Hello

total novice at this so I'm hoping someone can help. I have a USA

appliance and the spec is given as:

INPUT 110 volt fuse protected

OUPUT 12 volt DC as 3 amps, circuit breaker protected

the plug is a USA 3 pin type (2 thin plugs at bottom, one round plug at

top). Am I right in thinking that the round one means the appliance is

grounded/earthed?

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Yes, but the round pin is, arguably, on the bottom.

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I can't find anything that relates to watts. Do I calculate this by 12

(volts) * 3 (amps) to give me 36?

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Yes. That's the power your appliance can supply to a load designed

to operate on 12 volts DC.

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Also, if I buy a transformer which is too low (Maplins do one at

45watts) will I blow the transformer, the appliance, or both!?

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More than likely, just the transformer. But, before you buy a

transformer you need to determine what your appliance draws from the

mains. It should be on the nameplate somewhere and will either be

identified as "A" or "amperes", or "VA" (Volt-Amperes). If it's

specified in VA, then all you need to do is to get a transformer rated

to supply that, or a greater number of Volt-Amperes. However, if it's

specified in amperes, then multiply 120V by that number of amperes (to

get Volt-Amperes) and get a transformer which can supply at least

that.

If you can't find out how much current your appliance draws from the

mains, then what I'd do is assume that it's running at about 50%

efficiency under full load and get a transformer that can supply twice

the output power rating, or about 75VA.

If you're a total novice, though, the best thing to do would be to

take the thing to an electrical shop and have them measure the mains

current into it while it's fully loaded. That is, with a 4 ohm

resistor on its output. The resistor will be dissipating 36 watts, so

it'll get hot if the measurement takes much time at all to do. If you

have to buy one, a 50 watt resistor would be a good choice.