MOSFET Testing

[alpha_uma]

Is an oscilloscope the only way to test a MOSFET in circuit to see if it has
gone south?

Al-U

 

[alpha_uma]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:celpko$2o6$0@216.39.172.65@theriver.com...

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:msmPc.153352$Mr4.41748@pd7tw1no...
Is an oscilloscope the only way to test a MOSFET in circuit to see if it
has
gone south?

Depends on the circuit. You can sometimes use an ohmmeter. For power
(enhancement) MOSFETs, if the circuit is off, you should see something
reasonably large from gate to source/drain, and something similarly large
from source to drain. If you see only a couple of ohms it's probably
fried.
In my limited experience, MOSFETs usually fail shorted, not open.

Thanks for your advice, Walter. The MOSFETs that I want to test are in
circuit on my dead motherboard (socket 478, P4). BTW, I have already
replaced my system with a new motherboard. I just want to troubleshoot what
has gone south, and learn something along the way.

I did the ohmmeter test on the six voltage regulation MOSFETs near the ATX
power connector. I don't know if it makes any difference where the positive
lead of the meter should be placed. But I make sure that I did them
consistently by placing the positive lead on the first of each of the pairs
(G-S, G-D, D-S) listed below. For the sake of comparison, I performed
similar tests on a spare motherboard (socket 370, P3) that is known to be in
good functioning condition.

This is what I found on my GOOD socket 370 motherboard:

For one MOSFET (B3015L):
G-S: 21.3 mega-ohm
G-D: 20.9 mega-ohm
D-S: 238.6 ohm

For second MOSFET (B3015L):
G-S: 20.9 mega-ohm
G-D: 21.3 kilo-ohm
D-S: 14 kilo-ohm

This is what I found on my DEAD socket 478 motherboard:

For the three MOSFETs NEC K3296, all gave similar (incredibly low)
readings as follows:
G-S: 18.7 ohm
G-D: 21.8 ohm
D-S: 3.2 ohm

For the three MOSFETs JS214A, all gave similar (incredibly low) readings
as follows:
G-S: 0 ohm (this is bad, right?)
G-D: 29.7 ohm
D-S: 27.6 ohm

Do you think these six MOSFETs on my socket 478 motherboard are all dead?
BTW, the three electrolytic caps around these six MOSFETs are already
confirmed bad out-of-circuit, and I replaced them with good ones.

Thanks
Al-U

 

[Walter Harley]

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:q1JPc.158398$Mr4.33375@pd7tw1no...

I did the ohmmeter test on the six voltage regulation MOSFETs near the ATX
power connector. I don't know if it makes any difference where the
positive
lead of the meter should be placed.

A power MOSFET has a "body diode" (a diode that just happens to be there as
a result of the semiconductor process) from S to D. The polarity depends on
whether the MOSFET is n-channel or p-channel. Often in the manufacturer
datasheet it shows the diode explicitly. For instance, if you look at the
NEC datasheet for your 2SK3296, at
http://www.necel.com/nesdis/image/D14063EJ2V0DS00.pdf, down on page 6 you'll
see a diagram with the diodes shown. (There's also a protection diode for
the gate, on that MOSFET.) If you get your leads backwards, you'll see
conductivity through the diode, even though the MOSFET isn't turned on.

For the three MOSFETs NEC K3296, all gave similar (incredibly low)
readings as follows:
G-S: 18.7 ohm
G-D: 21.8 ohm
D-S: 3.2 ohm

In your case, when testing the D-S pair you put the positive lead on D,
which turns out to be correct for the 2SK3296, because it's an N-channel
MOSFET. So, yes, at least one of them is blown: the G-S and G-D readings
are way too low to be real (you should expect more than 100 ohms in any
circuit I've seen), and the D-S reading is also very low.

However, you probably can't tell which one is blown, because they may all be
in parallel with each other, meaning the blown one is shorting out the
others. You'll need to unsolder them till you find the one (or more) that
is blown.

For the three MOSFETs JS214A, all gave similar (incredibly low)
readings
as follows:
G-S: 0 ohm (this is bad, right?)
G-D: 29.7 ohm
D-S: 27.6 ohm

In this case, putting the positive lead on the D when you measured the D-S
pair was wrong; as you know, the JS214A (aka 2SJ214A, from previous
discussion) is a P-channel device.

But regardless, seeing a dead short from G to S indicates a fault. There's
a big smoking hole in that silicon. Again, it might only be one or two of
the three; they're probably all in parallel with each other.

By the way, the group sci.electronics.repair is also a good group for this
kind of discussion.

 

[Walter Harley]

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...

