Al wrote in message
...
Hi Guys,

I like to tinker with fixing computers, which seems to often be
ripping out a psu and putting a new one in or swapping ram etc.
Replacing caps etc I want to be able to test more at the component
level and MOSFETs seem to come up a lot.

So far the best description of a MOSFET is what I have seen at
http://www.youtube.com/watch?v=Te5YYVZiOKs - although maybe on a very
simple level

I have read the sci.electronicsrepair faq on MOSFET testing but that
has me confused as :
(a) the tests are all different
(b) some of them tell you to put a lead here and a lead there, but not
which lead on your DMM
(c) I brought a new MOSFET to experiment with - yet I still dont get
the same results as some of the tests????

So I have some questions that I hope the experts here can clear up.
They all relate to MOSFETs as used in computer PSUs and motherboards
1) Can you test them in circuit
2) I see some are though hole mounted and some are, "surface
mounted".Is there a special word to describe teh surface mounted ones?
Why are they surface mounted (like lieing flat on PCB)
3) I have been thinking of MOSFETs as a current gate that is turned
off/on by voltage - so with respect to all the MOSFETS you see on a
computer motherboard near the VRM circuitry - that are they actually
doing? how does a bunch of gates help?

Any help apreciated in trying to get my head around these things

Cheers

-Al


Crude basic test for checking MOS Fets , in isolation
The following is for n type mosfets and results do vary so it is probably
best
to try on known good matching FET before falsely discarding a suspect
one of same type number.
Starting by identifying G,D,S if unknown and assuming starting in
non-conducting state. Using a DVM on "diode" check with red and black
probes.
One pair of terminals only one way round should show a forward voltage x .
The black is on the drain (D) the red is on the source (S) leaving the third
as the gate.
Put red to G and black to D then swap probes to D-S . There should be
conductance less than x slowly increasing back to x in the limit.
Putting black on the G and red to D then should return D-S to full x value.
Or setting the D-S "voltage" to the low setting and just touching the G
may reset to the higher "voltage" state.


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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/