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

[HamishR]

I sometimes drop into my local music store with a newly built pedal as I have friends there who want to try things.  One thing I noticed when I first got into building pedals was that sometimes my pedals would make whining noises at the shop when plugged into cheap power adaptors in a room with fluorescent lighting, lots of amps and gear.  So much so that the pedals were unusable.  Weird, because it never happened at home.

The remedy is simple.  Just stick a 5817 or 4001 or whatever between the 9V into the amp and the rest of the circuit and the noise disappears.  So it would seem that as well as potentially protecting your pedal that diode is preventing noise from ruining your day.

I am no expert - I know next to nothing about electronics, but have built more than my fair share of tube amps and FX pedals.  So this is purely anecdotal.  I have no idea why this would work!  But I never hear this point being discussed.  The diode is only ever referred to as "polarity protection" in the sense that it may blow if the wrong power is applied.  But I have found it is essential in some pedals to avoid noise issues from (possibly poorly filtered) power supplies.  A 100ohm resistor can help in some cases too.

I'm posting this just in case it helps some of you guys with this issue.  I hope it helps someone!

[Blackboarcult]

Thanks for this useful bit of knowledge :) I also thought that it was just polarity protection.

[GrooveChampion]

Correct me if I am wrong, but isnt this called "Single Diode Rectification"?

[HamishR]

Haha - I have no idea!  Sounds good to me though.  I would say you were right.

It just shows that I need to understand more about this stuff.  I have only ever heard using a diode like this as "polarity protection", which is I guess what single diode rectification is.  But it does more than just protect the circuit.  If you only ever use good quality power supplies - like I do for myself - then you may never notice it missing.  But if you sell or lend pedals it's probably good to know that sometimes a diode can be helpful.  Just as a 1ohm resistor can cut down noise in the right place.

[Travis]

A rectifier converts AC to DC, the purpose of the reverse polarity protection diode is to prevent damage from connecting a battery backwards or using the wrong power supply (center positive)

If the diode is connected in series with the supply, it can help with decoupling which is why it might help with a noisy power supply

[GrooveChampion]

Oh, crap. Forgot that tiny part after watching all the ElectroBoom videos. Thanks Travis! I guess the decoupling part explains all.

Thing is I often saw the diodes facing either direction, what is the meaning behind this?

[Travis]

AC swings both positive and negative (hence the name alternating current)

A diode can be used to conduct current in one direction only. IE it can allow a positive voltage to pass through while blocking a negative voltage.

Or flip the diode around and it can conduct negative voltage and reject positive voltage

So if you take AC, connect two diodes in opposite directions, one diode can give you positive DC and the other can give you negative DC

The diode basically isolates the positive or negative swing of the AC wave depending on the direction the diode is connected

For more specific info and applications you can search “full wave rectifier” or “half wave rectifier” and check out those circuits

This isn’t a great explanation but I’m at work right now. I think it’s easier to look at an AC wave and see how the diode takes only the positive or negative swing

[GrooveChampion]

Well actually I was speaking solely in pedal polarity protection terms. I saw the anode connected to the 9V and the cathode to the ground, which I assume protects the pedal from voltages of over 9V. I also the the anode and cathode flipped, which I assume is for protection against polarity....if I correct...

[reddesert]

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).

[HamishR]

I'm glad I started this thread because I have learnt a lot.