Watt question

September 20, 2006 10:10 AM

A question about voltage, watts, transformers.

I have a transformer which steps up 125V Japanese electronic gear to run on a 220V mains outlet. It says on the transformer that the capacity is 300VA.

The three machines I want to plug in have written on their backs 100V 45W, AC 120V and 41W, and 100V 40W respectively.

If I plug them all in, will the transformer handle it? I don't want to blow anything up.

posted by dydecker to technology (8 comments total)

You're not stepping up the electronic gear, you're stepping down the 220V main.

That should all be okay on there, but: doesn't the transformer have a circuit breaker that will trip if it's overloaded or shorted out?
posted by aubilenon at 10:24 AM on September 20, 2006

I don't think there's a circuit-breaker. How do you know it'll be okay? Do I just add up the watts and if it's under 300 then it'll be ok?
posted by dydecker at 10:29 AM on September 20, 2006

Yeah.
posted by aubilenon at 10:34 AM on September 20, 2006

The machine that wants 120V AC, american style, isn't going to like being plugged into a british-style 220V.
posted by Mr. Gunn at 10:42 AM on September 20, 2006

Do I just add up the watts and if it's under 300 then it'll be ok?
Yes.
If you were talking about larger inductive loads, like a washing machine or a big electric motor, there might be other considerations.
posted by kc8nod at 10:53 AM on September 20, 2006

The Japanese electrical system is 100V, and mostly 50Hz; which is why some of your devices are rated at 100V, obviously.

I think you probably need a 220V:100V transformer.. The difference between 100V and 120V isn't that big, but it could easily be enough to cause damage, depending on the particular device. Lots of stuff nowadays is multi-voltage, so you can plug it in anywhere. Two pieces of your equipment are asking for 100V specifically, so..

Where did you buy the transformer (Japan, Europe, Middle Earth) and did you buy it specifically for Japanese gear?
posted by Chuckles at 2:25 PM on September 20, 2006

Yes, I bought it in Japan so I think it'll probably be okay. Thanks all.
posted by dydecker at 4:01 PM on September 20, 2006

Watts = VA x power factor.

You're going to be running 126W worth of gear off a 300VA transformer, so the power factor would have to be under 0.5 to get you near your transformer's rated limit.

Even the world's crappiest switchmode power supply should be over 0.5 power factor, so you should be fine.
posted by flabdablet at 7:06 PM on September 20, 2006