Transformer help

[Zane]

Ok i have a couple of transformers that I wanna test but, i don't know how to figure out the in out voltages and all that.

Can someone help me?

[DynamoBen]

I'm not sure exactly sure what you mean.

High voltage or low voltage transformers?
What are you using them for?
Any model number?

[Jeff Millard]

Zane,
This can be a touchy situation. Not knowing anything about the transformers or what their use is, I would be very careful testing them. A transformer is nothing more than a coil of wire wrapped around a metal core with another coil for the output. The size of the wire coil and the ratio of turns from the first to the second coil determines what the transformer does. IE, if the primary coil has ten turns and the seconday has 100 and you put 125vac to the primary you will have 1250 volts at the secondary. The term for that is a step-up transformer. It is very hard to say test the coils for a certain resistance, as the guage of wire used is also a factor.

Referring first to the disclaimer for this section of the forum, I want to emphasize that this post is not meant to direct you how to test your transformer. Rather, it's more of a request for safety. You are responsible for your own actions and this forum and I claim no responsibility for your actions.

I will tell you how I would attempt to test an unknown transformer. I would look online (Google etc...) for any information on any available part number. I would get a 10 to 1 transformer (it will have an output of around 12 vac) so I was not applying a full 120 vac to it. It must not be a DC supply. The transformer stripped out of a wall-wart power supply will do fine. I would fuse the output.

Some things that could complicate any testing are taps off of the coils. Taps can provide for multiple voltages at the input (Primary) or the output (secondary) You will find more than 2 wires on a side if you have taps.

A common use for a center tap on the primary of a 20 to 1 transformer is to facillitate 240 and 120 vac. You would connect 240 vac to the full winding to get 12vac at the secondary. You would connect 120 vac to the center tap, and one of the full winding legs to get the same voltage at the secondary. (I hope that makes sense)

Knowing these few things is still not enough to test an unknown transformer. There are way too many uses for them. A good example of one you wouldn't want to apply voltage to is a Current Aux transformer. The windings have very little impedence. To voltage, they act like a short. A very good short! (or bad depending on your proximity to the failure that might occur)

I wouldn't attempt to power any transformer I didn't have at least some knowledge about. And if I did try, I would certainly use another step-down transformer with a fused output for safety. What ever you do, be very careful. You can potentially (pun intented) create thousands of volts if you hook it up wrong.

jeff

[wjohn]

Very good advice Jeff,

a transformer is only worth a couple of dollars, a life is worth??

if you are not sure, bin them.

[Zane]

Thats what i am going to do. I couldn't find anything on them and they are most likely almost 10 years old and unreliable. I can get the one that i need from RadioShack for $20, no soldering needed. Thanks guys.

[ErnieHorning]

Here is something to keep in mind if anyone does attempt this in the future.

A 12 volt AC transformer is not going to be 12 volts unless you use it at its full current rating. A typical 12 volt transformer will output around 16 to 18 volts unloaded. You should also have a fuse inline with the output of the supplying transformer no higher than the rating on the transformer.

You may be able to guess the voltage and add a load, while watching the voltage drop, and guess the current rating but its still a crap shoot.

I’ve tested transformers at work with a Variac that I’ve changed the fuse to a half amp. This way I only need a couple of volts on the input to figure out if it’s anywhere near the ball park that I need. I can then try adding a load to the output while monitoring the temperature. I also only use unknown transformers for non critical and low current applications.