How to wire transformer?

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Ok so I'm doing this baby steps.

I got a dual primary & dual secondary transformer. Input is 120v, output 22v.

The input wires are red & black. There are 2 sets. I have isolated each set.

I have my AC coming in from the wall outlet, to a fuse holder. From the fuse holder to the transformer primary.

When I wire up 1 primary coil, I get 22v on either of the secondaries. If I try to parallel the 2nd primary coil (which I've gathered is what I'm supposed to do), as soon as I plug it in or throw the power switch I get a blown fuse.

I'm wiring the two reds together and the two blacks together. I'm on 120v mains.

Do I need to reverse one of the primary coil's polarity? In other words, connect the red of coil A with the black of coil B, etc.?

Or is it normal to blow a 1A fuse with a big (500va) transformer? I have nothing connected to the secondaries yet.

I'm beginning to think there's something wrong with my transformer. I've tried wiring the primaries up every way possible. It always blows a fuse.
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Hi Uzernaam,
I'm going through the same issues as you, AndrewT advised I wire a 40W lamp in series with the primary to act as a current limit.

It will be useful to connect an ammeter or one of those kill-a-watt meters to see if your trafo is drawing lots of current. If you have your primaries out-of-phase then you won't get much out the secondary side also you will burn out the primary. The lamp will help prevent that. If you have the primaries connected correctly, the bulb should get bright quickly and then dim or go out. If it stays bright, turn it off and change the phase.

You can dispense with the fuse whilst you're using a bulb in series. Of course replace the bulb for normal use.

For a 500VA trafo, P=IV, 500/120 =4.167amps, the switch on surge may be more than that even with no load. With rectifier and caps, it will definitely be higher.

Since you are from USA, a disclaimer is in order ;-)
My writings do not constitute advice and I am not a qualified engineer, use this information at your risk.
With a 500VA transformer you need to use some form of inrush limiting to avoid having to use a line fuse that is too big for safety. The simplest way is to use a negative temperature coefficient thermistor, like a CL60 in series with the primary winding(s).

As Firestorm points out, it is a good idea to USE A LIGHT BULB CURRENT LIMITER EVERY TIME YOU FIRST CONNECT A CIRCUIT TO THE MAINS. We all make mistakes from time to time, it is a good way to prevent damage to other parts and gives a quick visual indication of a fault.

Who is the manufacturer of your transformer? They probably have the wiring convention on their website. For Avel-Lindberg black and red are NOT primary winding colors...


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At US mains voltage, we wire the primaries in parallel. Of course we need to be sure that we have identified the primaries. Transformers and Power Converters From Avel Lindberg, Inc.

If you have another low voltage transformer that you know how to connect, use it to drive your test as Zen Mod implied above. Verify that the voltage ratios are as expected. If you drive one red/black winding, are the other windings at a higher or lower voltage?
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