The spot welder is finally working now! It was the fet driver signal from the processor that was not going back to ground after it sent the dual pulses. I added a couple of different values of resistor and placed them from the fet driver signal on the processor’s pin 27 (terminal block position 17) to ground. I ended up soldering the 2.2k resistor onto the right side of R27 and the left side of R22 for a permanent installation. 2.2k worked the best and draws only an additional 2ma. The I/O pins are rated at 25ma on the processor data sheet.

What also helped a lot was not connecting the ground lead for the fet driver to any of the grounds on the terminal block. When it was attached to any of the terminal block grounds, the spot welder kept sticking on. So I soldered it to the ground pad of C9, which is one of several parts not installed per the manual. It worked far better after that. This fet driver ground lead location was always contributing to the sticking on issue.

I also added a 1k resistor in series with the fet driver input signal so that when the fet driver fails shorted, it won’t take out the processor as well. When the first fet driver failed it was forcing 3.4v into the fet driver input circuit. Which forces pin 27 of the processor to 3.4v when it’s trying to stay at ground.

Now it’s time to get accustomed to using the settings to make good welds! All I have right now is .010″ thick nickel for testing. My aim is to spot weld braided cable for the combat robotics folks.

When I disconnect the fet driver wires from the control board, the fet driver pulse works fine. It no longer locks in the on state. Also when the ground lead for the fet driver output is attached near the grounds where the pots are attached, positions 6 & 7, it causes that pot to wander all over the place. So I soldered it to the ground hole near position 1 on P3. That corrected that wandering issue. Still the output gets stuck on. At least one press of the foot pedal corrects it. It almost looks like a ground loop.

Here is what the fet driver input signal looks like on the control board when the spot welder stays on. Trace #2 shows that it’s just stuck full on at 5vdc. This holds the fets in the on state and that’s why the probes are energized. The power supply is straining to keep up with the constant load instead of the milliseconds long pulses that it normally sees.

This is what the fet driver input signal looks like when operation is normal.

I found that the scope shot in the previous post represents when the spot welder is working correctly. When it’s not working correctly and the probes are constantly energized, the scope traces show 15vdc on the fet driver output. The entire time current is being moved into the cap using Q4. Almost as though the foot pedal was sticking closed if you will. But it isn’t, and it wouldn’t matter anyway since it just sets in motion some code to fire the fets. After the foot pedal is pressed just once more, the problem is corrected. It’s done this since it was first built. I wonder if the EMI from the cap charge leads are interfering with the system some how. The charge leads are running right past the terminal block and the 3 sets of pot leads. I’ll probe around with the scope to see if I can find something not doing it’s job.

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