transformer 110 to 240
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Posted by mystraightroad on May 6, 2005, 3:07 pm
Please log in for more thread options 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? I can't find anything that relates to watts. Do I calculate this by 12 (volts) * 3 (amps) to give me 36? Also, if I buy a transformer which is too low (Maplins do one at 45watts) will I blow the transformer, the appliance, or both!? Many thanks | ||||||||||||||||||||||||||||||||||
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Posted by on May 6, 2005, 3:46 pm
Please log in for more thread options mystraightroad wrote: : > top). Am I right in thinking that the round one means the appliance
is
> grounded/earthed?
> Yes. > I can't find anything that relates to watts. Do I calculate this by
12
> (volts) * 3 (amps) to give me 36?
> Yes. > Also, if I buy a transformer which is too low (Maplins do one at
> 45watts) will I blow the transformer, the appliance, or both!? > No. But why buy a transformer to power an adapter? Just but a 12V 3A adapter for 230V input. Maplins sell 'em. | ||||||||||||||||||||||||||||||||||
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Posted by John Fields on May 6, 2005, 5:50 pm
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On 6 May 2005 12:07:11 -0700, "mystraightroad" >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? --- Yes, but the round pin is, arguably, on the bottom. --- >I can't find anything that relates to watts. Do I calculate this by 12
>(volts) * 3 (amps) to give me 36? --- Yes. That's the power your appliance can supply to a load designed to operate on 12 volts DC. --- >Also, if I buy a transformer which is too low (Maplins do one at
>45watts) will I blow the transformer, the appliance, or both!? --- 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. -- John Fields Professional Circuit Designer | ||||||||||||||||||||||||||||||||||
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Posted by mystraightroad on May 6, 2005, 5:39 pm
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thank you so much for your replies. Ok, I have a feeling this is a stupid question but here goes.... If we assume that the draw is twice the max output (75w) would it hurt if I were to use a transformer rated at 300w rather than one at 100w. A friend has said they will lend me a 300w transformer, I'm just concerned it might be too strong. | ||||||||||||||||||||||||||||||||||
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Posted by ryan wiehle on May 6, 2005, 8:15 pm
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mystraightroad wrote: > thank you so much for your replies.
> > Ok, I have a feeling this is a stupid question but here goes.... > > If we assume that the draw is twice the max output (75w) would it hurt > if I were to use a transformer rated at 300w rather than one at 100w. > A friend has said they will lend me a 300w transformer, I'm just > concerned it might be too strong. you said the appliance at 12v draws 3 amps = 36watts if you now supply it with a 300watt supply you have enough power to connect about 8 of these devices. | ||||||||||||||||||||||||||||||||||
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> 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