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Old 31st May 2007, 10:17 AM   #1
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Default Question about autotransformer ( 230V to 115V )

Hi everyone,

I've go a question concerning an autotransformer which I want to use to convert 230V 50Hz AC to 115V 50Hz AC. I'll be using it to power a friend's Hello Kitty toaster (don't laugh) that was bought in the US. The mains here in Europe are 230V50Hz...I'm not concerned about the frequency nor the missing galvanic isolation from the mains voltage (provided that the autotransformer is wired correctly, the voltage taken from the bottom up, it should be ok). I assume that toasters don't count or require mains isolation...

Well, the toaster is said to consume 850W so I bought an 800W autotransformer (2:1, Siemens, 17 Euros), which hopefully will do the job.

My question is: Besides a switch, a fuse (230V4A slow blow @ mains in), a X2 capacitor (1uF, also @ mains in) and the wiring from them mains and to the toaster, do I need anything else to get this toaster toasting?

I'm asking because the guy who sold it to me via some internet-online-thingy said something about some diode I'd have to use but I've never heard of/seen/used such a thing along with an autotransformer. He sent some diode (metal case and the dimensions a little smaller than 1N5407, but can't read what is printed on it) but won't respond to any question.

Thanks for reading, Dave
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Old 31st May 2007, 10:52 AM   #2
AndrewT is offline AndrewT  Scotland
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I don't know what the diode is for.
The toaster will run at 800W continuously.
You may find that the autotransformer runs more than warm when loaded like this. The saving is that toasters only get used for minutes at a time (unless you run a B&B/hotel).

Consider the possibility of having to go to 1.5kVA.
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Old 31st May 2007, 11:14 AM   #3
paulb is offline paulb  Canada
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Maybe you can toast more bread on the transformer while it's running the toaster.
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Old 31st May 2007, 01:44 PM   #4
Eva is offline Eva  Spain
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In the good old days, a series diode was employed to get 110V RMS out of 220V AC mains. This works only for very simple loads like incendescent lamps and resistive heaters, though. Note that this is a really nasty practice because DC is effectively drawn from the mains line and a DC offset is created that leads to all kind of trouble with transformers and inverters.

There are other non-harmful ways to power 110V resistive loads from 230V mains without transformers, like a triac based circuit skipping every other mains cycle (power level may be further adjusted by changing the cycle skip ratio...)
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Old 4th June 2007, 11:42 AM   #5
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Originally posted by Eva

In the good old days, a series diode was employed to get 110V RMS out of 220V AC mains.
Wow, that's really nasty.

I finished the autotransformer-project and tested it successfully. I included a fuse (4A slowblow), a switch, a thermal fuse (117C) and a X2 cap. The autotransformer hums a bit but does not run hot at all. Like I said, the autotransformer is rated @ 800W, the toaster's bottom-print sez it consumes 850W. Hmm...

Thank you all for your replies. I'll post some pics later.

C ya, Dave
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Old 4th June 2007, 01:11 PM   #6
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I don't think you are going to have any problems with 5% overload for a intermittent duty load like your toaster Some transformers can take like 100% overload for several minutes due to high thermal mass... So 5% is nothing. Maybe if your hobby is toasting bread all day long it will start to complain
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