use wall power when plugged in, otherwise use battery power

[artifus]

memory fails but i'm almost certain that it was you who told me otherwise rg. ho hum. i'm not doubting anyone here but my own memory. and as i said - does not generally bother me or my builds - was just curious. apologies for any confusion caused.

[R.G.]

memory fails but i'm almost certain that it was you who told me otherwise rg. ho hum. i'm not doubting anyone here but my own memory. and as i said - does not generally bother me or my builds - was just curious. apologies for any confusion caused.

No worries.
I don't think it was me, unless I was in a senior moment. Reverse polarity protection is possible, within limits. There is always a voltage big enough to defeat any protection scheme. As we say here in Texas, never a horse that couldn't be rode, never a rider that couldn't be throw'ed.

A series diode is really good - unless you can't stand to lose the forward drop of the diode.

A semiconductor switch is good, as it can force itself to have less forward drop than a diode, as in the MOSFET protection circuits and the "Cheap - and good - polarity protection" article at GEO.

Shunt diodes are cheap, but not very good, as they have a huge vulnerability to high current reverse voltages. They're good for accidentally reversing your batteries, or current limited wall warts. 9Vac supplies will cook them dead.

[ashcat_lt]

And maybe that's where the confusion comes.

The shunt style is standard in damn near every commercial pedal and is the worst possible option.  I call it "warranty protection" because when they receive the pedal back for repairs they can point to the exploded diode and tell you that since you didn't use their officially sanctioned power supply you are SOL.

[artifus]

memory has poked me to say that it may have something to do with the amount of time the reverse voltage is applied... as in i read a thread that claimed that diodes were only so good for so long... but as i say - i don't care but am still curious as to whether or not i should...

*edit* ^ ashcat may have just pin pointed what has confused me - sorry guys - it's a big internet and i am of limited memory!

[ashcat_lt]

Still this think you're thinking about the shunt style.  In this case when the diode is reverse biased in normal operation, and can do so indefinitely as long as the input V doesn't exceed its reverse breakdown limit - but at that point you'll probably have other components fail first!  When power of the wrong polarity is applied, this diodes becomes forward biased, and acts as a short to hopefully keep that V from hitting other components downstream.  That short will pull as much current as the supply can provide.  If that amount of current exceeds the diode's safe current rating it will - in time - heat up and explode, and generally leave the rest of the circuit unprotected.

RG calls it a fight to the death between the power supply and the diodes, whichever can pass the most current the longest survives.

[icydash]

Thanks for the info.

I do have voltage regulation and filtering in my power circuit, but thanks for the heads-up.