazine siliconchip.com.au T he Capacitor Discharge Welder comprises three main electronic modules: the Power Supply, which is responsible for charging the capacitors; the Controller Module, which determines when voltage is applied across the welding tips; and an Energy Storage Module bank, typically made from around 10 modules joined to a common pair of bus bars, that hold the storage capacitors and Mosfets. Because of this modular approach, not only can you scale the system to meet your needs, but the PCB cost is kept down, and assembly is relatively straightforward. You build and test the modules, assemble them into the case, make the welding tips and cables, and finally wire it all up. Construction The first step in building the CD Welder is to assemble one Power Supply Module, one Controller Module and several Energy Storage Modules (ESMs). Each is built on a different PCB, but all the PCBs are the same size at 150 x 42.5mm. We’ll start with the Power Supply Module. Its PCB is coded 29103221 and Fig.6 is its overlay diagram, which shows where the parts mount on the board. Start by soldering the sole SMD ceramic capacitor (100nF) near the MC34167 regulator IC. Next, mount the INA282 current sense amplifier, which comes in an SMD package (SOIC). Watch its orientation; make sure its pin 1 is facing as shown before soldering its pins and then check for bridges. Follow with all the resistors and diodes (except for diode D1) with the diode cathode stripes facing as shown, leaving the taller shunt resistor until last. There are three different diode The CD Welder fully assembled and ready to be used in anger (or calmly, it’s up to you). types used: 1N4148, 1N4004 and zener, so don’t get them mixed up. Pay attention to the two different resistor value options shown in Fig.6. If you are using a DC power supply that can deliver at least 5A, you can use the values shown for 5A charging. Otherwise, stick with 2A charging. Now install the sole transistor facing as shown, then all the capacitors. Many of the latter are not polarised, but for those which are polarised (the electrolytics), these all have the longer positive leads going to pads on the right-hand side. Note that while you could use 100nF MKT capacitors, multi-layer ceramics will also work. Next come the connectors. There are two screw terminals, a polarised header for the Charge LED and a 2x5 pin header to connect to the other modules. Make sure the screw terminal wire entries face the outside of the board as shown. Mount the 6TQ045-M3 diode (D1) close to the board by pushing it down fully before soldering and trimming its leads. Also install the fuse clips (with the tabs towards the outside) and fuse, the LM358 op amp and 10kW linear voltage control potentiometer. Now fit the LM7815 regulator and attach a small flag heatsink using a machine screw, shakeproof washer Fig.6: the Power Supply board is built mainly using through-hole components. The only SMDs are IC2 and one 100nF capacitor near IC1, so fit those first. Watch the orientations of IC2, IC3, the diodes, electrolytic capacitors, REG1 and the terminal blocks. siliconchip.com.au Australia's electronics magazine April 2022  101 and nut as it gets warm during operation. Mount the 220µH toroidal inductor on the board, then finally the MC34167 switch-mode regulator. This also requires a small heatsink such as Altronics H0625 with an insulating bush and silicone pad. Hold this all together using an M3 machine screw, star washer and nut in the usual manner. Control board The Controller PCB is coded 29103222 – refer to Fig.7. Start by installing all the resistors and diodes, checking that the diodes are the right way around, then follow with the four NE555 timer chips, with their pin 1 notches/dots to the left. Next, fit all the ceramic MKT and electrolytic capacitors. Note the use of two different types of 1µF capacitor as well as different types for the 220nF capacitors. The electrolytics have longer leads for their positive connections, and these go to the side marked + on the overlay. Now mount the small transistor, facing as shown, followed by the 100kW linear potentiometer and the 2-way and 10-way headers. If you want to make the controller switchable for two pulses, make a cable with a switch at one end and a header plug on the other so that it can plug into CON8. Alternatively, you could install a jumper on CON8 and fix this setting, as we did. Energy Storage Modules The ESM boards are coded 29103223, and the components are mounted as shown in Figs.8 & 9. Presumably by now you will have figured out how many you need to build and obtained the appropriate capacitors. Generally, there are three caps per board, but some of the recommended configurations use two. In this case, fit the two closest to the headers. Start by fitting the surface-mount resistors and capacitors on the underside of the PCB. Make sure the 100nF capacitors are mounted either side of the Mosfet driver (IC8). Then solder that driver IC, being careful not to short any leads (you can clean up any bridges using flux paste and solder wick). Next, mount