Check Transistors and FETS in circuit for shorts or opens using a DVM ohm meter

[walters]

I'm trying to use a DVM meter in ohm mode to check transistors and mosfets in circuit

All I get is resistance measurements in the Meg ohms

Is there any other way to check or test transistors or mosfets in circuit?

Cause I can't make head or tails if they are open since its reading in the meg ohm or if it's shorted because of the resistance reading

The problem I'm having is that There is a SHORT or Multiple Shorts on PCB board circuits at my work and I'm trying to check the transistors and mosfets in the power supply section

The SHORTS causes the resistance readings to be different on every PCB board I check

I have Written down the resistance readings off of a GOOD working board a KGB, to reference off of.

But that doesn't do any good, why? because the transistors and mosfets can still be operating fine even if the resistance is way off because the SHORT somewhere on the PCB boards has changed the resistance value

So I have no Idea if the transistors or mosfets are GOOD or NOT , cause i can't make heads or tails because the SHORTS have changed everything

I'm just trying to verify if the transistors and mosfets work

How can I do this please?

 

[abhi_tandon80]

It's better to check transistor in diode mode careful in checking mosfet as it get ON while you test even board without power

you can find more info on

https://www.elexp.com/t_test.htm

 

[walters]

yes i turned the power off

The Diode mode is the Continuity check with the beeping right?

How do you check transistors and mosfets using the Diode mode?

 

[abhi_tandon80]

Keep your eye on multimeter means you should refer the value on multimeter which is more important then beeping

https://www.elexp.com/t_test.htm on how to check

 

[godfreyl]

The problem I'm having is that There is a SHORT or Multiple Shorts on PCB board circuits at my work.......

How do you know there are shorts?

The SHORTS causes the resistance readings to be different on every PCB board I check

Why do you think the differences are caused by shorts?

......the SHORT somewhere on the PCB boards has changed the resistance value

How do you know the resistance value was changed by a short?

......i can't make heads or tails because the SHORTS have changed everything

Why do you think all the changes are because of shorts?

Have you found any shorts?

 

[walters]

How do you check transistors and mosfets using the Diode mode on a DVM?

 

[abhi_tandon80]

check this page for explanation

https://www.elexp.com/t_test.htm

 

[walters]

it doesn't really explain how to use a DVM meter to test mosfets , transistors, diodes in circuit

---------- Post added at 06:16 ---------- Previous post was at 05:56 ----------

Whats the best pin to lift up on a Transistor? when it's in circuit to test with a DVM meter in Diode check? is it the base? emitter , or collector?

What is the best pin to lift up on a MOSFET? when it's in circuit to test with DVM meter in Diode Check? is it the gate? source? drain?

Ok I have my DVM meter set to diode check mode

1.) I lift up the Base ( pin #1) of the transistor
2.) I measure from base to Emitter it should read .7 volts?
3.) I measure from base to Collect it should read open circuit?
4.) I measure from Emitter to College it should read open circuit?

Does the Polarity of the red and black probes make a different which way they go when measuring?

I have my DVM meter set to diode check mode

1.) I lift up the GATE ( pin #1) of a MOSFET
2.) I measure from Gate to Source it should read .7 volts?
3.) I measure from Gate to Drain it should read open circuit?
4.) I measure from Source to Drain it should read open circuit?

Does the Polarity of the red and black probes make a different which way they go when measuring?

 

[fredd]

take note that some transistors have very low on-resistance (in mΩ). this value could be considered as short for some multimeters.

 

[abhi_tandon80]

ok

without lifting the pin first find the base

put one wire of DVM on any pin if that pin is base it must show value wrt base ie

base - emitter 0.765
base - collector 0.766 something like this then you can be sure to find the base of transistor in any other case it should not be same

For mosfet if gate is gud source and drain short and on reverse biasing they shoul open circuit need to lift pins any two

 

[walters]

For mosfet if gate is gud source and drain short and on reverse biasing they shoul open circuit need to lift pins any two

For Mosfets i have to lift up 2 pins? Which 2 pins?

The Gate to source would measure?
The Gate to drain would measure?
Source to Drain would measure?