A friend (who knows even less about electronics than me!) asked me if I could build a circuit that can convert 8 V 50 Hz AC into a DC power supply so he can power a small board that is normally driven by batteries. The load will have a low current and also the circuit will not need to run for more then a couple of minutes at a time.
The obvious solution is to make a full wave rectifier and to smooth the voltage out with a smoothing capacitor. I'm not quite sure what voltage he wants so I'm going to allow it to be tuneable via a trimming potentiometer. I also want to add an LED to show it works. What I've come up with is the following circuit:
The problem I've got is that I don't have access to the power supply he wants to use to drive it (the joys of lockdown). Which means I'll need to build it with out being able to test it at 50 Hz. I've built something on breadboard and tested it with a DC power supply. Based on that I'm happy with the values for R1, R2 and R3 and the diodes are spec'ed to switch fast enough.
What I can't work out is the value of the smoothing capacitor. If it's too small I think I'll get 50 Hz drops in the output voltage which will upset the down stream circuit. Too high and could I get trouble charging it?
Is there a formula or even a rule of thumb that will give me the right value?
Edit: I've updated the diagram. As people rightly pointed out I've had the LED in backwards. Fortunately this was just a mistake in the diagram. The breadboard prototype was the right way round - it did light up.