the RFN20NS flyback diode (D9) to the PCB. It’s easier if you spread a thin layer of flux paste on all its pads first. You will want to get a good lot of heat into the PCB; start by tacking down the two anode leads, then solder the main body of the diode. This will not dissipate much power, but you want a good solder joint here. Then fit the two Mosfets, keeping their leads short. Their metal tabs face away from the capacitors, and their source and drain pins connect to copper fills. These junctions will see very high current pulses, so be sure to get these properly hot when soldering to form nice-looking fillets. Now mount the 2x5 control header, the terminal block and finally, the capacitors. Make sure their positive sides go in the direction indicated, and the negative side stripes face away from this. Reversed capacitors will likely lead to an Earth-­shattering kaboom! Repeat the ESM assembly until you have enough of these modules, and are ready to test them and then proceed to final assembly. Testing Start by testing the modules individually, beginning with the Power Supply Module. To start with, solder the leads of one LED to a length of light-duty twin-lead cable (eg, two wires stripped from ribbon cable) and solder/crimp the other end into a pluggable header, and connect this to CON3, the charge LED header. Make sure the anode (longer LED lead) goes to pin 1. Connect the Power Supply board to a DC voltage source of at least 25V – up to 35V is acceptable. Make sure you have set the current limit (2A or 5A) to match your supply. Set your DVM to a DC volts range and put a 5W 82W resistor across CON2, “Power Output”. Apply power and check the following: • The output of the LM7815 is 15V ±0.25V. Its output is accessible on pin 2 of CON4, the control header. If not, check that it is the right way around and there are no shorts. • Check that pin 1 of CON2, the “Power Out” connector, is between 2V and 25V. Also check that this can be controlled using potentiometer VR1. If this is not working, check the following: • Check that you have the INA282 (IC2) in the right way around. • Verify that the 82W test resistor is connected correctly (eg, measure the resistance across the terminals of CON2). • Check that the MC34167 is oscillating; there will be a 72kHz signal at pin 2. • Check that D1 is in the right way around. • Check that the feedback pin 1 of the MC34167 has about 5.05V on it. If not, verify that the LM358 op amp is operating properly. Check the voltages at its power and ground pins (pins 8 & 1, respectively), and verify that the voltage at input pin 5 is an appropriate fraction of the output voltage, and that pin 7 is an amplified version of this. Check that diodes D4 and D5 are in the right way around. • Assuming that’s working, put an ammeter on its 10A range across the terminals of CON2 and check that the current is close to the expected 2A or 5A. If not, look for problems near the INA282 (IC2). Fig.7: the Control board uses all through-hole parts and assembly is straightforward. Again, be careful to orientate the diodes, electrolytic capacitors and ICs as shown. 102 Silicon Chip Australia's electronics magazine siliconchip.com.au The bus bar layout for 10 modules, five on either side of the bus bars. The holes at the end of the bus bars are drilled and tapped for M4 to secure the welding leads; all the other holes are M3 tapped. We have allowed enough length for the bus bars to protrude through holes in the case, as we do not want any joints in these. Testing the Controller To test the controller, ideally, you will need an oscilloscope. Make a 10-way IDC lead to connect the Power Supply module to the Controller module, ensuring that pin 1 connects to pin 1. Apply power and check the following: • Each NE555 chip has 15V at its pin 8. • The base of transistor Q1 is pulled up to within 0.6V of the 15V rail, turning it off. • The TRIGGER output of IC6 (pin 3) is close to 0V The next part is easiest if you assemble the foot pedal trigger by extending the existing lead with the two-metre length of microphone cable. You can simply snip off the screen wires as they are not required; just use the two internal conductors, then add liberal layers of heatshrink to protect the junction. Now temporarily soldering a length of light-duty twin lead to the other end (eg, stripped from spare ribbon cable) and solder/crimp this to a polarised header plug which connects to CON5. Connect your oscilloscope to the output pins (pin 3) of IC4, IC5 & IC7. If you only have a single-channel or two-channel oscilloscope, start with IC4 and/or IC5 and then test the rest later. Press the footswitch and check that IC4 generates a pulse of about 0.1ms and IC5 generates a pulse of about 5ms. Then check that IC7 generates a pulse length that is controllable using potentiometer VR2, from about 0.2ms to over 20ms. Next, check that the trigger output on pin 9 of the 2x5 header (or pin 3 of IC7) contains one or two pulses as set by the switch/jumper on CON8. If there are problems, check the power supply to the NE555 ICs; there should be 15V between pins 8 