Dynomotion Forum

One of the large 25 watt power resistors on one of my snapAmp crumbled and electrically split too. The board still works. I replaced it and within a few seconds the ceramic housing split poped and smoked. It got so hot that it de-soldered itself and fell out on one end, opening the circuit. The boards still works. Any ideas? Something seems shorted to ground and I’ll prbably find it but I’m not there now and maybe this is obvious.

Hi BillK,

Sorry to hear that.

The Power Resistors are only used for Voltage Clamping/regenerative braking protection. The Power Resistor can be damaged if the Clamp Voltage is set lower than the motor supply voltage.

The Clamp circuit is simply Motor Supply, Resistor, FET (Q5/Q14), GND.

Did the new resistor overheat before enabling Voltage Clamping? If so the FET may be shorted.

I don’t know if it would have fried before clamping or not. I did not think to turn clamping off before trying the new resistor.
Both clamps are set at 80 V and my supply is 77 V but I believe this is a case of closing the barn door after the horse has bolted.
Years ago I upgraded the power supply to one of those nice toroidal ones from eBay. I added about 20 V. I believe I did not pay any attention to the clamp voltages at that time and only adjusted them after several years when I was in the configuration file for some other reason.
Any recommended part numbers for the transistors I’ve likely damaged?

Hi BillK,

Those are 100V FETs rated at 100 or more Amps. Such as:

STB120NF10T4
IRFS4310ZPBF

Gate U11 Driver could also be bad
LM5110-1M

Good luck