Electronics > Projects, Designs, and Technical Stuff

Caps across chokes trick

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001:
Hi!

I hear about old trick to eliminate ripple in C-L-C filters after rectifiers
The idea is small cap adding across choke and trimming it for ripple resonant frequency (120hz)


but I see that it is not common at factory-made gear. Why? What about practical equations? What is affordable accuracy?

Berni:
Punch a few numbers into the calculator and see just how big of a L and C you need for this.

Then afterwards have a look at the ripple rejection graphs in the datasheets of even cheap crappy linear regulators and you will see why.

001:

--- Quote from: Berni on October 11, 2019, 02:55:16 pm ---Punch a few numbers into the calculator and see just how big of a L and C you need for this.

--- End quote ---

About 0.68uF will be Ok for typical 5H choke  isnt it? It is very cheap

Berni:
Go check how much a 5H inductor costs.

Then compare that to a cheap linear regulator IC that can be had for about 40 cents even at places like digikey in low volume. That cheap regulator IC gets you at least around 50dB of noise rejection at 120Hz. If you spend money more on a regulator IC you can get ones up about 80 to 100dB of noise rejection. This means that even the worst linear regulators can reduce the noise from 1 V to 0.003V. Or if you are after really low noise you could put two of those 80dB rejection regulators in series to get a theoretical 160dB of rejection bringing that 1V of noise down to 10nV (Of course you wouldn't actually get that because the regulator itself is probably generating more noise than that by its own internal reference)

I don't know how many dB of rejection you can get out of such a LC filter, but if its tuned like this that makes it most effective at the single tuned frequency while the mains sine wave does also have other harmonics on it, so at least some of those would get trough.

This method however was pretty common back in the days of vacuum tubes, since using an extra tube just to regulate one power rail was too costly.

duak:
I remember reading about this in one of the 1960's ARRL handbooks for radio amateurs.  It made more sense for vacuum tube equipment where the voltage is high and the current is low.

It can work but there are two problems to be aware of: unless an air core inductor is used, its inductance varies as a function of current due to core saturation.  The second is that a resonant LC circuit can be formed and if the load draws intermittant current at the resonant frequency, high AC voltages can build up.

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