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I have 2 identical 110V primary transformers (plug packs but with a small transformer inside) for charging batteries in a pair of US made walkie talkies. Only issue is where I live now is 220V mains.

If I connect the 2 110V primaries in series, and plug into 220V will there be a drop of 110V across each transformer?

Will small varying loads on the output, alter the primary voltage drop across each transformer?

Any and all suggestions appreciated.

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  • \$\begingroup\$ Buy a 230V battery charger that will charge those batteries, without any risk ... any method of joining two chargers together will be Heath Robinson... \$\endgroup\$ – Solar Mike Dec 27 '18 at 11:04
  • \$\begingroup\$ If you can't get 230V chargers for your radios, then look into getting a 230V to 110V transformer. Like this. Safer by far than playing funny games with 230V. \$\endgroup\$ – JRE Dec 31 '18 at 11:02
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You cannot guarantee that the primary current taken by one transformer will be close to the other transformer. This inevitably means that when primaries are put in series, one might hog more voltage than the other. You can try this on a 110 volt supply and one primary might receive 60 volts whilst the other only receives 50 volts.

The above describes the off-load scenario. It’s worse when loads are connected because if the loads are different values for each transformer, there can be a much bigger difference in received primary voltages and this could be a fire risk.

My advice is this: do not do it.

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  • \$\begingroup\$ Thanks for that. Transformers are really small, Motorola never made a 220V version, There is quite a bit of intelligent circuitry in the output as to flashing green LED for Topoff, solid green Complete, flashing red LED for charging, solid RED LED for rapid charging, flashiing Yellow Standby, etc, but the output rating on each transformer is only 4.2V DC at 400mA load, so believe impact on primaries would be low, even if only one was being used. \$\endgroup\$ – GLC Dec 31 '18 at 3:29
  • \$\begingroup\$ @GLC you are talking about power supplies that produce a DC output voltage it seems and NOT transformers. However, it is likely that the same principle given in my answer still applies. \$\endgroup\$ – Andy aka Dec 31 '18 at 10:04
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Since your two 110v transformers are identical, if you connect the two primaries in series the drops across each will be equal as well. You will then have two separate secondaries of the same voltage that the original separate 110v fed transformers had.

If the walkie-talkies are identical as well, go for it, end of story. You can plug in one walkie-talkie, or both.

What you have made is a new transformer (albeit with a split core) with 220v primary and two identical separate secondary windings.

Will small varying loads on the output, alter the primary voltage drop across each transformer?

Not much, as long as the load difference is small, but best to use it for your identical walkie-talkies, or any identical devices of similar, but no greater wattages.

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    \$\begingroup\$ No. As @Andy aka says, if the loads on the two transformers are not identical, the voltage across the primaries will not be identical, so it is not safe to do this. \$\endgroup\$ – Peter Bennett Dec 30 '18 at 2:27
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    \$\begingroup\$ As @PeterBennett said, your advice is questionable at best. You must justify and quantify your statement that varying loads will cause "not much" change in output voltage. \$\endgroup\$ – Elliot Alderson Dec 30 '18 at 3:08
  • \$\begingroup\$ Particularly since the loads are battery chargers, whose current demand will change with state of charge. \$\endgroup\$ – pericynthion Dec 31 '18 at 5:10

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