hFE on DMM

[[email protected]]

I found out that my DMM can measure hFE for transistors, but when a
correctly place a transistor in the slots the numbers wobble a little
then settle to a 1 as the only digit shown on screen. What am I
supposed to be looking for? Does this mean the transistor is not
working?

 

[Phil Allison]

I found out that my DMM can measure hFE for transistors, but when a
correctly place a transistor in the slots the numbers wobble a little
then settle to a 1 as the only digit shown on screen. What am I
supposed to be looking for? Does this mean the transistor is not
working?

** Maybe - do any transistors test OK ?

Sure you know which legs are what ?



........ Phil

 

[Chris]

I found out that my DMM can measure hFE for transistors, but when a
correctly place a transistor in the slots the numbers wobble a little
then settle to a 1 as the only digit shown on screen. What am I
supposed to be looking for? Does this mean the transistor is not
working?

I'd agree that you've probably got the E, B and C leads turned
around. It does make a difference. You should see a number greater
than one -- possibly 20 to 200.

Go look at the data sheet. The quickest way to do that would be to go
to the NTE site and punch in the manufacturer part number:

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm

It will then show the NTE equivalent repair part. Click on the data
sheets link to see the part pinout, and insert the leads into the DMM
accordingly.

Good luck
Chris

 

[Eeyore]

I found out that my DMM can measure hFE for transistors, but when a
correctly place a transistor in the slots

What do you mean by "correctly" ?

Graham

 

[Jamie]

I'd agree that you've probably got the E, B and C leads turned
around. It does make a difference. You should see a number greater
than one -- possibly 20 to 200.

Go look at the data sheet. The quickest way to do that would be to go
to the NTE site and punch in the manufacturer part number:

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm

It will then show the NTE equivalent repair part. Click on the data
sheets link to see the part pinout, and insert the leads into the DMM
accordingly.

Good luck
Chris

ha, i have a cheap meter that can turn the leads in the circuit to find
the highest Hfe. i don't have to make sure the leads are in the correct
hole!

 

[[email protected]]

What do you mean by "correctly" ?

Graham

I mean that I have correctly identified the pins. It is a 2N2222A
transistor. I looked up the datasheet and found the correct
information, but I still get the solitary "1" on the screen.

 

[John Fields]

I mean that I have correctly identified the pins. It is a 2N2222A
transistor. I looked up the datasheet and found the correct
information, but I still get the solitary "1" on the screen.

---
If you've connected the transistor to your DMM correctly and you
can't tell whether the transistor is good or bad, then you either
have a bad meter or a bad transistor.

In order to find out where the fault lies, (assuming your DMM has a
diode test function), do this:

Set your meter for diode test and connect it across the base to
emitter junction of the transistor. If it reads "OL", reverse the
connection of the leads. If, after that, it still reads "OL", then
the base to emitter junction has burned open and you have a bad
transistor. If it doesn't, and it reads something like "0.7", then
the base to emitter junction is OK.

However, if it reads close to zero, then the base to emitter
junction has been burned shut and you have a bad transistor.

If those tests are successful, then without disturbing the lead
connected to the base of the transistor, disconnect the lead
connected to the emitter and connect it to the collector. The
reading should still be around 0.7V.

If it isn't, and it's close to zero, then the collector to base
junction has been burned shut.

If it isn't and your meter reads "OL", then the collector to base
junction has been burned open.

If both of those preliminary tests were successful then, for a
final test, set your DMM to an "OHMS" range where 10k ohms can be
easily detected and connect the lead with a positive output to the
collector of the transistor and the other lead to the emitter of the
transistor. Your meter should read "OL".

If it doesn't, you've got a bad transistor.

If it does, then lick your fingers and pinch the collector lead
between the thumb and forefinger of one hand and pinch the base lead
between the thumb and forefinger of the other hand. If you do that
and you can change the resistance indicated by the meter by
squeezing and relaxing the pressure exerted on the base lead, you've
got a good transistor.