Thanks for this reminder. The bad news is that I'm not sure if I can
identify exactly which components on the motherboard belong to the driver
circuit. Near those MOSFETs, I can see three circular magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic, and two
transistors (whose labels I could not make out until I get a bigger
magnifying glass). These seem to be for driving the MOSFETs. And even if
these are the only items in the driver circuit, they will be testing the
limits of my soldering skill. At the very least, I need a finer tip. And
it
would have helped if they had directly soldered that IC on to the board,

Here's your KA7500B: http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

 

[Franc Zabkar]

On 5 Aug 2004 19:09:31 GMT, "Walter Harley"
<walterh@cafewalterNOSPAM.com> put finger to keyboard and composed:

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure if I can
identify exactly which components on the motherboard belong to the driver
circuit. Near those MOSFETs, I can see three circular magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic, and two
transistors (whose labels I could not make out until I get a bigger
magnifying glass). These seem to be for driving the MOSFETs. And even if
these are the only items in the driver circuit, they will be testing the
limits of my soldering skill. At the very least, I need a finer tip. And
it
would have helped if they had directly soldered that IC on to the board,



Here's your KA7500B: http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

FYI, the KA7500B is equivalent to the TL494. Many AT/ATX PSUs use
these.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Watson A.Name - \"Watt Su]

"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:5r35h0hvbhe5qd11kspnssq13ctjj94241@4ax.com...

On 5 Aug 2004 19:09:31 GMT, "Walter Harley"
walterh@cafewalterNOSPAM.com> put finger to keyboard and composed:

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure if I
can
identify exactly which components on the motherboard belong to the
driver
circuit. Near those MOSFETs, I can see three circular
magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic, and
two
transistors (whose labels I could not make out until I get a bigger
magnifying glass). These seem to be for driving the MOSFETs. And
even if
these are the only items in the driver circuit, they will be
testing the
limits of my soldering skill. At the very least, I need a finer
tip. And
it
would have helped if they had directly soldered that IC on to the
board,



Here's your KA7500B: http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

FYI, the KA7500B is equivalent to the TL494. Many AT/ATX PSUs use
these.

Thanks, that's good to know. I've found that Sharp pulls the shenanigan
of marking their equivalent chips with some oddball number, but I didn't
know that Fairchild did so, too. Usually they just change the TL (which
stands for TI linear, IIRC) to LM.

- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Franc Zabkar]

On Fri, 6 Aug 2004 08:34:37 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> put finger to keyboard and
composed:

"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:5r35h0hvbhe5qd11kspnssq13ctjj94241@4ax.com...
On 5 Aug 2004 19:09:31 GMT, "Walter Harley"
walterh@cafewalterNOSPAM.com> put finger to keyboard and composed:

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure if I
can
identify exactly which components on the motherboard belong to the
driver
circuit. Near those MOSFETs, I can see three circular
magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic, and
two
transistors (whose labels I could not make out until I get a bigger
magnifying glass). These seem to be for driving the MOSFETs. And
even if
these are the only items in the driver circuit, they will be
testing the
limits of my soldering skill. At the very least, I need a finer
tip. And
it
would have helped if they had directly soldered that IC on to the
board,



Here's your KA7500B: http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

FYI, the KA7500B is equivalent to the TL494. Many AT/ATX PSUs use
these.

Thanks, that's good to know. I've found that Sharp pulls the shenanigan
of marking their equivalent chips with some oddball number, but I didn't
know that Fairchild did so, too. Usually they just change the TL (which
stands for TI linear, IIRC) to LM.

It was originally a Samsung part, not Fairchild.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Franc Zabkar]

On Fri, 6 Aug 2004 19:54:11 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> put finger to keyboard and
composed:

"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:f5n7h0dfnrgosabv7aptufscpcgv7l70pb@4ax.com...
On Fri, 6 Aug 2004 08:34:37 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> put finger to keyboard and
composed:


"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:5r35h0hvbhe5qd11kspnssq13ctjj94241@4ax.com...
On 5 Aug 2004 19:09:31 GMT, "Walter Harley"
walterh@cafewalterNOSPAM.com> put finger to keyboard and composed:

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure if I
can
identify exactly which components on the motherboard belong to
the
driver
circuit. Near those MOSFETs, I can see three circular
magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic,
and
two
transistors (whose labels I could not make out until I get a
bigger
magnifying glass). These seem to be for driving the MOSFETs. And
even if
these are the only items in the driver circuit, they will be
testing the
limits of my soldering skill. At the very least, I need a finer
tip. And
it
would have helped if they had directly soldered that IC on to
the
board,



Here's your KA7500B:
http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

FYI, the KA7500B is equivalent to the TL494. Many AT/ATX PSUs use
these.

Thanks, that's good to know. I've found that Sharp pulls the
shenanigan
of marking their equivalent chips with some oddball number, but I
didn't
know that Fairchild did so, too. Usually they just change the TL
(which
stands for TI linear, IIRC) to LM.

