Transformer help

[DuWong]

Need help with this transformer I just picked up...
Transformers in text book are easy but here's the real thing and I don't understand why there is 4 connection on each coil.
Unless there is 2 coils each? But why would they be wired up that way?
Also how would I tell which is primary or secondary? How would I find the turns ratio?

[rstofer]

It is very common to have two primary windings and two secondary windings.  This allow the primary voltage to be 120 or 240, for example.  The secondary could be 12V at X amps or 24V at X/2 amps or the secondaries could be entirely different like the old vacuum tube transformers with 12V for filament voltage and some much higher voltage for the plate circuit.

Nothing unusual

http://solderman.dapj.com/2005/09/transformers-series-and-parallel.html

[DuWong]

So in my case I have 120 VAC and would want the primary to be wired in parallel? What would happen if it was in series?

[WackyGerman]

If you wire it in series the secondary voltage would be the half of the voltage you get if you wire them in serial

[Benta]

From your pictures, it already has paralleled primary and secondary.

[DuWong]

Okay I think I understand now, so if the primary is in series then the turns is double? And same for the other side?
So if I want to step down a lot I should have primary in series and secondary in parallel? Would this yield the lowest voltage step down?

[Benta]

Okay I think I understand now, so if the primary is in series then the turns is double? And same for the other side?
So if I want to step down a lot I should have primary in series and secondary in parallel? Would this yield the lowest voltage step down?

No. The primary should be wired for your mains voltage, which you mentioned is 120 VAC. Leave the wiring as it is. Turns/iron/copper is optimized for this.

You can play around with the secondary, getting either one voltage with the windings in parallel, or double that voltage with them in series. Or even two floating/independent supplies, which can be useful.

[Seekonk]

Best to test a transformer on line power with a 60W lamp in series.  If power is applied in this method to a 24V winding or two parallel windings out of phase, the lamp will glow as the transformer saturates and just turns into a resistor or out of phase windings short each other.

[Zero999]

Best to test a transformer on line power with a 60W lamp in series.  If power is applied in this method to a 24V winding or two parallel windings out of phase, the lamp will glow as the transformer saturates and just turns into a resistor or out of phase windings short each other.

That's a good idea but beware of potentially high voltages (possibly much higher than the mains voltage) on other windings.

[orolo]

To test a transformer the guetto way, with little peril and power, get a synchronous motor (like the one rotating the plate in a microwave) and use it as a low voltage, low frequency AC generator. Put one voltmeter at each winding under test, and gently rotate the shaft with the help of pliers. Not very precise, but you get an idea of which is primary, secondary and the turns ratio.

[tautech]

With only #'ed terminals the only way to be sure is to have a spec sheet, the maker and model # is clearly visible on the laminations, Google them.

Should you still draw a blank there may some little guidance on this page:
http://www.radioremembered.org/xfmr.htm

[strawberry]

May disapoint, but it seems insolation transformer with turns ratio 1:1. wire colors are same on both winding terminals and winding wire diameters are identical gauge.

[basinstreetdesign]

Okay I think I understand now, so if the primary is in series then the turns is double? And same for the other side?
So if I want to step down a lot I should have primary in series and secondary in parallel? Would this yield the lowest voltage step down?

The answers to your questions are yes and yes.  It would provide the greatest step-down ratio.  This arrangement would have the greatest turns ratio from primary to secondary.

[DuWong]

Thanks for the help every thus far.
So how much current could flow through the primary with no load on the secondary?

[tautech]

Thanks for the help every thus far.
So how much current could flow through the primary with no load on the secondary?

Who know what the magnetizing current will be, if you select the wrong connections you might destroy the transformer and at best trip your circuit breaker.

Do what Seekonk suggested in reply #7.
It's called a Dim Bulb tester, Google it. 

[DuWong]

Thanks will do.

[Cerebus]

If you use the 'lamp in series' test it's worth underlining that it must be a good old fashioned incandescent lamp, not a modern compact fluorescent or LED lamp. That's understood by the old hands, but a beginner could quite easily think "a lamp's a lamp".