A Fused Reverse Polarity Protection Circuit?

[karesplat]

Hi all,

I'm a newbie so before I go and modify a circuit, possibly for the worse, I thought I'd ask about my plans on here. I prefer using a DC power supply to power my pedals and my primary one has jacks whose polarity I can change. An absent minded moment therefore presents the risk that I apply a reverse polarity to my pedals. I was thinking of adding a reverse polarity circuit module that hangs off my dc sockets on my home made pedals. I noticed that most reverse polarity protected circuits use a diode from the 1N400X series installed such that it conducts when a reverse polarity is applied. This would, of course, lead to a short circuit if it were not for the series resistor that some of these circuits also employ. Standard enough so far but I would like to replace the resistor with a PTC type resettable fuse. When circuit amperage trips these they go to a really high resistance and only return to their usual 1 or 2 ohm resistance level when power is removed and they're left for 20 seconds to reset. This might sound like a pain considering the original circuit doesn't have this 20 second limitation but it seems to make more sense to me than a fixed resistor and would protect my components against short circuits that I might mistakenly cause while breadboarding my ideas. It's also an opportunity to play with a new kind of component.

What is the opinion of the DIY Stompboxes forum? Is this a waste of time, an interesting idea, a newbie misunderstanding something etc. Feel free, I need some input even if it is just to say that, yes I can do that.

Thanks!

[GibsonGM]

I see no problem with trying it out, karesplat. You'll likely have to modify how it's wired, but it could work out ok.    BUT, will it trip fast enough to protect your circuitry?  A diode goes pretty quickly, as I'm sure you know.  Esp. a 1N914 applied in parallel across the PS input to the board (reverse biased) on EACH effect.  So the question is, is it a good use of time to implement the idea?

[Processaurus]

Hello, you understand everything right, but you can achieve your goal (extended duration polarity protection), with, rather than the shunt diode arrangement that cooks a power diode when the polarity is wrong, a forward conducting diode in series between the power jack + and the circuit +.  You can have the polarity wrong all week and nothing will happen.

The only possible drawback to the series diode is that the circuit always sees a voltage drop (from the PS voltage)  of .7v (for Si diodes), however a Schottky diode will  drop less than .3v, that's what I use on all my pedals...  

[R.G.]

Even better, read geofex.com.

See:
http://geofex.com/Article_Folders/mosswitch/mosswitch.htm
http://geofex.com/Article_Folders/cheapgoodprot.htm

Both of these offer series protection, like a series diode, and do not suffer the voltage loss that a diode gives you.

The MOS power switch has been there since November 1999.