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I recently acquired a Japanese oven toaster, with a maximum power consumption of 1000w at 100v, and I would like to use this appliance in USA.

I was told that for safety reasons I should get a step-down transformer with wattage at least double the appliance's wattage.

Another source told me that if the appliance generates heat, I should get a step-down transformer with wattage at least 4 times the appliance's wattage.

Do I really need 4 times the appliance's power consumption? For example, can I get away with using a 2400w or 3000w step-down transformer?

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closed as off-topic by brhans, Michel Keijzers, laptop2d, DoxyLover, Lior Bilia May 1 '18 at 20:40

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  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – brhans, Michel Keijzers, laptop2d, DoxyLover, Lior Bilia
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  • \$\begingroup\$ No issues. Only heavy and expensive. \$\endgroup\$ – winny Apr 25 '18 at 20:55
  • \$\begingroup\$ Chances are that device is temperature-controlled. If yes, it will just heat up more quickly at 120V. Check if it has a thermostat. \$\endgroup\$ – Janka Apr 25 '18 at 22:08
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The Japanese standard is 100 V, whereas in the US it is 115-120 V. The higher voltage will cause more heating.

Power into a resistor is proportional to the square of the voltage. Let's say worst case we compare 120 V to 100 V. The resulting power ratio is (120/100)2 = 1.44, or 44% higher.

If it were just 10% or 20% higher, I'd probably just try it directly. 45% more power is enough I'd worry about over heating. If this toaster oven has a thermostat, then maybe you can still get away with it. The heating elements may burn out, and they will get hotter than intended, but they will be on for shorter amounts of time to result in about the same oven temperature.

All that said, why not just get a toaster oven intended for the power voltage you have? They are relatively cheap, and don't come with the same risk of burning your house down. Sell the Japanese oven in Japan and get a US oven in the US. It may even be cheaper overall if you consider the cost and hassle of shipping it.

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The recommendation is either excessively conservative or assumes lying on the part of the transformer supplier. Appliances which have large heaters in them are easier to power since their power factor can be assumed to be close to 1. I can state that no-name stepdown transformers are sometimes "optimistically" rated.

A step-down autotransformer good for 1000W can be made with a 120:20VAC 10A transformer, wired to buck the input voltage. That's only 200VA, so a fairly small and light transformer (relatively speaking). A universal autotransformer that handles 120/240 as well as 100/200 will be much heavier, but I see local suppliers offer 1500W no-name Chinese non-CSA-approved units for around $100 US so not insane (unless you are safety-conscious, of course).

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While you are not looking to make something, this can also be done with a triac, zero-crossing switch (moc3063), and skipping every 5th (110-115V) or 4th (120V) cycle (or 1/2 cycle).

You might also be able to use a simmerstat from a stove, and set it to 80% duty cycle. Simmerstats have the subtle advantage that they also work by heating, so they compensate for the actual mains voltage.

(I am not sure what duty cycles a simmerstat can acheive i.e. does it do 80%?)

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Double the wattage rating is being conservative, but makes sense. Four times is just excessive, You will end up with a big, heavy and expensive transformer.

When you look at the rating of a transformer, check whether the seller is quoting a continuous rating or an intermittent rating. The latter is often the big headline figure, but the transformer may only be able to handle it for a few minutes before overheating.

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