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I have two transformers (220v to 110v) which has a nominal power of 300VA. I want more than that for a specific application. Can I use both together to supply more current like this (?):

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Any chance of going wrong or anything that I should take care?

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  • \$\begingroup\$ If you are looking for more current why are you connecting the windings in series rather than in parallel? \$\endgroup\$
    – alexan_e
    Feb 20, 2014 at 21:03
  • \$\begingroup\$ Hard wire the primaries in parallel so the output polarities are fixed. Then connect only one wire from each secondary together and with power on, measure the voltage between the remaining secondary wires. If zero volts, those wires when connected will be in phase and parallel. If at double your output voltage, reverse one pair of wires. Have spare fuses on hand, those transformers only have one. Be careful, death can come at .1 amp. \$\endgroup\$ Feb 20, 2014 at 22:42

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That won't hurt anything (until you put a load on there) but it also won't work so well. As you can see, the same current is going through both coils. The lower voltage on the primaries and secondaries means that the core is going to be less utilized, so they might be a bit more efficient, but basically you'll be stuck with roughly the same current rating and poorer regulation.

If (and this is a huge if) both transformers were identical you might be able to parallel the two secondaries and primaries (taking great care as to phase, or you'll release the magic smoke). The slightest difference between the two transformers (even a slight one or two turn error in manufacturing the primary or secondary) and a lot of current could flow between the secondaries (resulting, potentially, in a lot of heating).

It would be better if you could split up your load between the transformers, or just use a bigger transformer.

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Who is to say the two transformers will be exactly identical? The primary inductanc eon one will not necessarily be the same as the other. For instance, with no-load on the secondary, the primary takes a small current because basically it acts like a big inductor across the AC voltage. It might be 10 henries and at 50Hz, the impedance will be: -

\$2\pi f L\$ = 3142 ohms - this will take an RMS current from a 220V AC supply of 70mA.

Bigger transformers will generally have bigger magnetizing inductance but just because two transformers look the same it doesn't mean they'll have the same inductance - one transformer could have 10H inductance and the other might be 5H.

The result is that two-thirds of the primary voltage will be across the primary with the higher inductance. The cetre point of the two series connected secondaries will also be offset by the same amount. Remember this is on no or light loads.

This is important to remember if you think that the centre point of the two secondaries could be used as a "mid-point". You might be lucky but more likely it'll be out by at least 10% or more. Why do I say 10%? A mains transformer doesn't really take care to control its primary magnetizing inductance and as there are no air-gaps to "manage" the flux in the core, the resultant inductance can vary substantially.

One of the mechanisms that can control flux density is the actual core saturating a little bit - both cores will be roughly operating at half flux level and although it is a benefit not to have saturation, this in turn can exacerbate the primary inductance differences even more.

OK I've exhausted the problem of the primaries not being well-matched and this basically means you can't use the centre-point of the two secondaries for anything meaningful because it will be unpredictable as to what percentage voltage it has on relative to the full voltage developed across both secondaries.

As load current increases this becomes less of a problem because the dominant primary current starts to become the load current reflected onto the primary and this will be substantially greater than the magnetizing current.

As for other reasons, Spehro's answer covers this.

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