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

"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

You can do a lot with a battery, LED and resistor.

If you apply reverse voltage from drain to source, the "body diode" will
conduct and light the LED. (can be useful to identify leadout on small TO92
MOSFETs).

Swap the leads around so that the correct polarity is applied to the MOSFET
and the degree of conduction will depend on any static charge on the
insulated gate.

Keep the negative lead on the source and touch the positive lead to the gate
to charge the gate capacitance positive, this will make the channel conduct
and light the LED.

Keep the positive lead on the drain and touch the negative lead on the gate
to charge the gate capacitance negative, then any channel conduction means a
faulty MOSFET.

HTH.

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.


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

Crude basic test for checking MOS Fets , in isolation

So this test must be done out of circuit?

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.

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

Thanks - will try this.

Incidentially what do you use for MOSFET removal from PCB? The type of
MOSFET's I am trying to deal with are all mounted "flat" on the PCB of
the motherboard.

Cheers

-Al

You can do a lot with a battery, LED and resistor.

I have all these parts....;-)


If you apply reverse voltage from drain to source, the "body diode" will
conduct and light the LED. (can be useful to identify leadout on small TO92
MOSFETs).

I am pretty new to this, can you explain what you mean by reverse
voltage? +ve terminal to ? and -ve to ?

I assume the resistor is just for limiting the current to the LED?



Swap the leads around so that the correct polarity is applied to theMOSFET
and the degree of conduction will depend on any static charge on the
insulated gate.

Keep the negative lead on the source and touch the positive lead to the gate
to charge the gate capacitance positive, this will make the channel conduct
and light the LED.

Keep the positive lead on the drain and touch the negative lead on the gate
to charge the gate capacitance negative, then any channel conduction means a
faultyMOSFET.

HTH.

Will try that, as soon as I know what you mean by reverse volage.

It sounds like there are really no useful test that can be done in
circuit?

What I have been doing is getting a lot of motherboards and in diode
test mode measuring between gate and source. One way it has open
circuit, the other way it has a reading of about 790 ohm. This is for
the MOSFETs the are around the edge of the CPU as per here (they are
60T03H):

http://img188.imageshack.us/i/asus11.jpg/

All mosfets with Black on G Red on S measure 790 Ohm. With Red on G
and Black on S they all measure open.

The two other MOSFETs are here, near the Vdimm for RAM. They are
APM2054N:

http://img17.imageshack.us/i/asus12.jpg/

One them ith Black on G and Red on S it takes about 2 seconds to climb
to open. With Black on S and Red on G it just measures open.

The other with Black on G and Red on S it takes about 2 seconds and
climbs to 1466 Ohm. With Black on S and Red on G it just measures
open.

Does the fact that these two test differently between G and S mean
anything or could it be because this is being done in circuit?

Cheers

-Al

"Al" wrote in message
...
You can do a lot with a battery, LED and resistor.

I have all these parts....;-)


If you apply reverse voltage from drain to source, the "body diode" will
conduct and light the LED. (can be useful to identify leadout on small
TO92
MOSFETs).

I am pretty new to this, can you explain what you mean by reverse
voltage? +ve terminal to ? and -ve to ?

I assume the resistor is just for limiting the current to the LED?


If you need that explaining, you shouldn't be messing with power supplies! -
However, what do I care if you kill yourself so here goes.

Its best to use a 2mA ultra-efficient LED, the battery should be a MN21/A23
12V car keyfob battery, calculate the resistor to limit the current to suit
the LED.

Assuming an N-channel MOSFET connecting the negative lead to the drain and
the positive lead to the source, the body diode will conduct and light the
LED.

Next, connect the positive lead to drain and negative to source - the LED
may partially light due to static charge on the gate capacitance.

Keep the negative lead on the source and touch the positive lead on the
gate, this charges the gate capacitance positive and makes the N-channel
conduct - so if you then connect the positive lead to drain the LED will
light.

Next, keep the positive lead on the drain and touch the negative lead to
gate, this charges the gate capacitance negative and should make the
N-channel completely non-conducting - so put the negative lead on the source
and the LED should not even glow the slightest bit - if it does the MOSFET
is leaky and will go bang in a PSU!

Assuming you can get the MOSFET at a reasonable price, why not just replace
it, and not worry about testing it? It isn't worth the trouble.

