How to correctly wire this transformer for 240V?

I have an old transformer (without any details about brand and model) with dual primary windings. I need to set it up to take 240V, but I am unsure about the correct way to connect the windings in series.

Here is a photo of the primary side:

So what is the correct way to connect these for 240V input?

Do I connect the two 0 connection points and supply 240V at the two 120 points or cross over a 0 to a 120 pin?

Note: the windings is on each side, i.e. there is no connection between 0 and 0, and 120 and 120. On each side I have about 8 Ohm between the top 0 and the bottom 120 pin.

• Are you sure the transformer is designed to take 240V? It may as well be that the insulation isn't rated for it Commented Feb 2, 2016 at 19:56
• Yes, I'm sure - it is in an amp with a 120/240 switch, but the wiring were too messed up to make any sense from ;-) Commented Feb 2, 2016 at 19:58
• Electrically speaking putting two identical windings in series will double the voltage of those windings. (Unless you put them in series with opposite phase, then the voltage will cancel to zero.) Based on what you have said so far, I would guess that you want to jumper diagonally from 0 on one side to 120 on the other side. Since you have so little information about the transformer, I have to caution you that there is some chance that things could go wrong, leading up to a fire or shock or other catastrophe. It is up to you to be safe, and I make no representation in that regard. Commented Feb 2, 2016 at 19:58

The primary windings go in series, but get it wrong and the transformer turns into a (bifilar wound) resistor. It is important to not connect them in "anti-series". It cannot be seen from the outside how the windings are internally connected. You 'll have to wire it, then carefully test if the transformation ratio primary-secondary is about right.

I think it'd be connected as below, but you'll have to carefully verify the result before using it at full power.

As @jms suggests, add a (regular old fashioned incandescent) 240V light bulb in series when testing. If it lights up while the transformer is unloaded, then you've wired it incorrectly.

~ ---o   0  o
/
/
/
/
/
/
o  120 o--- ~


I understand from your comments that the transformer is rated for 240V, but do be careful, a new transformer with decent spec sheet is cheaper than rebuilding a burned down lab.

• Well, that is why I'm asking really. Tried that before, smells bad ;-) Is there no way telling from the photo? Commented Feb 2, 2016 at 20:00
• If you already tried it (and you know it didn't work) just invert one of the coils and test again, it should work. For safer testing you can add a 230V light bulb in series with the transformer, which limits the current.
– jms
Commented Feb 2, 2016 at 20:05
• Also, in future, if you already tried something, consider adding it to the question (unless the question is already ridiculously long and complicated). Commented Feb 2, 2016 at 20:15
• @jms' incandescent lightbulb is a very good tip. Commented Feb 2, 2016 at 20:20
• @mkeith Sorry - what I meant was in an earlier project - not this one. Havent tried anything yet here. I'm trying to make some sense of the wiring (or what was left) to see if that brings any clues. Commented Feb 2, 2016 at 20:26

This type of transformer has two 120 V primary windings. You connect them in parallel to drive it from 120 V, and in series to drive it from 240 V.

To connect the windings in series, you first need to figure out which pins the two windings are connected to. This can be done easily with a ohmmeter or a continuity tester. Put one lead on one pin, then go around to the others. Two of them will show open connection. One of the other pins will show a few Ohms or tens of Ohms. That's the other end of the winding the first probe is on. The second winding is connected between the remaining two pins, although it is good to check this to make sure you didn't make a mistake.

Now that you know which pins go to each of the windings, the only question is which way they are connected. You can't tell polarity with a ohmmeter or continuity tester, because the windings themselves have no inherent polarity. The polarity is relative between the two windings.

Arbitrarily pick one pin of one of the windings and call it 1+, then the other pin of that winding 1-. Ultimately you are going to connect 1- to 2+, then apply 240 VAC between 1+ and 2-. You only have two choices for the remaining pins. They are either 2+ 2-, or 2- 2+.

Unfortunately, you can't just try it and see which way it works. If you happen to get it right, then the expected voltage will come out of the secondary, and all will be fine. However, if you get it wrong, the two primary windings will cancel each other's magnetic fields and together act like a shorted transformer. This may cause a puff of smoke to be emitted, or at best, a breaker to pop.

The way to test it is to apply a lower voltage or limit the current so that nothing is damaged. When the two primaries are connected backwards, then the secondary will have very little voltage. The secondary voltage will be much larger when the two primaries are connected with the right polarity.

There are several ways you might have available to you to achieve this. If you have some other low voltage transformer, like something designed to run a doorbell, a old model train, and the like, use it to drive the primary directly. Using a low voltage is preferable, but if you don't have anything like that available, connect the tranformer to line power but with something else in series. This something else should be able to run with full line power so that it won't get damaged in case your transformer is wired to look like a short. It also should be something "dumb" and preferably resistive. A desklamp with a old fashioned LEB (light emitting bulb) would be good. A dumb toaster would likely be OK, or some dumb heater. The more smarts and the associated electronics it has, the less likely it is to work well when the transformer drops some voltage.

• Well, if you have 120 available, you can wire the two windings in series (guessing about the polarity), and then excite only one winding with 120. This is safe, even if you guess wrong. If the total series voltage is around 240, then this is the correct configuration. If the total series voltage is close to zero, then it is not the correct configuration, so swap and confirm. Commented Feb 2, 2016 at 23:46

If you connect wrongly the windings in series, then the magnetic field will cancell each other == no inductance == short circuit. You can wire a light bulb in series with transformer primary and play with correct sequnce. The light bulb will limit the current, if the fields cancell each other then the bulb will lit on full bright, if the windings form an inductance then the bulb shall lit less bright.

• would a soldering iron do? I only have low energy bulbs nearby :-D Commented Feb 2, 2016 at 20:27
• @mhbuur With a light bulb you have a visual interactivity, I don't know how you will check with iron, you will hold it in hand and count seconds until burning hand or so? Commented Feb 2, 2016 at 21:04