#### csdave

##### Member level 5

as per my other threads, I am at my first experiences with designing small power supplies for my circuits and I feel I need to understand transformer ratings a little better.

Say I have a transformer that is rated 6V @ 3VA.

My understanding is that if I connect a purely resistive load to this transformer, it will be able to provide up to 500mA(rms) at which value, the voltage will be of about 6V (rms).

With reactive loads, things get more complex, but the key is that the apparent power should not exceed 3VA.

So far so good, right?

Here however, where I start getting confused. With a sinusoidal waveform, apparent power is the product of RMS voltage and RMS current, but what happens when the load is such that the voltage/current waveforms are no longer nicely sine shaped?

Consider the same transformer feeding a full-bridge rectifier with a filter capacitor at its output. What will happen is that the capacitor will draw high bursts of current when it charges. The biggest will be when it charges from the empty state at the beginning. But there will also be bursts at each half-cycle of the initial waveform if the load circuit is drawing enough current to significantly discharge the cap.

- LTSpice allows me to calculate the RMS current required by the circuit, but how should I go about calculating this myself?

- Is it correct to say that once I have the RMS current, then the apparent power is RMS current times the (RMS) Voltage of the transformer?

- Is the RMS current the only thing that can cause problems? Since we no longer have a sine wave, the same RMS current can be obtained with a variety of peak current values. How do I compute the peak current in practice? How large a peak can I assume the transformer to handle?

thanks

Davide