I mean it only as a neutral, impersonal statement when I say that you don't
know much about electronics, and this sort of time-wasting trouble-shooting
is not likely to help you learn.

On Aug 23, 9:55*am, "ian field"
wrote:
"Al" wrote in message

...

You can do a lot with a battery, LED and resistor.

I have all these parts....;-)

If you apply reverse voltage from drain to source, the "body diode" will
conduct and light the LED. (can be useful to identify leadout on small
TO92
MOSFETs).

I am pretty new to this, can you explain what you mean by reverse
voltage? +ve terminal to ? and -ve to ?

I assume the resistor is just for limiting the current to the LED?

If you need that explaining, you shouldn't be messing with power supplies! -
However, what do I care if you kill yourself so here goes.

Its best to use a 2mA ultra-efficient LED, the battery should be a MN21/A23
12V car keyfob battery, calculate the resistor to limit the current to suit
the LED.

Assuming an N-channelMOSFETconnecting the negative lead to the drain and
the positive lead to the source, the body diode will conduct and light the
LED.

Next, connect the positive lead to drain and negative to source - the LED
may partially light due to static charge on the gate capacitance.

Keep the negative lead on the source and touch the positive lead on the
gate, this charges the gate capacitance positive and makes the N-channel
conduct - so if you then connect the positive lead to drain the LED will
light.

Next, keep the positive lead on the drain and touch the negative lead to
gate, this charges the gate capacitance negative and should make the
N-channel completely non-conducting - so put the negative lead on the source
and the LED should not even glow the slightest bit - if it does theMOSFET
is leaky and will go bang in a PSU!

Hi Ian,

Thanks - and to put your mind at rest its on a motherboard, so its all
DC.

I stay away from AC power supplies :-)

I assume all this must be done out of circuit?

-Al

Al wrote in message
...

Crude basic test for checking MOS Fets , in isolation

So this test must be done out of circuit?

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.

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

Thanks - will try this.

Incidentially what do you use for MOSFET removal from PCB? The type of
MOSFET's I am trying to deal with are all mounted "flat" on the PCB of
the motherboard.

Cheers

-Al


test out of circuit, unless comparing known good board to questionable, from
the same model, even then depends on circuitry

For small ones I tend to score around and angled under a bit, with a scalpel
and prize off, pad by pad , with a small jeweller's screwdriver. For larger
ones ,I loop some fine nichrome wire under a couple of pins and then ( not
owning a micro-jet hot air system) I shroud around the item (to protect
surrounding devices) and blast with paint-stripper hot air, tugging at the
wires to reduce the heating/loosening time. Blasting the device with freezer
spray beforehand, if wishing to salvage undamaged from heat.


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

On Aug 23, 9:55*am, "ian field"
wrote:
"Al" wrote in message

...

You can do a lot with a battery, LED and resistor.

I have all these parts....;-)

If you apply reverse voltage from drain to source, the "body diode" will
conduct and light the LED. (can be useful to identify leadout on small
TO92
MOSFETs).

I am pretty new to this, can you explain what you mean by reverse
voltage? +ve terminal to ? and -ve to ?

I assume the resistor is just for limiting the current to the LED?

If you need that explaining, you shouldn't be messing with power supplies! -
However, what do I care if you kill yourself so here goes.

Its best to use a 2mA ultra-efficient LED, the battery should be a MN21/A23
12V car keyfob battery, calculate the resistor to limit the current to suit
the LED.

Assuming an N-channelMOSFETconnecting the negative lead to the drain and
the positive lead to the source, the body diode will conduct and light the
LED.

Next, connect the positive lead to drain and negative to source - the LED
may partially light due to static charge on the gate capacitance.

Keep the negative lead on the source and touch the positive lead on the
gate, this charges the gate capacitance positive and makes the N-channel
conduct - so if you then connect the positive lead to drain the LED will
light.

Next, keep the positive lead on the drain and touch the negative lead to
gate, this charges the gate capacitance negative and should make the
N-channel completely non-conducting - so put the negative lead on the source
and the LED should not even glow the slightest bit - if it does theMOSFET
is leaky and will go bang in a PSU!

Thanks - have done all those tests on a known good mosfet and got the
expected results.

Now to try some other mosfets whose status is unknown.