and 1 of each chip. Verify that the trigger input (pin 2) is being pulled low on IC4, and that the inputs to subsequent NE555s have a short negative-going pulse (this is capacitively coupled, so look closely with the scope). Check also that the diodes are in the right way around, that Q2 is indeed a PNP device and that the INHIBIT line is not pulled low by the Power Supply. Make sure that you are happy with the operation of the power supply and controller modules before assembling the CD Welder. Testing the ESMs To check out each Energy Storage Module, connect one at a time to the Controller and Charger modules. Use medium-duty hookup wire (0.7mm diameter copper/21AWG) such as Altronics Cat W2261/W2260 or Jaycar Figs.8 & 9: the ESM has parts on both sides, although the underside components are limited to a few SMDs near the Mosfets; mainly, the driver IC and associated passives. Fit all those first, then flip the board over and solder the remaining components to the top side. Be very careful with the electrolytic capacitor and Mosfet orientations, as putting them in backwards would be disastrous. siliconchip.com.au Australia's electronics magazine April 2022  103 Cat WH3045/WH3046 to connect the Power Out connector on the Power Supply board (CON2) to the Power In connector (CON10) on the Energy Store Module. You’ll also need a control ribbon cable with three 10-way IDC line sockets to connect the Power Supply, Controller board and ESM together. Connect an 82W 5W test resistor across the ESM output using 16mm M3 machine screws, nuts and washers. Apply power and check that the capacitors charge and that you can adjust the voltage using VR1. The “Output -VE” connection (right near the edge of the PCB) will be pulled 104 Silicon Chip up to the same voltage by that 82W resistor. Use an oscilloscope to watch the voltage on that pin and press the trigger. There is a convenient ground on the power header; we also added a ground via on the board between the capacitors. After triggering, you should be able to see the output pulled to ground in two pulses (with dual pulse mode on). If this does not work, use the scope to check for the trigger pulses on the control cable, check the +15V rail and check that the TC1427 is sending pulses to the Mosfet gates. Check all cabling and the orientation of the components. Australia's electronics magazine Now swap that 82W resistor for a 0.27W 5W resistor. Repeat the test, and check that everything works. At 25V, this will pass close to 100A. You will see the Charge LED come on, especially with long pulse lengths and high voltages. You will also feel the 0.27W resistor get hot after several shots. This is normal. You may blow this resistor, so if things look odd, check it is still 0.27W. At this point, Dr Evil is smiling. Bus bars Once you’ve tested the modules, it’s time to put them all together. We have laid these boards out such siliconchip.com.au that they can mount back-to-back on two 260mm-long bus bars. Fig.11 shows where to drill holes to allow M3 screws to hold pairs of modules into common tapped holes. Mount the modules to the bus bar using 6mm-long M3 panhead machine screws and star washers. As you assemble the modules to the bus bars, put 10mm M3 spacers, 6mm screws and star washers between the holes at the far end of the PCBs from the bus bars, securing pairs of boards to one another, stabilising the assembly. Now tighten the screws well; these will be carrying a lot of current. You may find another way to lay the modules out. While it might be possible to run machine screws right through holes drilled in the bus bars with nuts on the other side, we feel that using threaded holes into the aluminium is important to keep the resistance down. So we strongly advise you to take the time to tap all these holes (aluminium is soft, and you can use a through-tap, so it isn’t that hard). Cabling We have endeavoured to keep cabling as simple as possible. Fig.10 shows the complete layout. We extended the ribbon and power cable from the Energy Store Modules to the Charge and Control modules to suit our application. Try not to make these more than a few hundred millimetres long, though. Fig.12 shows the layout we came up with to fit the modules inside the case and how most of the wiring is routed. Note that it is necessary to cut the Inhibit line in the ribbon cable so that it only connects the Controller and Power Supply modules. This is to prevent it from acting as an antenna and picking up pulses during welding. You will need to make up a cable for the enable switch similar to the one you made before for the charge LED. This will plug into CON6 at one Fig.10 (left): this shows the required cabling for the complete system, which is relatively simple. You can have more or fewer ESMs, but six is the minimum. All cables connect to headers or terminal blocks, except the optional voltmeter we added, which tacks onto a solder pad that joins to the +15V supply rail. Fig.11 (below): to make the bus bars, cut 10mm square aluminium bar to two 260mm lengths and drill and M3 tap holes in the locations shown. Use