cap disch welding (was X10, X100 Readout Resistor)

John Griessen

On 10/29/20 3:53 PM, Brent W8XG wrote:
Capacitive welding of the wire to the SMD is an interesting idea however. I've never done that, but the circuit looks easy
enough to work up. A little jig might help?? Thanks,
A circuit to do that small wire welding can be engineered, or easily brute forced with a big contactor relay.
You don't really care if some contact bounce happens -- use a 3 pole contactor that comes off of junk like old A/C units with all 3 poles in parallel. A variac, resistor and diode
can be the mildly charging part of the brute-force-system for super rapid discharge.


As I understand the OP's original problem, he had to join a very fine copper wire to a component or assembly. As a graduate student (50+ years ago), I used CD welders extensively. The best candidates for CD welding are low thermal and electrical conductivity materials. Thermal, so the hot zone remains at the point of contact for as long as possible and electrical, so the energy is deposited in the materials to be joined. Copper and silver are bad candidates in both respects. Also, there are problems joining very fine wires; determining the correct pulse energy is very tricky - too little and a weak or non-existent weld results- too much and the fine wire gets blasted away. In my work, I successfully welded 0.002" tungsten wires to support structures. In order to do this, an intermediate layer of platinum foil was necessary. Even though both tungsten and platinum have very high melting points, durable welds were possible. Much trial and error is necessary to get things right. Both mechanical pressure and pulse energy are critical parameters.

I'd suggest that, for the OP's application, a dab of silver-filled conductive epoxy might work. The circuit resistance is probably much higher than the joint; so, the electrical resistance of the epoxy joint might not affect electrical performance. For mechanical strength, the entire area could be potted in a high quality potting compound or epoxy.