Building A Spot Welder From 500 Junk Capacitors

[Kasyan TV] over on YouTube was given a pile of spare parts in reasonably large quantities, some of which were useful and allocated to specific projects, but given the given the kind of electronics they’re interested in, they couldn’t find a use for a bag of 500 or so low specification 470uF capacitors. These were not low ESR types, nor high capacitance, so unsuitable for power supply use individually. But, what about stacking them all in parallel? (video, embedded below) After a few quick calculations [Kasyan] determined that the total capacitance of all 500 should be around 0.23 Farads with an ESR of around 0.4 to 0.5 mΩ at 16V and packing a theoretical energy total of about 30 joules. That is enough to pack a punch in the right situation.

A PCB was constructed to wire 168 of the little cans in parallel, with hefty wide traces, reinforced with multiple strands of 1.8mm diameter copper wire and a big thick layer of solder over the top. Three such PCBs were wired in parallel with the same copper wire, in order to keep the total resistance as low as possible. Such a thing has a few practical uses, since the super low measured ESR of 0.6mΩ and large capacitance makes it ideal for smoothing power supplies in many applications, but could it be used to make a spot welder? Well, yes and no. When combined with one of the those cheap Chinese ‘spot welder’ controllers, it does indeed produce some welds on a LiPo cell with a thin nickel plated battery strip, but blows straight through it with little penetration. [Kasyan] found that the capacitor bank could be used in parallel with a decent LiPo cell giving a potentially ideal combination — a huge initial punch from the capacitors to blow through the strip and get the weld started and the LiPo following through with a lower (but still huge) current for a little longer to assist with the penetration into the battery terminal, finishing off the weld.

[Kaysan] goes into some measurements of the peak current delivery and the profile thereof, showing that even a pile of pretty mundane parts can, with a little care, be turned into something useful. How does such an assembly compare with a single supercapacitor? We talked about supercaps and LiPo batteries a little while ago, which was an interesting discussion, and in case you’re still interested, graphene-based hybrid supercapacitors are a thing too!


Thanks to [Danjovic] for the tip!

5 thoughts on “Building A Spot Welder From 500 Junk Capacitors

  1. I’ve made quite a few welders using this principle. A single or multiple 250 volt 200 mfd capacitors are charged up with a variac and bridge rectifier for power control. Then discharged into the primary of a large filament transformer. The switching was done with large mercury relays for the current surge and no contact burning.
    The usual spring loaded copper welding contacts made the connection to power resistor end caps. We made high precision wire wound resistors from 1/4 watt to 100+ watts. I don’t think we ever had a failure in these welders after years of service and millions of cycles.

  2. The cleverest, due to its simplicity, spot welder I’ve seen, intended for welding cell straps between cells, used a car or motorcycle lead acid battery, a car starter solenoid operated by a cheap adjustable timer module driving a power MOSFET module.

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