Cheers

-Al

"William Sommerwerck" wrote in message
...
Assuming you can get the MOSFET at a reasonable price, why not just
replace
it, and not worry about testing it? It isn't worth the trouble.

I mean it only as a neutral, impersonal statement when I say that you
don't
know much about electronics, and this sort of time-wasting
trouble-shooting
is not likely to help you learn.



What a complete and utter load of cobblers!!!

Experimenting with how components behave in the real world is exactly how to
learn.

"ian field" wrote in message
...
"William Sommerwerck" wrote in message
...

Assuming you can get the MOSFET at a reasonable price, why not
just replace it, and not worry about testing it? It isn't worth the
trouble.
I mean it only as a neutral, impersonal statement when I say that you
don't know much about electronics, and this sort of time-wasting
trouble-shooting is not likely to help you learn.

What a complete and utter load of cobblers!!! Experimenting with
how components behave in the real world is exactly how to learn.

I'm not sure that applies when you're trying to _fix_ something.

My experience with repairs -- which I've been doing on and off for 50
years -- has generally been that the only thing I learn when fixing things
is how _stupid_ I am, for not catching the "obvious".

In my opinion, you learn more from a combination of reading books, building
projects, and experimenting with known-good components.

"William Sommerwerck" wrote in message
...
"ian field" wrote in message
...
"William Sommerwerck" wrote in message
...

Assuming you can get the MOSFET at a reasonable price, why not
just replace it, and not worry about testing it? It isn't worth the
trouble.
I mean it only as a neutral, impersonal statement when I say that you
don't know much about electronics, and this sort of time-wasting
trouble-shooting is not likely to help you learn.

What a complete and utter load of cobblers!!! Experimenting with
how components behave in the real world is exactly how to learn.

I'm not sure that applies when you're trying to _fix_ something.

My experience with repairs -- which I've been doing on and off for 50
years -- has generally been that the only thing I learn when fixing things
is how _stupid_ I am, for not catching the "obvious".

Which is bound to happen if you don't appreciate the importance of
understanding how components behave!

My experience with repairs -- which I've been doing on and off
for 50 years -- has generally been that the only thing I learn when
fixing things is how _stupid_ I am, for not catching the "obvious".

Which is bound to happen if you don't appreciate the importance
of understanding how components behave!

But that isn't necessarily something you learn by performing repairs.

I had a problem fixing an electronic crossover that used a op amp and
current-driver chip in its output stage. There was severe DC offset at the
output, and it wasn't clear whether the op amp or current driver was
defective. (The circuit board was almost impossible to remove from the unit.
Had it been easy, I would have simply replaced both ICs without worrying
about it.)

If I'd read the spec sheet for the current-driver chip carefully, the answer
would have been obvious. It was John Curl ("Jesus Christ" in the audio
industry) who had to explain it to me: "That current driver has a fixed gain
of -1." Aha! There was -1V on its input, and something like 5V on its
output. So the current driver _had_ to be defective. *

It was. I replaced it and that was that. (The driver had only five,
well-spaced pins, so I was able to snip them off, suck out what was left,
and solder in a new driver without having to remove the board. I lucked out.
Had the IC been bad, the board would have _had_ to have been removed, as I
would not have risked "snipping and sucking" fourteen leads from the top.)

A side remark... If you own oddball equipment you can't easily replace,
especially stuff made by out-of-business companies, it's a good idea to
study the schematic and purchase uncommon parts. The op-amp and current
driver were not easy to find. I bought a few of each, "just in case".

* The possibility that the op amp was blown and somehow pulling the current
driver's output to the wrong voltage seemed unlikely. The current driver is
the electrical "alpha male" in this circuit.

"William Sommerwerck" wrote in message
...
My experience with repairs -- which I've been doing on and off
for 50 years -- has generally been that the only thing I learn when
fixing things is how _stupid_ I am, for not catching the "obvious".

Which is bound to happen if you don't appreciate the importance
of understanding how components behave!

But that isn't necessarily something you learn by performing repairs.

I had a problem fixing an electronic crossover that used a op amp and
current-driver chip in its output stage. There was severe DC offset at the
output, and it wasn't clear whether the op amp or current driver was
defective. (The circuit board was almost impossible to remove from the
unit.
Had it been easy, I would have simply replaced both ICs without worrying
about it.)

Sounds like you miss the old days when you could just keep swapping tubes
and see if anything improves.