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Hi - forgive me if this is very basic, but I have a transformer for 240 - 240 with a center tap which I intend to use to provide two 120VAC lives, summing to 240VAC, with the center-tap providing an earth reference. Diagram attached. On running this transformer, I get some strange readings and want to check that I have a) identified the leads correctly and b) that my understanding of what it does is right.

There are four leads for the secondary (two red, two black), and I assume that two of these need to be wired together to make the center-tap. Is there an easy way to identify what two wires these should be? Using a VM, I see that the two black wires have a very tiny voltage developed across them, while the reds have 240V and the paired black/reds have 120V, so presume that the blacks are the center-tap. Is this likely to be right?

Now, If I've got the above right, joining the two black wires (E1 in the diagram) should effectively be at zero voltage with reference to a real ground - but I am showing circa 80V, which indicates that there is some fault. Any clues as to what I might be doing or understanding wrong here?



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Is there an easy way to identify what two wires these should be?

David, with no electric connected to the transformer, use an ohmmeter to determine which wires are going to the two individual secondary windings. You have two windings, which you hope are the same. You should have two combinations that give you the same or very close to the same ohm reading, where other combinations are infinite or some very high number, depending upon ohmmeter and how it is configured. It is possible to have a transformer that can give you two different ohm readings, which relates to two different voltages from the two secondary windings. If this is the case, this transformer will not work for your purpose.

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Think of the windings as resistors. With power turned off, you measured two sets with the same ohm readings, right? These most likely each have a red and a black wire. Mark them as secondary red A and secondary Black A for the one set and secondary red B and secondary black B for the other set. These are your two 120V sections. Hope that makes it a little clearer.


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I've worked out which are the A and B pair, but not which two wires to join to have these wired in series.. is there a simple way to work this out?

If all else fails, I presume I can put a scope on both pairs and they will be out of phase with each other when they're wired series and in phase when they're wired parallel. I'd try this now but my scope has died and I've just bought another on ebay!

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Hi David,
To avoid too much smoke if the primary winding is the wrong one, connect a 240V/40W incandescent lightbulb in series with one of the primary wires. If the connections are wrong then the light will be bright and limit the current.

Connect the two black secondary wires together and measure the voltage between the red wires. It will probably be very low.

Connect one black wire from one secondary to the red wire from the other secondary together and measure the voltage between the free black and res wires. It will pfobably be 240VAC.
Measure the voltage across each secondary winding to make sure they are both the same at 120VAC. The junction of the black wire and the red wire is the center-tap and can be grounded if you want.
The light bulb should not have been bright if the transformer didn't have a load.

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