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Old 26th Sep 2022, 7:44 pm   #4
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Smoothing capacitor value ?

Simple answer:

It depends.

If you have a rather small smoothing capacitor, your circuit will discharge it further between peaks of the mains voltage. So you get more ripple. On an unregulated supply, this translates to more hum.

A regulated supply will take off the ripple UNLESS the smoother voltage falls so low that the regulator can't handle it. This can be very bad.

If you opt for a massive smoothing capacitor, it won't discharge much between peaks of the mains. This means that the mains waveform will be almost at the top before the rectifier on the positive-going side turns on and starts to recharge the smoother. This means that the smoother is recharged in a very short amount of time. The same amount of charge has to go in, in order to drive the load for the period between peaks, and this in turn means that the charging current is dramatically increased.

So with too big a C, the transformer, the rectifier and the capacitor all have unnecessarily hard lives from the bigger current surges. Efficiency falls and heating increases as resistive losses and diode drops are all scaled up by the increased current.

So like goldilocks, you want a capacitor which is not too small and also not too large. Both cause problems, though different problems.

Finding where the upper and lower limits are depends on things you'd think of as mere details or stray effects. They are actually important. There are some web-based calculators which have been mentioned on here, as being good. I can't steer you to them as I just use LTspice. It's a full-blooded circuit simulator and some people are scared of it, seeing it as excessive for the job. The advantage of using a sledge hammer to crack a walnut is that you not only have a cracked walnut, you also get to keep the sledge hammer and you've started to learn how to use it on things where it really makes a difference.

David
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