Re: [CAD_CAM_EDM_DRO] How do I check the MOSFET's in my stepper driver

washcomp wrote:

>I blew a fuse in my API CMD-260 stepper driver. Before I put it back
>into my CNC mill, I'd like to make sure that one of the MOSFET's
>didn't blow and cause the fuse to go. How would I test them without
>de-soldering? (I didn't find any consistancy in resistances with my
>ohm meter, probably because of external circuitry or interconnection
>between them). There are ten of them in there.
>
>

If you have the transistors with the leads down and their heat sink face
away
from you, then the common pin out from left to right is Gate, Drain, Source.
If you apply the positive ohmmeter probe to Drain, and the negative to
Source, you should not show conduction. If it does, that may indicate a
bad FET. It can be hard to get a meaningful reading, depending on snubber
and protection circuits on the drive.

If you get a wierd reading, like 1-25 Ohms in the above test, you might
unsoder
the questionable transistor and check it out of circuit.

Here's a more comprehensive test.
1. connect + probe to S, - probe to G - if not infinity, transistor is
shorted.
2. connect + to D, - to S, if not infinity, transistor is bad.
3. connect + to G, - to S, should again read infinity
this test leaves abuot +3 V on the Gate
4. Connect + to D, - to S, should read lower than step 2, showing that
gate charge is partially turning transistor on.
5. Connect - to D, + to S, should read like a forward-biased diode.

>Or should I just change the fuse and hope for the best?
>
>

Not real likely to be a success. The fuse blew because something is
shorted.

Jon