kerosene or light machine oil to lubricate the tap and if it sticks, withdraw it and clear out the swarf before continuing. You don’t want to break the tap off in the bar. siliconchip.com.au Australia's electronics magazine April 2022  105 Fig.12: this diagram shows how we mounted the modules in the recommended case and wired them up (62.5% scale). end and go to the terminals of a toggle switch at the other end. Now would also be a good time to disconnect the twin lead from the microphone cable in the footswitch assembly you made earlier, and instead solder these to the microphone plug (footswitch end) and socket specified in the parts list last month. In our application, we started with 300mm lengths of twin lead and trimmed them as required. The power connection from the chassis DC socket to the Power Supply board needs to be made using 5A-rated cable; the type of wire used earlier to connect the Power Supply to the ESMs should be suitable. While the ribbon cable connects the output of the Power Supply to each ESM, it is only rated at 1A per wire. Two wires are used for power, plus two for ground, limiting charging over the ribbon cable to 2A. So if you want to charge at 5A, the IDC headers will ‘need help’. This is the purpose of CON10 on each ESM. You will need to wire all those headers back to CON2 on the Power Supply using 5A-rated cable. We used Altronics Cat W2109 for this job. Don’t use thicker wire if you can avoid it, as you need to fit two pairs into each terminal block to daisy-chain them. For this, we cut nine 60mm lengths plus one long length, stripped and tinned these together and used a bit of heatshrink to make it look tidy. This is a little fiddly, but it is the best approach we could come up with that was not big or too expensive. By paralleling the ribbon cable, this heavy-duty wire will take the majority of current during charging. Make sure you connect each terminal with the same polarity; otherwise, it will short out the Power Supply! To make the ribbon cable that connects all the modules, assuming you have 10 ESMs, you need 12 10-way IDC line sockets and about 610mm of Fig.13: we used 610mm of ribbon cable to connect our 12 modules as shown here. Adjust the total length and connector positions if you aren’t using 10 ESMs or want to arrange them in a different layout. 106 Silicon Chip Australia's electronics magazine siliconchip.com.au The finished Capacitor Discharge Welder, with the welding cables attached. 10-way ribbon cable, depending on your layout. Fit the IDC connectors as shown in Fig.13. We crimped the IDC connectors using a vice, although specific tools are also available to do this. If using a vice, add timber blocks or sheets on either side of the connectors to avoid marring them and make it less likely to break them when squeezed. As mentioned earlier, we recommend cutting the inhibit line (wire 7) between the Power Supply Module and the Energy Store modules. Simply slit the ribbon cable on either side of wire 7 over a 10mm section and snip a 5mm section from it using side cutters. This reduces the chance of EMI being picked up. Cables The footswitch is our solution to keeping your hands free to weld, but you could place a button on one of the leads as an alternative if you wish. The recommended footswitch comes with a short lead, hence our earlier instructions to extend it with about two metres of microphone cable. Now that you’ve added the plug and socket, this cable should be complete. For the all-important welding cables, we crimped Altronics H1757B non-insulated eyelet lugs at the Welder end (Jaycar PT4936 is equivalent). We were lucky and our crimping tool worked on these, but we know from experience that you can also solder them (with a powerful iron) or crimp them in a vice. We put 10mm heatshrink over the terminal to ensure nothing shorts to it. We made the welding handles and tips as shown in Fig.14. These comprise a 100mm length of 10mm square aluminium bar with a 4mm hole drilled in the end to accept the welding cable. Two additional M4 threaded holes allow 6mm-long M4 screws to fix the welding cable. After making them, we applied Fig.14: a cross-section of the welding probes we made from 10mm square aluminium bar. The welding tips are 3mm copper rods ground to a sharp point. A close-up of one of the tips is shown adjacent to this diagram. siliconchip.com.au Australia's electronics magazine April 2022  107 many choices out there, and the wiring is pretty straightforward. Welding! To illustrate the energy involved, and potential danger, this shows the result of placing the probes across the tab between two AA cells. The capacitors were charged to 15V, so this is about 127J of energy. A look inside a can used for testing, which shows the damage caused by excessive voltage. The higher energy welds have made holes right through. 