One reason could be is that the Semi Cond. Tab bolted to the heat sink may originally be insulated from the H.S. and you inadvertently shorted the tab to the heatsink proper?
Max.

 

Another possibility is that static electricity, generated by you or by the grinding process, killed the MOSFET on the heatsink. Did you take anti-static precautions when handling the controller?

 

Another possibility is that static electricity, generated by you or by the grinding process, killed the MOSFET on the heatsink. Did you take anti-static precautions when handling the controller?

Not really. Maybe it's better to modify the box - PITA that it will be - and avoid these issues entirely. So there's little chance the the LED indicator lamp somehow caused the board failure?

 

Sorry if stating the obvious, but Presumably you had a suitably rated resistor in series with the LED.
Although if it was on the power in lead it should not have had an effect on the drive.
Does the LED still work?
Max.

 

Not obvious to me! No, I did not install a resistor and, yes, the LED still works. I wired the LED leads together with the power in leads on the PMW board terminals...

 

Do any of the heat sink'ed semi's have any numbers? You could check input to output terminals to see if anything is shorted?
Unless it is a self contained LED it will normally need a series resistor for a discrete LED.
Max.

 

Well, fried another one. I can only attribute the failure now to the LED wired in parallel on the power in side of the board. (The LED packaging says it's good to 28VDC w/o a resistor.) I took every precaution wiring up this board, including modifying the project box to eliminate grinding the heat sinks. Maybe it would be better to solder the LED leads to the power-in jack tabs?

 

Wired to the power input side of the controller I would not have expected it to blow the controller, only the LED itself maybe?
If it states good to 28vdc then it most likely has a built in resistor, going higher would need an additional series resistor.
Have you attempted to confirm the motor itself is OK?
Max.

 

If you are enclosing the controller in a tight-fitting project box then the heatsink won't be very effective. Perhaps the controller died from being cooked .
Does the motor/controller have a back-emf protection diode or snubber fitted?

 

If you are enclosing the controller in a tight-fitting project box then the heatsink won't be very effective. Perhaps the controller died from being cooked .
Does the motor/controller have a back-emf protection diode or snubber fitted?

I apologize but I have no idea what those things are or what they look like. However, the board failed (single-speed output) instantly upon power-on. No time to even warm up the heatsinks.

Wired to the power input side of the controller I would not have expected it to blow the controller, only the LED itself maybe?
If it states good to 28vdc then it most likely has a built in resistor, going higher would need an additional series resistor.
Have you attempted to confirm the motor itself is OK?
Max.

The motor seems fine. It still varies its speed using a resistance-type controller. Maybe I'll just give up and buy an AC/DC controller.

 

I have no idea what those things are

A quick google will give lots of info on those.