Power reversal protection idea

[Paul Perry (Frostwave)]

http://www.blacet.com/tech.html
halfway down this page. John Blacet has been in the game a long time (selling kits to modular synth builders). One of the good guys (I'm a happy customer).

[R.G.]

Cool. I didn't know PTC thermistors had a low enough cold resistance to do that.

[Paul Perry (Frostwave)]

The magic word is 'PolySwitch'.
http://en.wikipedia.org/wiki/Polyswitch
The only place I have seen them used, is in speaker protection.

[grapefruit]

We've been using these in circuits at work for short circuit current limiting protection for a while.
I just tried it for reverse polarity protection yesterday. It work great.

I used the Altronics R4545A
Self resets when fault condition is removed. Hold Current is the maximum current at which the device will not trip at 20°C. Trip Current is the minimum current at which the device will always trip at 20°C.
Specifications
Hold Current: 0.30A
Trip Current: 0.60A
Pitch: 5.08mm

Polyswitch in series with the input V+ into a reverse 1N4007 diode. With correct polarity It only dropped a couple of millivolts with a 9V input. With reversed input polarity at 9V it limited current to 200mA almost instantly and then dropped closer to 100mA. With 18V input the reverse polarity current was even lower. I'm using this on a pedal I want to operate from a regulated 9V supply, or 18V supply for higher headroom. It's perfect.

RG's mosfet switch is a great idea, but I can get a polyswitch cheaper than a mosfet, and it offfers the bonus of short circuit protection!

Stew.

[Peter Snowberg]

I would suggest using a 1N5818 (or 1N5817 or 1N5819) rather than a 1N400x.

The polyswitch is a great idea. :icon_cool: 

[gez]

Thanks Paul, missed this one the first time round!  Just checked some data sheets and these things have really low 'initial' resistance! 

[Paul Perry (Frostwave)]

There appear to be 1A polyswitches in the "clearing" catalog at Rockby mail order in Melbourne Australia.
http://www.rockby.com.au  stock number 29825 (min quantity 10) for 20c Aust each.

[gez]

Rapid Electronics in the UK have some 0.1A polyswitches, which would be useful for preventing PSU fuses blowing:

http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=80010&CTL_CAT_CODE=&STK_PROD_CODE=M29182&XPAGENO=1

Plus a few others:

http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=80010&CTL_CAT_CODE=&STK_PROD_CODE=M31870&XPAGENO=1