13mm heatshrink tubing over the handles to make them easier to hold and act as strain relief for the cables. At the welding tips, we have again drilled 3mm holes in the end of the handles and drilled and tapped an M3 threaded hole to hold the tip. We tried various copper welding tips and feel that 3mm rod filed to a point are pretty good. We used small pieces of 20mm heatshrink to ensure the positive and negative welding cables remain close to one another along the bulk of their length. We do this to minimise the inductance in the welding cable loop. If there is a lot of inductance, then there will be much energy stored in this that the Mosfets have to switch, and the flyback diodes need to redirect. store to the case and put firm foam under the lid to hold it all together when the lid is attached. We folded and mounted a sheet of Presspahn between the output bus bars (visible in the lead photo) to ensure that accidental shorts cannot easily occur. Note that there is no danger here unless the “trigger” footswitch is pressed, but we do not want any chance of accidentally firing into a dead short. The cutting & folding details for this are shown in Fig.15. We cut two square holes in the front of the case to allow the bus bars to poke through, shown in Fig.16, along with the other front-panel cutouts. All controls were placed in locations that felt convenient, and we used four holes to fix the Presspahn sheet to the front panel. We found a cheap voltmeter on eBay and decided to add this – these are available on your favourite auction site for a few dollars if you go looking. We will leave the selection and integration of this to you, as there are Case assembly There are many ways of packaging this up. By avoiding mains wiring, we don’t need to be so worried about Earthing and suchlike. We used an Altronics H0364A case, which is just large enough to fit all the modules. This allows us to mount the ESM ‘bundle’ on its bus bars in the base with the Power Supply and Controller modules just behind the front panel, secured to the side of the case. The photo of the case with the lid off shows this arrangement pretty clearly. We found that the potentiometer shafts were only just long enough – you might find a better way of mounting these. As our application is stationary in the lab, we used long tie wraps (thick cable ties) to secure the energy 108 Silicon Chip You will need to experiment to find the settings that work best for you. We used flat AA and D cells to test the system out, and found that with 0.12mm nickel strip, setting the pulse width to maximum and voltage to about 12-14V gave extremely solid welds. We started with a low voltage and increased the voltage until the welds just stuck, which was about 8V. From that point, we increased the voltage to get a solid weld (in our case, at around 12V), then added a bit. To test your welds, take pliers and try to pull the tab off. It should be exceptionally well attached and require you to tear the weld ‘beads’ off. You will find the copper weld tips wear and get dirty if you experience arcing. Clean them up with sandpaper or a sharp knife for consistent results. Once you have worked your settings out, this CD Welder should provide solid service and consistent weld energy. Some tips • We found 12-15V to be the sweet spot for welding. While we did install 25V capacitors, if you are welding only light gauge battery tabs, you will probably find that you need to charge them no higher than 16V. Then again, you gain a lot of headroom for the slight cost increase of using 25V capacitors. • To check the effect of weld energy, we welded tabs to the top of a soup can, using this as a battery surrogate. From the outside, the 15V welds are reasonably light ‘dimples’, while with the 25V welds, some of the tab material has clearly been blown away. This was accompanied by sparks and a flash. The photo of the inside of the can shows that all the welds are visible, Fig.15: cut, drill and fold the Presspahn as shown here to make the bus bar insulator. This ensures that the Welder cannot be accidentally fired with a short circuit across the bus bars. Holes A are 3mm in diameter. All dimensions are in millimetres. Australia's electronics magazine siliconchip.com.au but with significantly more damage on the 25V welds. • Never short the output bus bars directly (say with a screwdriver); this will lead to dangerous arcing and probably break something expensive. • Always wear safety glasses. • Do not use welding leads with copper wider than 3.3mm in diameter (8 Gauge) or shorter than 1m, as this forms part of the design. • Always keep the leads parallel and never curl them into a coil. Coiling them will increase inductance in the system and give the flyback diodes a hard time. • Note that some plug packs have their negative output connected to mains Earth. Be careful of these packs as the output leads are at your weld voltage. Finally, for those interested, we have a couple of spreadsheets available for download from siliconchip. com.au/Shop/6/6306 that include many of the calculations used to verSC ify this design. Fig.16: the front panel cutting diagram for the layout used in our prototype. This box suits our application in the lab, but you might be able to come up with a better arrangement. siliconchip.com.au Australia's electronics magazine April 2022  109