Why is smaller cap better for noise filtering?

[waveboy]

Dear Engineers,

At Interview, i was recently shown a circuit involving a small cap (about 100n, ceramic) at the output of a voltage regulator (linear one). It was for filtering out noise....it was connected from the output of the regulator to ground.
-The input to the regulator was supplied from a transformer, going through a full wave diode bridge.....then there was a big electrolytic cap to filter out the mains ripple...then there was the regulator......at the output of the regulator there was (as i said) a small 100n ceramic cap, as well as another big electrolytic.

I was asked what the small ceramic cap was for. Thinking about it, wouldnt the big electrolytic at the output of the regulator filter out the noise well enough on its own....noise is higher frequency, i admit, but a big electrolytic with this higher frequency would have a very small reactance to ground...so why was the small ceramic necessary to filter out noise??

Any comments gratefully received.

I dont know what the circuit was supplying...but the company makes LED light displays for theater and restaurants etc...as well as making lighting control consoles.

 

[OperaHouse]

Big capacitance generally comes with higher inductance so it works better at higher frequencies. In the case of the regulator output, a larger cap would have worked well enough to supress oscillation of the regulator. These can sing at a couple MHz without one. Unless the output is supplying a high current pulse application, a small cap on the output is probably chosen for cost, size, and reliability reasons.

 

[BrianG]

Hi waveboy, I think OperaHouse mis-typed his post. True, a big capacitance has a higher inductance due to its method of construction, but this means it does NOT work as well at high frequencies.

A smaller capacitor, such as the 100n ceramic cap you were asked about at the interview, definitely work better at high frequencies because the overall inductance (internal construction plus connecting leads) is very much lower hence it is a more effective capacitor. This is essential to filter any h.f. noise, and also to stabilise the regulator and stop it from oscillating.

 

[europus2002]

Hi waveboy,
The High electrolytic caps are for filtering as well as smoothing out DC from ripples and 100nF capacitor is for bypassing the supply transients (mostly high frequency and bipolar) present during turning ON/ OFF the circuit and for bypassing thermal noise that becomes apparent once you load the regulator above 70-80% or so.... For high frequency bipolar transients, your electrolytic caps are not as efficient since they are polarised. In many places like pulsing circuit applications , electrolyic caps can be used as a reserved current source stuck along DC supply line. The advantage of doing that is you need to used less incoming DC power while you can deliver high current pulses. You calculate the duty cycle of your pulse for application like that.