Filtering voltage regulators . . .

[mlabbee]

I'm a little confused about voltage regulators - if you are using them after a rectifier, do you still need the big filtering caps or does the regulator do the same thing?  I know you need a couple of caps around the regulator itself - I'm wondering if these take care of everything. The schematics I've seen for regulated supplies are all over the place - some show several capacitors before the regulator, others just show one . . .   any thoughts?

[R.G.]

There are two different jobs that capacitors do, and in power supplies with regulators, they do both but are connected the same. Confusing?

Let's clear it up a bit.

In a rectifier/capacitor power supply, the capacitors act like big charge storage buckets. Since the AC power reverses polarity ( it is, after all Alternating Current), the rectifiers dump charge into the big capacitor charge buckets only twice each AC cycle, once for positive polarity, once for negative polarity.

Between those dumps, the circuit has to run from the stored charge in the capacitors. The capacitors act like short-term rechargable batteries. So at each charging pulse, the battery-capacitors charge up to the peak of the AC waveform, then as the voltage gets lower, the diodes turn off and the battery-capacitors are on their own to support the load until another charging pulse gets there several milliseconds later. It's like water in the old water-tower water systems - you pump a bunch in every now and then and the users use a little at a time, draining the tank until you have to pump som more in.

So the voltage on the battery-capacitors jumps up when the charging pulses get there, then drains down slowly til the next charging pulse. That is the power filter cap ripple voltage you hear about.  The more load on the caps, the faster they drain down. The bigger the caps, the slower they drain down.

A regulator after the main battery-capacitors needs a certain minimum voltage to run. Most cheaper 3-terminal regulators need two volts above their regulated voltage, so a 9V regulator will have to be fed a voltage greater than 11V or the ripple will "show through" to the output, the regulator no longer able to maintain regulation. That is on an instant by instant basis.

You have to size the transformer voltage, diodes and filter capacitors so that the instantaneous ripple voltage leading to the regulators never gets below the minimum input voltage for the regulator to keep regulating. (See "power supplies basics" at GEO for more of this stuff and how to calculate the sizes).

There is another job that capacitors do, and that's more like filtering in the high frequency/low frequency sense. Power supply bucket caps are good at holding quadzillions of electrons, but they don't have very good high frequency characteristics. Regulators are internally opamps with a reference voltage, and they need power supplies that don't look like inductors to keep from oscillating like a poorly designed opamp circuit. The smaller caps are to keep them from oscillating. They do this by being faster at charging and discharging than the main bucket-capacitors.

So, the answer to your question is - you need both in the general case. You need big bucket caps; 470uF is about as small as I've ever seen for a mail power bucket cap, and that's for tiny load currents. Usually you start with 1000uF and get bigger - sometimes much bigger.

Most 3-terminal regulators need a 10uF right at the regulator, maybe one or two at the outputs.

[mlabbee]

Very nice explanation - thank you!