AC / DC transformer

[horst]

Hi,

I am searching for a may simple answer: If I have a switched power supply or a transformer, 230V AC in and 12 DC out. If I use eg a Halogen light, 12V, 50W, the current is about 4.1 A. How do I calculate how much current this will draw on the AC side of 230V ?

Thanks for the help

[phonoplug1]

Its not too difficult to work out, but you need to make some assumptions:

If the loads is 50 watts and the power supply was 100% efficient (which it isn't), then it would consume 50W at the input. Ok, so make an assumption that its 80% efficient, so therefore it will consume 62.5 watts at its input. At 230V(rms), 62.5 watts equates to 272mA.

Another point to bear in mind is that switch mode power supplies to not consume current as a sinewave thats in phase with the voltage - though this might not be relevant to you.

[michael kiwanuka]

I wish to comment on the answer put forward by the previous reader and this is:-

The power input to the unit cannot be less than the output power from the unit. This would be against the laws of nature. So the correct steps should be as follows

Output power approx. = 50W

Assuming 80% efficiency, Input power = 50/(.8) = 62.5W
Assuming that this power is available at the primary side of the transformer, Input current = 62.5/(230) =271.7 mA. I have assumed that the mains voltage is 230V rms.

Best regards,

Michael Kiwanuka

[michael kiwanuka]

Sorry, I misread the answer put forward by the previous reader. His approach is right and his answer is right. My sincere Apologies.

Michael Kiwanuka