Re: "polarity protection" diodes - more useful than you might think

Posted by reddesert on Nov 26, 2018; 9:52pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/polarity-protection-diodes-more-useful-than-you-might-think-tp44848p44872.html

Many effects have a power supply filter capacitor connected between +9V and ground. Ideally, this cap charges up to +9V and then charges/discharges through the circuit if there is any fluctuation in the power supply level. This filtering often works better if there is a small resistance in series with the +9V before it reaches the filter cap, like the 100 ohm filter suggested by the OP. Sometimes in complex circuits like amps, you'll see different parts of the circuit having their own V_supply line and filter cap, each separated by a 100 ohm resistor to decouple fluctuations in the power rails.

The resistor and cap form a RC lowpass filter on the supply voltage. For example, for R=100 ohms and C=100 microfarads, the corner frequency of the filter is 16 Hz, and it's a simple first-order filter with slope -6 dB/octave. Any 60 cycle hum is two octaves above the corner frequency, so it gets attenuated by a lot.  

If you put a diode in series with the +9V supply before the filter cap, then the voltage after the diode is 9V minus a diode drop (about 0.7V for silicon, 0.1 to 0.3 V for a Schottky diode). The capacitor will charge up to this voltage, say +8.7 V.   If the power supply was really noisy and dropped below 8.7V, then the series diode would block conducting in reverse, so in that sense it is rectifying some of the noise out of the supply voltage (but that's really a pretty terrible power supply to have so much fluctuation).