How to test a small audio transformer to find out the specs?

[wblackledg]

I have a small transformer that came out of a piece of cheap Chinese equipment.

I looked on ebay and what comes up as "audio transformers" for about $2 each seem to look about as close to what I have as I can find.

I would like to what the specs are on it, so how would  I go about finding out out?  putting voltage on one of the pins and testing the output on the others?

no real ID numbers on it.  Just the number 4 and a Chinese character.

thanks.

[ivaylo]

Do you have a link? What's an "audio transformer" nowadays? Vacuum tube audio amplifiers have/had transformers but not sure what other audio equment does. If you don't find anything about it applying some (not power grid!) AC will show you the primary/seconday ratio. Then wire thickness and resistances the rest of the parameters. It's clunky though, better ask the seller...

[UnsatisfactoryResult]

An audio transformer refers to a small transformer that is used in the signal path of a piece of audio equipment for various purposes. You can increase or decrease signal strength with step up/down transformers. These transformers have differing impedances between windings. If the windings are of the same impedance, then they are referred to as isolation transformers. These are used to drop any DC voltage present on a signal line, such as with a ground loop between multiple pieces of equipment. The latter are what seem to be most marketed to consumers. Most of the cheap ones will be taken from the telecommunications industry, and as I understand it they have poor low frequency response.

For more info than you ever wanted about audio transformers:

http://jensen-transformers.com/wp-content/uploads/2014/09/Audio-Transformers-Chapter.pdf

[German_EE]

Feed the signal from an audio signal generator into one side of the transformer and measure the output on the other winding using an oscilloscope or (if you have one) an audio millivoltmeter. I just tried this on a transformer pulled from a junk radio and got a response of 250 Hz to 3500 Hz.

The AC volts range on a multimeter might work but this isn't guaranteed.

[wblackledg]

Do you have a link? What's an "audio transformer" nowadays? Vacuum tube audio amplifiers have/had transformers but not sure what other audio equment does. If you don't find anything about it applying some (not power grid!) AC will show you the primary/seconday ratio. Then wire thickness and resistances the rest of the parameters. It's clunky though, better ask the seller...

here is a link to something similar, but it maybe the same because  I don't know the specs on the one I have.

http://www.ebay.com/itm/3pcs-Audio-input-output-transformers-kit-/131320023257?pt=LH_DefaultDomain_0&hash=item1e934888d9

[wblackledg]

Feed the signal from an audio signal generator into one side of the transformer and measure the output on the other winding using an oscilloscope or (if you have one) an audio millivoltmeter. I just tried this on a transformer pulled from a junk radio and got a response of 250 Hz to 3500 Hz.

The AC volts range on a multimeter might work but this isn't guaranteed.

thanks, one thing I should mention, is this one is not used in an audio application, its used to ramp up voltage.

[TimFox]

The frequency response of a transformer depends on the termination impedance.
One way to characterize a transformer is to measure the magnetizing inductance (inductance at the primary with secondary winding open-circuit) and the leakage inductance (inductance at the primary with secondary short-circuited) if you have an impedance bridge or LCR meter.  See an electronics textbook to relate those two measurements to the high-frequency and low-frequency roll-off frequencies (ignoring capacitance).

[wblackledg]

The frequency response of a transformer depends on the termination impedance.
One way to characterize a transformer is to measure the magnetizing inductance (inductance at the primary with secondary winding open-circuit) and the leakage inductance (inductance at the primary with secondary short-circuited) if you have an impedance bridge or LCR meter.  See an electronics textbook to relate those two measurements to the high-frequency and low-frequency roll-off frequencies (ignoring capacitance).

great thanks.  yes, I have a cheap LCR, which should do.

should I take it out of the circuit first or not?

[TimFox]

To get a proper measurement, the transformer should be removed from the circuit.  Otherwise, you won't get good "open circuit" measurements.

[wblackledg]

To get a proper measurement, the transformer should be removed from the circuit.  Otherwise, you won't get good "open circuit" measurements.

ok, very good.

thanks for all the help to everyone. If I get stuck, I'll post the problem.

[Richard Crowley]

It may provide useful clues to know what kind of circuit it is operating in now.
What kind of gadget is it?  Do you have the circuit diagram?
Can you trace out at least the immediate circuit around the input/primary side of the transformer and the output/secondary side?

Note that after you determine the primary and secondary impedance of the transformer, then you need something to measure the frequency response with. There are several software products available (some commercial and some free) which use the sound card in your computer to perform a frequency-response test and generate a plot.

No, you cannot effectively perform any of these tests "in-circuit".  You must remove the transformer to test it.

Also note carefully that it makes a huge difference whether this is a traditional steel-laminated core transformer (suitable for mains and audio frequencies) or whether it is a ferrite-core transformer (suitable for super-sonic and RF frequencies).  If it is a ferrite-core transformer, your LCR meter may yield incorrect results.

[wblackledg]

It may provide useful clues to know what kind of circuit it is operating in now.
What kind of gadget is it?  Do you have the circuit diagram?
Can you trace out at least the immediate circuit around the input/primary side of the transformer and the output/secondary side?

Note that after you determine the primary and secondary impedance of the transformer, then you need something to measure the frequency response with. There are several software products available (some commercial and some free) which use the sound card in your computer to perform a frequency-response test and generate a plot.

No, you cannot effectively perform any of these tests "in-circuit".  You must remove the transformer to test it.

Also note carefully that it makes a huge difference whether this is a traditional steel-laminated core transformer (suitable for mains and audio frequencies) or whether it is a ferrite-core transformer (suitable for super-sonic and RF frequencies).  If it is a ferrite-core transformer, your LCR meter may yield incorrect results.

a cheap bug zapper
http://www.ebay.com/itm/400724840398?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[Richard Crowley]

So you don't really have an "audio transformer" at all.
You have an ultra-sonic, extreme-ratio, step-up, switch-mode power-supply transformer.
You gadget takes the 3V from the batteries and steps it up to 1500V to electrocute the bugs.
The circuit operates at a single frequency and so the transformer was not designed to have anything but the most minimal "frequency response"
Furthermore, it is quite possibly a ferrite-core transformer since those are typically more efficient for ultrasonic-frequency circuits like that.

Your transformer is probably a lot like this one...
http://www.goldmine-elec-products.com/prodinfo.asp?number=G13599

[mtdoc]

What do you want to use the salvaged transformer for?  Trying to make a stun gun perhaps?