will a 100uf power cap alone protect my trannies?

[csmatt45]

Was just wondering, is a diode a must if you are concerned about a backwards battery harming your trannies, or can a filtering cap alone (100uf) do the job, or does a cap not do the job at all??

thanks,

matt

btw i already read the geo article about polarity protection so no need to post a link.

[Peter Snowberg]

The cap is only there to filter variation in DC, not to provide any protection against reverse polarity.

It's almost a trick question because electrolytic caps will conduct if reverse biased so they may provide some protection. A tantalum cap will do that about once before emitting toxic smoke as it dies. Aluminum electrolytics are a bit more forgiving; reverse polarity will slightly damage them but they have the ability to 'heal' if the polarity is corrected.

[csmatt45]

thanks peter,

so the answer is (regarding the trannies) yes some protection, but no guarantees??use a diode just to be sure???

is that right?

matt

[R.G.]

The right answer is yes, there is some protection, as long as the power is reversed for less time than it takes for the capacitor to die. This could be from a couple of seconds right on down to a few milliseconds depending on the cap and the power supply trying to kill it.

A better answer is as you said - there may be some protection, but it's so short and unreliable that it might as well not be there.

By the way, a reverse connected diode has much the same story, excepting that it will usually hang in there for many seconds to minutes before charring the PCB and shorting if you use a 1A rectifier type like the 1N400x series.

[markm]

Some of the builds show using a 1N914/4148 for the polarity protection.
Are they less safe than say the 1N400X series?
  MarkM

[Processaurus]

Some of the builds show using a 1N914/4148 for the polarity protection.
Are they less safe than say the 1N400X series?
  MarkM

The small signal diodes (1n914, 1n4148) are used for series polarity protection.  The + power supply voltage must pass through it to get to the rest of the circuit.  The beefy 1n400x diodes are used for shunt polarity protection, where they are tied from + to ground, but reversed biased, so they'll only conduct if the ground is more positive than the power supply, shorting out the power supply until someone figures out whats going on. 

I personally use a schottkey diode (now stocked by small bear) for the series polarity protection, they have a lower voltage drop (.2v - .3v) than the Si types.  The shunt protection seems like a good idea for battery only effects, because you can't actually get the 9v snap on backwards.

I learned about that from the first part of this Geofex article.

[csmatt45]

wow,

thanks everyone for commenting/helping. RG your site has been immensley helpful in my pedal-lust/no-$$$ life. I'm sure many can relate. 

You said a few seconds would be the max the cap would probably last, but the reason this topic came up for me, was i had a brain fart and hooked up my 9v batt NPN style, for a PNP circuit. i had the batt plugged in a good 10-15 seconds before I caught it and was actually surprized to find the batt  hadn't heated up, the 100uf cap was ok, and the Ge trannies (for a "stock" spec FF) seemed fine. I've watched a tranny or 2 go up in smoke before when i didn't have a cap on the power rail in the past and made a similar mistake. I think the trannies are fine but now i'm obsessively doing the thing where i'm listening to it and going "does it really sound the same...etc??" (my bet would be it's fine, but that obsessive part of me can be a nag...). I don't have a proper meter to check it, just my ears.

was it the cap, luck, or a divine intervention??

thanks again,

matt

[mac]

My choice was to build a well-filtered 9.6V supply and to put a 400X in series with the supply. This way I have about 9V after the diode.
Some 'purist' may argue about the effect of the series diode in a Fuzz Face or other circuits that are sensitive to the impedance of the power supply.
I can live with that.


mac