I'm going to make 12V DC power source for my project, using a transformer (230V AC to 15V AC) and full wave bridge rectifier, followed by LM7812.
The load will be 200mA DC.

This is the simulation circuit:
enter image description here It is a standard bridge rectifier circuit, nothing special.

This is the Vout and current through Rload:
enter image description here Clean 12V and 200mA as expected.

It is known that capacitors in bridge rectifiers are charged by current spikes. This is the current through Rsense2:
enter image description here Peaks are about 450mA high.

That is all perfectly OK, I understand it pretty well.
But there is one thing that is not clear to me. Do I have to select transformer that is capable of providing 200mA or 450mA?
Transformers specifications usually say:
Power rating: 3VA
Secondary voltage: 15V
Secondary current: 200mA
example: TR Z03.00/15

I know that I should not pick transformer on the edge of current rating and have some margin. But my question is whether to use RMS value (200mA) or peaks value (450mA) for the selection.


1 Answer 1


There are several considerations to take into account, the two main ones being transformer heating, and output voltage regulation.

For transformer heating, you're interested in the RMS current. You could compute the RMS of the current waveform drawn, or you could use the mean DC output as a 'correct ballpark but likely under-estimate' estimate.

Transformers specifications will usually give you an amount by which to derate their rated current for rectifier/capacitor loads. Here is a guide from a random transformer manufacturer giving typical formulae for transformer current rating versus the type of load. They recommend derating to 62% for a rectifier/capacitor load.

For voltage regulation, you're interested in the peak current, as this is what reduces the voltage the reservoir capacitor charges to. However the difference in the effect between rms and peak is usually minor compared to the poor total regulation of small transformers, so a detail not worth worrying about.


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