Capacitors on a Bridge Rectifier


#1

Hi Guys,

I have been looking at building a Linear Power Supply for some time now, starting with the AC transformer and working through to the regulator etc. My question is, some of the circuits I have found online use a 0.1uF capacitor across each of the diodes in the bridge rectifier - so four caps - one for each diode. What purpose are these caps filling ? In each of the circuits, there are filter caps downstream of the rectifier which is fine, but are the caps across each of the diodes actually doing anything ? Is this best practice ?

Thanks in advance.

Andrew


#2


#3

Have to say that I’m curious too. I simulated it in LT Spice really quick and couldn’t see a difference (for a pure sine wave at least). If I could hazard a guess… it would be something to do with blocking either DC components to the wave (really unlikely), or high freq responses. If I have some time, I might simulate it some more, but that might be what you can look into.

As a side note, I’ve looked at a lot of linear supply designs and I can’t say I’ve ever seen caps across the diodes. Not that that means anything.


#4

Thanks Scott. I might have found the answer to my own question. Should have asked Google before… Apparently power supply transformers have leakage inductance and parasitic capacitance, and when the diodes in a bridge rectifier switch off these “non-ideal” elements form a resonant circuit that can oscillate at high frequency. This high frequency oscillation can then couple into the rest of the circuitry. Snubber circuits are used in an attempt to mitigate this problem. Just using capacitors doesn’t damp the ringing completely, but does cause it to drop to a lower frequency where the coupling effect is less. An RC circuit across the diodes can damp the ringing almost completely. I assume it is only an issue in audio circuits and it might be overkill for a power supply but I think that is a good explanation and I will include it in my circuit for now. It cant hurt… right ?


#5

There is a very good explanation here : http://www.hagtech.com/pdf/snubber.pdf


#6

I’m not sure, but if you are using a linear regulator, it wouldn’t need the caps since the regulator would take care of a good chunk of high freq noise. I could see it being more of an issue for a SMPS. I’ll have to read that link later.