I am trying to test two RFG70N60 Mosfets, with my meter set on testing
diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this
seems kind of low.

These are both out of a UPS that is having problems, does anyone know
what the readings for the Mosfets should be? I emailed Fairchild
Semiconductors but they never replied.


Thanks,

-Landon

Landon,

Put your meter on ohms and measure between the drain and the source. These
are the righthand two leads. You should get a high reading, either no
reading or high megohms. If not, it's shorted, the usual failure mode.

John Musselman

"Landon" wrote in message
...
I am trying to test two RFG70N60 Mosfets, with my meter set on testing
diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this
seems kind of low.

These are both out of a UPS that is having problems, does anyone know what
the readings for the Mosfets should be? I emailed Fairchild
Semiconductors but they never replied.

Hi, thanks for replying.

I found a procedure online somewhere that basically said the following:

------------------------------------------------------------------------
-Set meter to check diodes.
-Put positive lead on the GATE, put negative lead on the SOURCE (this
will charge the gate, before doing this the DRAIN and SOURCE pins should
give a HIGH reading).
-Then move the positve lead to the DRAIN, it should give a LOW reading.
-Touch the GATE and SOURCE pins with finger tip, the gate should
discharge, then the DRAIN and SOURCE pins should again give a HIGH reading.
------------------------------------------------------------------------

This is where I am stuck, the Mosfets are acting the way the test says
that they should but I am used to seeing diodes reading at 0.5xx to
0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too
low or right where it should be.

-Landon

John Musselman wrote:
Landon,

Put your meter on ohms and measure between the drain and the source. These
are the righthand two leads. You should get a high reading, either no
reading or high megohms. If not, it's shorted, the usual failure mode.

John Musselman

"Landon" wrote in message
...

I am trying to test two RFG70N60 Mosfets, with my meter set on testing
diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this
seems kind of low.

These are both out of a UPS that is having problems, does anyone know what
the readings for the Mosfets should be? I emailed Fairchild
Semiconductors but they never replied.

Landon,

That's a cute procedure and may even work.

In the "diode test" position, your DVM is generating a small current and
reading what voltage is generated. So a diode will read out the voltage
across the junction (the "forward voltage" or Vf) with this current passing
through. In a diode, this tends to be 0.5 to 0.7 for a silicon diode, and
varies little with the current.

In a MOSFET, there isn't a diode junction. It's just like a resistor, so a
low reading is a low resistance. (A low voltage is generated by the test
current). The low number is OK, if you have, indeed, charged the gate. You
should get a high reading (or overload, or infinity) when the gate is
discharged or shorted to ground.

John


"Landon" wrote in message
...
Hi, thanks for replying.

I found a procedure online somewhere that basically said the following:

------------------------------------------------------------------------
-Set meter to check diodes.
-Put positive lead on the GATE, put negative lead on the SOURCE (this will
charge the gate, before doing this the DRAIN and SOURCE pins should give a
HIGH reading).
-Then move the positve lead to the DRAIN, it should give a LOW reading.
-Touch the GATE and SOURCE pins with finger tip, the gate should
discharge, then the DRAIN and SOURCE pins should again give a HIGH
reading.
------------------------------------------------------------------------

This is where I am stuck, the Mosfets are acting the way the test says
that they should but I am used to seeing diodes reading at 0.5xx to
0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too
low or right where it should be.

-Landon

Thank you for the reply...

I believe that answers my question (that the 0.001 to 0.003 readings
are normal), and yes when the gate is discharged it is giving an
infinity reading.

I guess I will keep troubleshooting this UPS, my next guess is that the
LM324 that is driving the two MOSFETS isn't giving them the correct
waveform or isn't being told to give them the correct waveform.


Thanks again,

-Landon

John Musselman wrote:
Landon,

That's a cute procedure and may even work.

In the "diode test" position, your DVM is generating a small current and
reading what voltage is generated. So a diode will read out the voltage
across the junction (the "forward voltage" or Vf) with this current passing
through. In a diode, this tends to be 0.5 to 0.7 for a silicon diode, and
varies little with the current.

In a MOSFET, there isn't a diode junction. It's just like a resistor, so a
low reading is a low resistance. (A low voltage is generated by the test
current). The low number is OK, if you have, indeed, charged the gate. You
should get a high reading (or overload, or infinity) when the gate is
discharged or shorted to ground.

John


"Landon" wrote in message
...

Hi, thanks for replying.

I found a procedure online somewhere that basically said the following:

------------------------------------------------------------------------
-Set meter to check diodes.
-Put positive lead on the GATE, put negative lead on the SOURCE (this will
charge the gate, before doing this the DRAIN and SOURCE pins should give a
HIGH reading).
-Then move the positve lead to the DRAIN, it should give a LOW reading.
-Touch the GATE and SOURCE pins with finger tip, the gate should
discharge, then the DRAIN and SOURCE pins should again give a HIGH
reading.
------------------------------------------------------------------------

This is where I am stuck, the Mosfets are acting the way the test says
that they should but I am used to seeing diodes reading at 0.5xx to
0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too
low or right where it should be.

-Landon

What symptom is the unit showing?

Paul