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I need a transformer to get 18VAC ~ 30-40W for a power supply. Living in Europe, I am looking around for a 220V - 18V transformer. As far as I understand transformers, I should also be able to use tranformer specified for 110V - 9V. Are there any technical reasons that should not work or I should not do that?

(I know varranties will be voided, but I do not care about that) (And yes, I have been building a few mains powered things before, so I know how to handle that safely)

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  • \$\begingroup\$ No, it will not work. What you can use is a split primary winding 110/230 V transformer or a simple 220 V only transformer. \$\endgroup\$ – winny Aug 3 '16 at 10:24
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Keep looking for a 220/18 V transformer. You can not plug a 110 V transformer into a 220 V supply and expect anything other than smoke.

You can, however, use a transformer with dual 110 V primaries.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1.

In your case you would use the configuration of Figure 1a.

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  • \$\begingroup\$ Well, you can expect a good deal of heat as well as smoke. Perhaps some reddish light too, and a bit of noise... \$\endgroup\$ – Brian Drummond Aug 3 '16 at 13:09
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AC power transformers carry a lot of iron inside them and this makes them relatively big. That iron will magnetically saturate and cause significant loss of performance and heat if the correct number of turns are not used. That correct number of turns is chosen for the nominal supply voltage it is attached to.

Of course a transformer manufacturer could put more iron in the core and make it usable for 220 V and, transformers will weigh a bit more and cost a bit more. The other option is to wind more turns on the primary in order to reduce the magnetization current that would cause core-saturation. Again this costs more.

So, you could use a 220 V transformer in the US and there would be no core-saturation problem but not the other way around.

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  • \$\begingroup\$ The necessary core size depends on power and frequency, but not on voltage. You need a larger core for higher voltages only if there is not enough space for the windings. A transformer for 60 Hz only is a little bit smaller than one for the same power and 50 Hz. The transformer dimensioned for 50 Hz may be used with 60 Hz too, but a transformer dimensioned for 60 Hz only will be saturated when used with 50 Hz. \$\endgroup\$ – Uwe Aug 3 '16 at 13:47
  • \$\begingroup\$ @Uwe Saturation is caused by applied voltage forcing a current through the primary inductance that is too high. A core that runs on a higher supply voltage must either have more primary turns or greater primary inductance by using more iron. Power throughput has nothing to do with core saturation. \$\endgroup\$ – Andy aka Aug 3 '16 at 14:03

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