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I have two 24VAC transformers. If I hook the voltmeter to contacts 1 & 2, or 3 & 4, it measures about 26 volts. If I connect it to 1&3 or 1&4, or 2&3 or 2&4, I get zero. I would have thought that it wouldn't matter, just as if I would have plugged one half of a power cord into one outlet, and the other half in the other half of another outlet.

Background of how this question arose if interested - I have a thermostat that had one of it's wires shorted, so I needed to add a 2nd transformer near the thermostat. The first transformer (the transformer originally providing the power before the wire shorted) is near the boiler. The problem I had was that I was powering the thermostat entirely with 2 wires from the 2nd transformer, but the boiler valve was powered by 1 wire from the thermostat, and 1 wire from the first transformer. Which didn't work.

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    A volt meter shows potential voltage between two points. #1 and #2 are in a circuit/loop/connected together, same as #3 and #4, but #1 and #3 are not in the same circuit so no voltage potential between them.
    – crip659
    Mar 2 at 21:25
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    take two batteries ... do not connect them together ... measure voltage between the two + terminals ... you are doing the same thing with the transformers
    – jsotola
    Mar 3 at 4:54

3 Answers 3

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To measure voltage, you need a complete circuit.

Connect a voltage meter between a hot 120V wire anywhere in your house and a neutral wire anywhere in your house and you will get 120V because the current makes the round trip. A multimeter uses very little current. But if you pull too much current - e.g., connect a light bulb that way - and the wires are on different circuits and one of them is protected by a GFCI then you will trip the GFCI.

However, a transformer is isolated from the hot and neutral of your main AC wiring. If it wasn't isolated, strange/dangerous things would happen. Since each transformer is isolated, there is no path for electricity to flow between them. Hence, 0V.

There are actually two ways to get 0V on a multimeter.

  • Two points connected (e.g., neutral and ground in a properly wired house) - there is no potential difference between the points so there is no voltage. In that situation, if you switch from Volts to Ohms you will measure very low resistance.

  • Two points totally disconnected. There may actually be a potential difference between the two points, but since electricity can't flow between them, there is no measurable voltage. That is what is happening with your two separate transformers. In that case, if you switch from Volts to Ohms you will measure "infinite" resistance.

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  • That is not how a transformer works
    – Ruskes
    Mar 3 at 1:59
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    @Ruskes I am not trying to explain how a transformer actually works. I am explaining why you don't get voltage between the secondaries of two separate transformers.
    – manassehkatz-Moving 2 Codidact
    Mar 3 at 2:09
  • I must say I am a bit taken aback by the wording of your second paragraph. I am not a native speaker, but I would interpret that as shorting hot with neutral, at least looking at the first couple of sentences :D
    – Vladimir Cravero
    Mar 3 at 12:40
  • I meant using a multimeter one probe in hot and the other one in neutral
    – manassehkatz-Moving 2 Codidact
    Mar 3 at 13:30
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    I think the correct wording here is galvanically isolated.
    – Vladimir Cravero
    Mar 3 at 18:05
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Transformers are magnetic devices. There is no electrical connection between the secondaries (the low voltage side), so what you are seeing makes sense.

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In a nutshell, the secondary voltage is a result of the field created in the windings around the core of the transformer. If you leave the core, you lose the voltage.

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