# Wiring a single-phase auto transformer

I have a 220 V to 110 V transformer but it's not the type which comes with the plug and outlet built-in so I need to make the connections myself.

The only problem I have is that I'm not sure which terminal is live and which is neutral. I don't see any signs indicating which is which. So does it matter?

• 3 good answers. ............................... Sep 15, 2022 at 11:22
• Also, if you open the case, you may find that you can somehow tell that one of the 110 V wires is directly connected to one of the 220 V wires. If so, those two wires that are connected together, they are the two neutrals. Sep 19, 2022 at 6:07

You should be able to determine which terminals are neutral (or rather common between input and output) by measuring resistance between each of the 220V terminals and each of the 110V terminals. All resistances will probably be very low, but the lowest resistance combination should be the common terminals,and you would use those as Neutral.

• What if I were to mess that up and connect live and neutral from the mains onto the wrong terminals of the 220v side would that cause a problem?
– blah
Sep 12, 2022 at 17:51
• It should be safe, as you should not be able to contact any current-carrying conductors or anything connected to them. Sep 12, 2022 at 20:08
• @blah It should be essentially impossible to get that wrong. Given 220A 220B 110A 110B then you have 4 cominations of type 220x-110y. One of those will be a hard connectio internally. || IF it was a transformer rather than an autotransformer then ALL 220x-110y combintions will be open circuit. || See vu2nan's circuit. Sep 14, 2022 at 11:49

You would need to check the resistance between the terminals on the 220 V terminal block and the 110 V one.

The terminals between which the resistance is the lowest would be the neutral terminals.

If it's a 220 volt in and 110 volt out auto transformer then there are only 3 effective connections and there should be symmetry like this: -

simulate this circuit – Schematic created using CircuitLab

So, all you have to do is find the middle centre-tap connection. You can do this by applying a 50/60 Hz AC voltage to the 220 volt side (up to 220 volts) and looking for half that applied voltage on the 110 volt winding using a voltmeter.

• I'm not sure what are the three possible connections. There are two terminals on the 220v side and two terminals on the 110v side. You said to apply a 50/60 Hz AC voltage to the 220 volt side, but which terminal connects to live and which one connects to neutral?
– blah
Sep 12, 2022 at 17:54
• The autotransformer windings are probably isolated from ground so, it won't really matter. The ground connection just connects the chassis. Sep 12, 2022 at 18:08
• What you have shown in the diagram is correct but somewhat confusing in the context. However the text is wrong as he has two wires labelled 220V and 2 wires labelled 110V - while they match what you have shown internally your text does not answer the question. Sep 14, 2022 at 11:46
• @RussellMcMahon yes I wasn't that clear so I've amended the diagram. Sep 14, 2022 at 12:05

I use a no contact ac voltage tester. The tester flashes and beeps only when the tip is close to the hot lead. This is true on both the 220 and 120 connections. This is also the same, with the more common isolated primary and secondary transformer.

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Sep 19, 2022 at 4:58