Transformer testing

[DrF]

Would anyone please be kind enough to explain a good, quick way to test a output (and power) transformer? I read so many ways that I am 101% confused. LOL!
MANY thanks!

[Enzo]

The various ways are not mutually exclusive, you can do it more than one way.

GO over to RG's Geofex site and look up his transformer tester circuit.

[Chuck H]

What are you testing for???

Good / bad? Primary / secondary voltage (PT)? Primary / secondary impedance (OT)? Wattage / mA rating? Color codes? What do you need to know about the transformers?

Chuck

[DrF]

Thank you!

Thank you for the info! I'll look at that site.

[DrF]

Transformer testing

Geofex was not in depth enough. I'd really like to learn a simple test(s) to determine if a OT or PT is bad. I've read a bit... it's gotten to be confusing. I like to learn some tips from the experts.
Especially if it's wired to a PCB.
Thank you for your assistance.

[pontiacpete]

The only two ways I know of, is to check for an open in the windings with an ohm meter and to check for a short - the last time I shorted a transformer I checked it with a light bulb placed in series in the primary, if the bulb glows bright it's shorted.

[Enzo]

What do you want to know about the transformer? The Geofex transformer tester (under tech tips) will tell you if any of the windings are shorted. As pete mentioned, your ohm meter will tell you if a winding is open or shorted to the frame.

What else do you want to test? "Bad" doesn't specify anything.

[R.G.]

Geofex was not in depth enough.

Without wishing to be argumentative, how did you find it too shallow? I've debugged failing transformers for decades based on those tests plus the newer one for internal shorts.

There are only a few ways that a transformer that ever worked properly CAN fail. These are the following:
1. Open winding. A wire has broke/burned open and no current can flow. The test for this is to use your ohmmeter on every single winding all by itself with no external loads. As an alternative, you can test it with external connections if you are 100% certain you can tell the difference. But it's far safer to identify a winding, pull one end loose, then ohm it out. You only need to be able to tell the difference between normal wire resistance (usually under 500 ohms) and infinitely many megohms. When you have identified every winding and tested them for open, you're done with this one.
2. Shorts between windings and between winding and core. Again an ohmmeter test. Just test for open circuit between every single isolated winding and the others, and also between windings and the core. When you have used the ohmmeter, you have eliminated all of these.
3. Internal short in a winding. The pulse-inductance test with a battery finds these every time.
4. AC voltage test. If it passes 1-3, put an AC voltage on one winding and measure the AC voltage on every other winding. If these are what they should be and it passes 1-3, it's almost certainly good.

What exactly does that miss?

I've read a bit... it's gotten to be confusing.

That's a problem, all right. The internet lets anyone set up shop as an expert. That's possibly why you think testing transformers is complicated.

I like to learn some tips from the experts.

I'm working real hard on that right now.

Especially if it's wired to a PCB.

Ah. This is a huge problem. To do a good test of a PCB mounted transformer, you're simply going to have to either pull it out of the PCB or butcher the PCB to open windings. It is unreasonably difficult to test a transfomer while it's on a PCB, and this carries a huge possibility of errors. I would never "test" a transformer on a PCB if I had any suspicion that it was damaged.

I'd really like to learn a simple test(s) to determine if a OT or PT is bad.

Pretty much everyone who works with transformers does. That's what's at GEO.

Thank you for your assistance.

You're welcome.

[Chuck H]

Nice work cutting and pasting in DrF's "Thank you" on your own behalf

I think "thank you" is small praise for the enormous effort you put into the GEO pages I personally have the whole thing in three ring binder for referencing (in case anything ever happens to the site ).

After DrF's implication of mediocrity I thought you should know, and I think I speak for everyone here, that your efforts are appreciated.

Chuck

[slidincharlie (Carlo P)]

Where is the transformer test page in Geofex?

Where is the transformer test page in Geofex?
I don't see a link to transformer testing in the Tube Amp Debugging homepage.
Thanks.

[pdf64]

Geofex.com
Left hand menu list
- Tube Guitar Amp Tech Pages - Tech Tips

Then the link titled
'Transformer Short Tester', under Tube Amp Tips

http://www.geofex.com/FX_images/xform_test.gif

[Prairie Dawg]

Without wishing to be argumentative, how did you find it too shallow? I've debugged failing transformers for decades based on those tests plus the newer one for internal shorts.

There are only a few ways that a transformer that ever worked properly CAN fail. These are the following:
1. Open winding. A wire has broke/burned open and no current can flow. The test for this is to use your ohmmeter on every single winding all by itself with no external loads. As an alternative, you can test it with external connections if you are 100% certain you can tell the difference. But it's far safer to identify a winding, pull one end loose, then ohm it out. You only need to be able to tell the difference between normal wire resistance (usually under 500 ohms) and infinitely many megohms. When you have identified every winding and tested them for open, you're done with this one.
2. Shorts between windings and between winding and core. Again an ohmmeter test. Just test for open circuit between every single isolated winding and the others, and also between windings and the core. When you have used the ohmmeter, you have eliminated all of these.
3. Internal short in a winding. The pulse-inductance test with a battery finds these every time.
4. AC voltage test. If it passes 1-3, put an AC voltage on one winding and measure the AC voltage on every other winding. If these are what they should be and it passes 1-3, it's almost certainly good.

What exactly does that miss?


That's a problem, all right. The internet lets anyone set up shop as an expert. That's possibly why you think testing transformers is complicated.


I'm working real hard on that right now.


Ah. This is a huge problem. To do a good test of a PCB mounted transformer, you're simply going to have to either pull it out of the PCB or butcher the PCB to open windings. It is unreasonably difficult to test a transfomer while it's on a PCB, and this carries a huge possibility of errors. I would never "test" a transformer on a PCB if I had any suspicion that it was damaged.


Pretty much everyone who works with transformers does. That's what's at GEO.


You're welcome.

I'm enamored of my insulation tester (megger, if you will). It's real good for checking out transformer windings for high resistance shorts.

[R.G.]

I'm enamored of my insulation tester (megger, if you will). It's real good for checking out transformer windings for high resistance shorts.

Yep, meggers are very good for that kind of thing. Anyone who regularly debugs on transformers might want to consider some kind of high-resistance insulation tester like that.

I tried to limit my advice to tests that can be done with a minimum of equipment because most people asking the question will have nothing more than a DMM to work with.

But advanced test equipment is really nice!

[Prairie Dawg]

Yep, meggers are very good for that kind of thing. Anyone who regularly debugs on transformers might want to consider some kind of high-resistance insulation tester like that.

I tried to limit my advice to tests that can be done with a minimum of equipment because most people asking the question will have nothing more than a DMM to work with.

But advanced test equipment is really nice!

I cannot tell a lie. I went to an auction and I collected my IRT-2 insulation tester for $5 and my Radiotron Designer's Handbook 4th edition for a dollar.