It was originally a Samsung part, not Fairchild.

Ah-hah, I see. Fairchild's is a clone of the Samsung, which is a clone
of the original TI part. )

It's more like Fairchild *is* Samsung.

See "Fairchild Semiconductor Completes the Acquisition of Samsung
Electronics' Power Device Division (Apr 14, 1999)":
http://www.fairchildsemi.com/news/1999/9904/ssclose.html


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Watson A.Name - \"Watt Su]

"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:eft8h0pu4cdapm801oo89doc0ejsmntcp8@4ax.com...

On Fri, 6 Aug 2004 19:54:11 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> put finger to keyboard and
composed:


"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:f5n7h0dfnrgosabv7aptufscpcgv7l70pb@4ax.com...
On Fri, 6 Aug 2004 08:34:37 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> put finger to keyboard and
composed:


"Franc Zabkar" <fzabkar@optussnet.com.au> wrote in message
news:5r35h0hvbhe5qd11kspnssq13ctjj94241@4ax.com...
On 5 Aug 2004 19:09:31 GMT, "Walter Harley"
walterh@cafewalterNOSPAM.com> put finger to keyboard and
composed:

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure
if I
can
identify exactly which components on the motherboard belong
to
the
driver
circuit. Near those MOSFETs, I can see three circular
magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF
electrolytic,
and
two
transistors (whose labels I could not make out until I get a
bigger
magnifying glass). These seem to be for driving the MOSFETs.
And
even if
these are the only items in the driver circuit, they will be
testing the
limits of my soldering skill. At the very least, I need a
finer
tip. And
it
would have helped if they had directly soldered that IC on to
the
board,



Here's your KA7500B:
http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

FYI, the KA7500B is equivalent to the TL494. Many AT/ATX PSUs
use
these.

Thanks, that's good to know. I've found that Sharp pulls the
shenanigan
of marking their equivalent chips with some oddball number, but I
didn't
know that Fairchild did so, too. Usually they just change the TL
(which
stands for TI linear, IIRC) to LM.

It was originally a Samsung part, not Fairchild.

Ah-hah, I see. Fairchild's is a clone of the Samsung, which is a
clone
of the original TI part. )

It's more like Fairchild *is* Samsung.

See "Fairchild Semiconductor Completes the Acquisition of Samsung
Electronics' Power Device Division (Apr 14, 1999)":
http://www.fairchildsemi.com/news/1999/9904/ssclose.html

Oh. I thought National acquired Fairchile, and then later spun it off
as independent. Cornfusing. Whatever makes a buck, I guess. As long
as I can get datasheets, that's mainly what i'm concerned about.

<<
A guy goes into a restaurant/lounge wearing a shirt open
at the collar and is met by a bouncer who tells him he must
wear a necktie to gain admission. So the guy goes out to
his car and he looks around for a necktie and discovers
that he just doesn't have one. He sees a set of jumper
cables in his trunk. In desperation he ties these around
his neck, manages to fashion a fairly acceptable looking
knot and lets the ends dangle free. He goes back to the
restaurant and the bouncer carefully looks him over for
a few seconds and then says, "Well, OK, I guess you can
come in - just don't start anything."

- Franc Zabkar
--

 

[alpha_uma]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:ceu0lb$908$0$216.39.172.65@theriver.com...

"alpha_uma" <no_one@nonesuch.com> wrote in message
news:HIjQc.14687$gE.13675@pd7tw3no...
Thanks for this reminder. The bad news is that I'm not sure if I can
identify exactly which components on the motherboard belong to the
driver
circuit. Near those MOSFETs, I can see three circular magnet-looking
coils,
an IC (labeled F E151A KA7500B), a tiny tiny 10uF electrolytic, and two
transistors (whose labels I could not make out until I get a bigger
magnifying glass). These seem to be for driving the MOSFETs. And even if
these are the only items in the driver circuit, they will be testing the
limits of my soldering skill. At the very least, I need a finer tip. And
it
would have helped if they had directly soldered that IC on to the board,



Here's your KA7500B: http://www.fairchildsemi.com/ds/KA/KA7500B.pdf

Thanks. I saw it too.

Does anyone have the documentation or a cross reference for the MOSFET
2SJ214A? Who makes the chip? I can't find it on the web, and without it, I
don't know how to find a replacement MOSFET. Any help would be much
appreciated.

 

[alpha_uma]

alpha_uma" <no_one@nonesuch.com> wrote in message
news:Fc1Rc.33783$gE.8615@pd7tw3no...

Does anyone have the documentation or a cross reference for the MOSFET
2SJ214A? Who makes the chip? I can't find it on the web, and without it, I
don't know how to find a replacement MOSFET. Any help would be much
appreciated.

OK, I'm going to start a new thread on the search for JS214A and hope that
more people would be able to help.