The parts to fix the spot welder showed up this afternoon. I was thrilled.

After putting in a new fet driver, I put my DVM in series with the power supply
to the fet board. The fet board draws only 28ma ac. So it’s basically in fine
shape, so far.

The first time I took the probes and pushed them up against a piece of nickel
strip to test the spot welder, there was a large flash and pop! The probes had
voltage across them!! Almost like the foot pedal was being pressed ahead of
time. Yet it wasn’t. I put the scope across the output of the control board that
drives the fet driver. I couldn’t get the scope to trigger right. So I put the
probe on the output of the fet driver. The scope was triggering on something
kind of random. I adjusted the scope and there is a pulse on the fet driver
output that is 18.6v p-p. But only 500 nanoseconds long. Something around
2.5mhz. Wow. The normal pulses are between .1-35 milliseconds. But now I finally
got the scope to sinc up nicely to this odd pulse. So I adjust the other scope
channel to the signal that triggers the fet driver on the control board, and
it’s the same frequency, but only a 4v p-p pulse that I have not identified the
source. So the fet driver is actually being triggered to fire the 4 farad cap by
this micro tiny pulse. When I hit the foot pedal I get the correct pair of
pulses. So now I have found out why my spot welder probes are hot before I even
hit the foot pedal. This happened yesterday too! Some how this must have smoked
the fet driver. I whipped out my EMI probe and am now sniffing over the system
for a source that fits this pulse. The processor is 20mhz. A 4:1 harmonic
perhaps? I disconnected the fan and the lcd hoping they were involved, but no
such luck.

Oh brother, is this how I am spending my Friday night?

Party on Garth!

Here is the phantom pulse. Ch 1 is the output of the fet driver. Channel 2 is the input of the fet driver.

I did order some fet drivers from Newark. But there is not much of a chance that they’ll get here by Saturday USPS. So I called TI and they are overnighting the fet drivers to a local office. So I should have them in my hot little hands soon. I spent the evening drawing up a schematic for the fet board so I have a better view of what caused the fet driver to smoke.

The one thing I will modify is the output that goes directly from the processor to the fet driver should have a 1k ohm resistor in it in case the fet driver melts down like it did. I also learned this with motor control hardware. The versions with the resistor withstood much more abuse than the versions that had no resistor. This was particularly true of the self balancing scooter project. When I discovered that the fet driver was very hot, I measured the voltage on the enable line and it was 3.5vdc, coming from the fet driver! It had shorted internally and was pushing voltage out it’s input, to the processors output pin. The normal signal is only an enable line, so it’s very low current anyway. I also saw some noise on this same line with my scope. A little more isolation for the processor is a good idea I’ve learned from projects past.

I also ordered some 45v schottky’s for the protection board that Fritz designed. He calls out 100v parts and then says it provides 60v limit. So I ordered 45v parts at $1 each.

I painted the leaking light sources on the lcd tonight. A few more areas and the stray light will not show through the bezel any more. This cleans up the look of the lcd behind the smoked bezel. I just used Tester’s flat black enamel.

Using some 6ga silicone wire from my combat robotic days, and some nice electrical connectors from Home Depot, these probes came out great. This set is 23.5″ long. I have enough wire to make another set at 40″ long.

I finally decided on taping the 1/4″ copper probes with an 8-32 thread and screwing the connector to it with a brass screw. This makes the probes universal so I can change them out for a different configuration of probes. The connector is plated, solid copper. So these connections should not be a bottleneck for the high amperage current flow.

Here are some practice spot welds. The first couple blew holes in the .010″ thick nickel strip. I dialed down the power and time. With more practice I could make strong welds what took pliers to tear apart, and only then the material would fail, not the weld. Then I started to see a vertical trail of smoke so fine, I thought it was my vision was messing with me. It was real smoke coming from the fet driver. It and the voltage regulator were blazing hot. I grabbed my IR temperature probe and the driver read over 400F. The input to the fet driver was at 3.4vdc. It should be at zero. The processor output was working, but not putting out any voltage. My DVM confirmed that the fet driver was shorted internally. So I ordered more drivers from Newark. Probably won’t get here until Monday. There is a fellow here in San Jose that made one of these welders too. Maybe I can borrow a driver from him.

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