# transformer 110 to 240

Discussion in 'Electronic Basics' started by mystraightroad, May 6, 2005.

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

2. ### Guest

:
No. But why buy a transformer to power an adapter? Just but a 12V 3A
adapter for 230V input. Maplins sell 'em.

3. ### Tom BiasiGuest

Yes, the round pin is earth ground, it should be connected to the chassis of
Your output can supply 36 Watts.
You didn't say what the device is. I suspect a power supply for something.
be fine.
Regards,
Tom

4. ### John FieldsGuest

---
Yes, but the round pin is, arguably, on the bottom.
---
---
Yes. That's the power your appliance can supply to a load designed
to operate on 12 volts DC.
---
---
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.

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.

6. ### ryan wiehleGuest

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.

7. ### DwayneGuest

Just buy a power adapter with a 12 Volt output capable of at least 3 Amps.
For example if you buy a 48Watt adapter with a 12V, it can suppy a maximum
current of 48/12= 4A. So you can connect any 12V device, and draw a MAX of
4A. There is no minumum draw, your 3A device will work fine, so will a
device rated at 12V and 2A or 1A or 500mA, etc...

Dwayne

8. ### Kitchen ManGuest

A transformer cannot be too strong, but it can be too weak. In the
trade, acquiring items with greater ratings than minimum is called
"pro-rating." If you use the 300VA - it's more correct to use the
term "Volt-Amps" (VA) when speaking of AC power - your design will be
just fine.

Here's a very good article that discusses just the subject you are
concerned with:

http://www.toddsystems.com/geninfo.html

Have at it, sparky!

9. ### JamieGuest

go to your local electronic supply store and get your self a
AC(240 Vac) to DC (12volts DC)converter that will supply 3 or more amps.
in the long run, it will be cheaper and easier for you.

10. ### Dr Engelbert BuxbaumGuest

It will work just fine, however, power losses (nothing works with 100%
efficiency) will be slightly higher. For something used only