How to test a transformer with only a DMM?

[drdavidge]

Hey, does anyone know how to test a transformer with only a digital multimeter?

[gastorgrab]

Re: How to test a transformer with only a DMM?

Test for open circuit?

1. Test for continuity between the input pins.
2. Test for continuity between the output pins.
3. Done!
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[drdavidge]

Re: How to test a transformer with only a DMM?

Sorry, never really got into transformers so i'm lost even on the basics. Do you happen to have a pin diagram showing which pins are the input ones and output ones? And would I use the diode test to test for continuity on the multimeter? Thanks for the help.

[drdavidge]

Re: How to test a transformer with only a DMM?

Here are the transformers I want to test. There are four of them but the picture only has 3 in them:

[drdavidge]

Re: How to test a transformer with only a DMM?

Also, how about capacitors? My DMM doesn't have a function for testing capacitance but is there some way to do it without that?

[gastorgrab]

Re: How to test a transformer with only a DMM?

Sorry, never really got into transformers so i'm lost even on the basics. Do you happen to have a pin diagram showing which pins are the input ones and output ones? And would I use the diode test to test for continuity on the multimeter? Thanks for the help.

Sorry, but you've contacted another beginner.
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You could still get away with a continuity test but the reading won't be absolute. For 'go', 'no-go' results you would have to isolate the the transformer. The results now would be 'Almost No Resistance', and 'Some or Infinite Resistance'. (and may still not give an absolute determination)

You'll need to identify the input and output sides so you can eliminate some possibilities. The transformers shown have multiple pins, not just two pins in, and two pins out. Since i don't see any reason that a circuit would be designed to use separate voltage inputs i would assume the side with the most pins is the output side.

There should also be some diodes downstream of the output since A/C is so rarely used in these circuits.
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[starfury1]

Re: How to test a transformer with only a DMM?

OK playing blind mans bluff here

You would do better to post make model of the device and what the problem is

That way if there is a common type failure mode for this then someone may know what you need to do to fix it


From what I can see they appear to be inverter transformers for CL tubes?
but I am no guru on these

and as above "go /no go" but you need to know whats what on the windings anyway
inverter transformer I believe can go shorted turns. not sure if a MM will help you find them

4 going at once is doubtful.

there has been posts of failed transistors and electrolytic capacitors depending on make model.

you can get an idea with a MM on Electro caps but that does depend on failure
u can also do go no go on transistors as well

more info would be helpful.

[drdavidge]

Re: How to test a transformer with only a DMM?

Its actually for a dell e172fpb monitor and there is a whole thread about it here: https://www.badcaps.net/forum/showthread.php?t=1303. Read my last 3 posts there at the very end.. I replaced the transistors and then blew again a few days later so they said it could be the capacitors or transformers, but nobody has the transformer part number so I am shooting in the dark here.

[starfury1]

Re: How to test a transformer with only a DMM?

Humm OK thanks, and for linking in the other thread too.....

[starfury1]

Re: How to test a transformer with only a DMM?

Seems the multitude of possible issues with this has been well explored with the above thread.

in fact this is what came to mind but not sure how well it works with inverter transformers....(not the guru of inverters here)
you may find it via DC resistance measurements but that really depends on how many "shorted turns" I suppose....not conclusive thought, I think
So yeah DMM may not be a great deal of help on this
Temperature of each transformer may be a guide?
like does 1 or 2 seem to get a lot hotter then the others...but not sure how accurate this would be.

Iam not really up on how they work ...forgotten info..dont use it tends to be forgotten
but for a refresher and for those wanting to know here,see Bob's project LOPT tester...grab the PDF Here
(Kit no longer avail)
google shorted turns or fly back lopt tester for more info on the subject

If Q759 and Q760 are getting very Hot; chances are that one of the switching transformer coils are shorted (T751 or T753). Also check (C754 C755) which lies across the collectors of Q759 and Q760. These caps dampen the voltage and current across the tranny's (V*I=P (Watts=heat)). I am under the assumption that Q759 and Q760 are not failing (have you removed, and metered them individually?).

Resistance from Emitter to Collector (Ohm'd out in both directions) should be infinite. Also, to test the switching transformers you will need to perform a "Ringer" test using a special tester (which a TV repair shop or University would have). Be sure to check the primary and ALL (3) of the secondary terminal pairs.

See the attached schematic which I made based on my inverter board. Keep note that this schematic is NOT complete. There are many other subsidiary circuits indicated by ">>". It does however show the critical aspects required for proper operation. Best of Luck!!!

See UMD_EE post (thanks UMD_EE)
PAGE 7 of thread, he has also posted a basic PDF odf the circuit he has drawn up
This does show the Transformers and windings.

Fix advice, range from reflow PCB to transistor capacitors and inverter transformers.

By the sounds of it all four and hope it stays working!

there is a bit on monitoring a diode thats seems interesting I have to find it again re read it
dry joints are always a problem and a resoldering of heat stress joints is always a good idea...I think anyway

The whole thread should be condensed into a trouble shooting guide
As this seems to be a real issue out there for this model.

Goodluck with it , its sounds like you are going to need it

[drdavidge]

Re: How to test a transformer with only a DMM?

Thanks for the help so far. Any idea how I can check capacitors? My DMM doesn't have a capacitor option.. know of any tricks? I heard of something with 555 timers and I have some of those lying around.

I really just want to find out of those transformers are bad or not and check the capacitors too.

[starfury1]

Re: How to test a transformer with only a DMM?

the old analog meters used to be good for checking, go no go (more or less) on caps you can with a DMM
Electrolytic capacitors are a little easier cause they take time to charge up.
but you need to make sure of the polarity of the Resistance range on the meter
(see URL below)

You will find in this page has how to use a Multimeter to check caps
but its not going to be the best way to check caps
Sure you can build a little capacitance type meter or a circuit to hang off your DMM

these are out on the net but you may have to do some searching...
I couldn't quickly find one as an adapter for the DVM but I know many have designed and build circuits for this

The only 555 circuit I found used an analog microamp meter Here and doesn't go above 10uF tops


Here's one its stand alone and yeah some what complex

Anyway if I find a simple one I'll post it...they are not that expensive to buy but not worth it if you not intending to use it a lot either

found one circuit Here but not much in the uf again.


HTH

Cheers

[drdavidge]

Re: How to test a transformer with only a DMM?

Thanks.. I was able to build a simple 555 timer circuit on a breadboard and use the capacitors in the circuit to test to see if they worked, and them seemed to work fine. I am guessing one of the transformers are bad but I am still not sure how to be positive about this. I also can't find a part number for replacement transformers either. Maybe I should just get a new additional monitor when it finally dies again, haha.