I need help with transformer

I have transformer see picture need to connect to power supply 220V AC
transformer is from UPS
Is 1-3 input
4-5 output

resistance between
1-2 0,4 Ohm
1-3 2,1 Ohm
2-3 2,3 Ohm

4-5 0,2 Ohm

which wire need to connect to power supply 220V AC

Thanks for any help

jurilica said:

Is 1-3 input
4-5 output

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That is the most likely wiring, but with no markings the voltages are guesswork...
Be careful!

Where did you get the transformer? Do you know for sure that it really has a 220 Volt primary?

I would expect the 1-3 resistance to be the 1 - 2 resistance plus the 2 - 3 resistance, but I know that low resistances like that can be difficult to measure accurately.

I suggest that you put a standard incandescent light-bulb in series when you first turn on. If the 1 - 3 winding isn't rated for mains, the current will be limited by the bulb and nothing will be damaged.

You could also run from a shaver transformer. That lets you try at 110 V, and a shaver transformer probably won't be damaged if the current is too for a minute or so.

How big is the transformer? The size is closely related to the power rating of a transformer. The primary resistance of a mains transformer changes a lot with the size of the transformer, so it's not possible to say if 2.3 Ohms is about right unless the size and rating are known.

It's a bit confusing because the thinner wires look like a center tapped winding, and the heavier wires are just a signle winding. You are more likely to find a CT secondary and not a CT primary. So from that I would expect the heavier wires to be the primary. But in a transformer like this, the secondary is usually lower voltage/higher current so would use heavier wire. Conflicting....

You will need to do some experimenting with lower voltages to determine the turns ratio. And when you do think you have identified the primary and the expected secondary voltages, using the series light bulb is a good idea.

Ylli said:

It's a bit confusing because the thinner wires look like a center tapped winding, and the heavier wires are just a signle winding. You are more likely to find a CT secondary and not a CT primary. So from that I would expect the heavier wires to be the primary. But in a transformer like this, the secondary is usually lower voltage/higher current so would use heavier wire. Conflicting....

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Sometimes, it's also very common to find a centre tapped primary, for 110/220V selection, or even a 'slightly' tapped primary for 220/240V selection - although the EU 230V product standard has pretty well killed those off now.

Assuming it's a mains transformer at all? (and it looks to be) then the thin wires are most likely the primary.

I also concur that the mains bulb idea is a good one - but what are we going to do in the future?, now that incandescent bulbs are ancient history?.

Nigel Goodwin said:

I also concur that the mains bulb idea is a good one - but what are we going to do in the future?, now that incandescent bulbs are ancient history?.

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Resistor and / or PTC self-resetting fuse. Or, if price is no object, use a variac and start at 0 V with an ammeter in series.

How about 20 off car headlight bulbs in series? What about a separate 230 - 12 V transformer in series, with an incandescent bulb load on that?

Loosing a bit of test gear for a few enthusiasts is probably a small price to pay for not having incandescent lamps everywhere.

I've got a pretty good stock of 100W incandescent lamps that I bought when they started to discontinue them. At the time, the LED replacements were dim and off color. They have improved a lot since, and I've just about totally changed over to LED, but i still have that stock of 100Wers.

What I do with unknown inputs/outputs is use a standard 24V AC transformer as a feed, then measure the voltages on the other side. Then reverse... That way, you know by a factor of 10, what the voltages will be with 240V (or 220V pro-rated). You can use just about any transformer, 12V, 18V, etc. and thus relatively safely measure the voltages at the other end.
If the transformer is from a UPS, odds are high that the switched battery input would be the heavier wires, and the smaller wires would be the output.