How to identify a transformer without blowing it up?

[Dan123456]

Hey all 

I have I whole bunch of transformers that I have previously liberated from old scrap electronics (picture of some of the smaller ones below - 18650 for scale).

I found this pretty useful YouTube video that explains what to do pretty well.



I was thinking of using my 12VAC wall wart power supply to test all the ones I have 

However, as hooking up the power directly to one side of the transformer is a dead short, wouldn’t that be likely to exceed the power supplies output current rating?

My AC power supply’s output is 12V 6VA which, if I understand correctly, would be 0.5A? To limit this, I believe I just need to put a resistor in the circuit.

Using an Ohm’s law calculator, if I were to add a 47ohm resistor (I chose this just because I have a 47ohm 10W resistor on hand), this would limit the current to 0.255 right so that would be the solution?

Apologies if I am misunderstanding here and this is an absolutely stupid question! I just wanted to double check my understanding so I don’t blow it up 

P.s. I do have a recently acquired variac I can use if this route is completely stupid but I think I would have a similar issue as am worried that the output current would be too high for some of those tiny transformers to handle and think I would need to limit its output anyways 

[Roehrenonkel]

Hi Dan123456,

on your picture is not one transformer,
just cm-chokes.

Best regards

[TizianoHV]

Hi,
All the transformers in the photo are for high frequencies (probably tens of kHz). They have a rather small inductance and, at 50Hz, they'll be as a short-circuit.

(https://en.wikipedia.org/wiki/Electrical_impedance#Device_examples)

At 50Hz you can test iron core transformers (not ferrite).

The variac is an extremely useful tool but remember, when testing an unknown component, always connect something in series (an incandescent lamp, a resistor...) to protect the variac. Otherwise, you'll risk burning the wiper.

[Dan123456]

Hi Dan123456,

on your picture is not one transformer,
just cm-chokes.

Best regards

Ah thank you!

I didn’t know what the little toroidal ones were called  I had only heard “choke” being used before for the inductor type ones (I.e. only two exposed ends - like the one in the picture that has masking tape on it) 

Some of the big guys I have (but not pictured) look much more like the normal transformer I am used to 

That said, I imagine you would test these the same way? Feed AC in one side and measure the voltage from the other to work out the winding ratio?

[mariush]

Only the ones wrapped in yellow tape may be transformers, the others are just inductors (chokes) or common mode chokes used on ac input for filtering.

As for those transformers, they're designed to work at high switching frequencies, above human hearing range, like at least 37-40 kHz.... on modern hardware 300kHz and above is common.

[wasedadoc]

Hi Dan123456,

on your picture is not one transformer,
just cm-chokes.

Best regards

Not strictly true.  None of them are AC mains transformers that work on 50Hz or 60Hz.  Some, possibly the majority, of the yellow taped ones, are high frequency transformers from switch mode power supplies.

[jonpaul]

First learn the basics of transformers and magnetics.

Many excellent sites and text books.

My first was Hunt and Stein Static Electromagnetic Devices, 1971!

Jon

[u666sa]

Transformers rarely break in flyback power supplies. The only thing I've seen break was twice and years apart. Recently I've seen one of the wires melt off a leg. Something you have to look for under microscope. Easily soldered it back. Then years ago I've seen fuse being put inside windings. Also easily fixed by replacing the fuse wire.

[MathWizard]

The big yellow wrapped ones with lots of leads, are from switching power supplies, and probably would have 170DC applied, ans get stepped down to 12V or so, but as other have said, at 50kHz type frequencies. Not the same as a linear transformer.

I have a signal generator, that makes it easy to test normal, linear transformers. But a few things I've noticed over the years, probably all in the video

)The DC resistances can give clues about the the output voltages of say a tapped winding, but that's assuming the wire gauge is all the same. And once you get a feel for it could can often tell the primary side from the secondary side by knowing the resistances and if it's a step up or down transformer.

)If you have a good step down transformer, just apply it's LV to windings of the unknown trans, and just measure the outputs, to get the ratio's

For load testing windings, (I already knew or figured out the AC mains input winding), I get the open circuit voltages, then measure the voltages with smaller and smaller resistors. Ending up with big high power resistors, and high currents. I don't test very long with them, in case I pull too much current and melt the wires.

Some people do short circuit test of a transformer, that's ok sometimes, but for an old unknown transformer, I'd be too worried about cooking some insulation or even melting the wires, if there's no external current limiting to keep that from happening.

From that it's easy to plot the load-line response of the transformer.

[tridac]

You can use a variac with ammeter in series. Magnetising current should be very low at rated voltage and no secondary load, but will rise when the core starts to saturate. Also, measure the inductance of a winding then work out the reactance at the frequency of interest. Finally, measure winding resistance to get some idea which is the highest voltage winding. None of that is foolproof, but it can help...

[jonpaul]

Great question! Bravo for curiosity about magnetics.

Wound my first transformer in 1950s....worked up to 12 KW and 100 kV over a long carreer.

 here are  a few transformers!

You have a hard time to "blow up" the first two...

Bon chance!

Enjoy,


Jon