PTC's/Polyfuses.

[raulduke]

Polyfuses/PTC's: I have seen these for quite cheap recently over at Farnell.

http://uk.farnell.com/multicomp/mc33169/fuse-ptc-reset-radial-60v-100ma/dp/1800726

You don't see them much in commercial fx pedals as far as I can see, but at the right rating (eg. 100ma or even lower if available) they could be another usefull protection component for shorts, damaged IC's pulling excess current etc, and also from preventing any damage to external PSU's etc. in the case of a pedal pulling too much current under fault.

Any of you guys got any thoughts on em?

I'm thinking I may plonk a few on my upcoming PCB's PSU filtering/protection stage.

[RobA]

I picked some up a few months ago and I tested them on a power supply input for limiting the current flow through a crowbar protection scheme. They did blow and reset like they should. But, there was something that made me think it wasn't going to work. But, I can't remember what. Damn senility! .

I'll have to pull them out again and see if I can figure out what the issue was.

[RobA]

OK, so I pulled them back out to play with. I used the MF-R005 in this spec sheet,
http://www.mouser.com/ds/2/54/mfr-72330.pdf

First thing is that my main goal was protection from a reversed voltage on the input. I wasn't thinking about other short circuit situations.

The setup I envisioned was to put one of these right at the input of V+ and then put the 1N4001 on its opposite leg and tied to ground in the usual way from there. This does work. There are two issues I see this time. The first is that it's not an instantaneous shutdown. There is a noticeable lag before my voltmeter drops the negative voltage. The second thing is that the little guy gets hot. So, I was worried that if someone plugged in and the power supply didn't trip, then someone might just leave it plugged in for some time.

So, I figured that I would just go with a 1N5817 on the input with the 1N4001 to ground right after that and that's what I've been doing.

It occurred to me that you could get the short circuit protection with the reverse bias protection by doing all three together, 1N5817 with a following PTC in series and then the 1N4001 to ground.  I set this up and then dead shorted the power rail to ground. It works. The PTC does heat up, but the rest of the circuit is protected well and the power supply didn't see the short enough to trip it's protection.

Then the reason I didn't stick with this last time came back and reminded me . When I pulled the short circuit off, the power came right back on and my circuit powered back up, but the voltage was about 1V lower than before I did the short circuit. It took about a full 5 minutes for the V+ rail to recover all the way.

If you can live with that last limitation, I think the idea would work well.

[rullywowr]

You don't see them much in commercial fx pedals as far as I can see, but at the right rating (eg. 100ma or even lower if available) they could be another usefull protection component for shorts, damaged IC's pulling excess current etc, and also from preventing any damage to external PSU's etc. in the case of a pedal pulling too much current under fault.

This.  Polyfuses have a voltage drop and when they come back online (presumably after they cooled down) they can take a while to get back up to full voltage.  This may or may not work for the original intended purpose.  In addition, they do not like to be stuffed very close to other components (traps the heat) - they need space to cool off to run properly.  IMHO think they are best suited for devices which draw in excess of 250mA...places where you would want to put a fuse but a "auto-reset" type instead.

Since most modern 9v power supplies (one spot, pedal power, etc) have some sort of protection already built in...they are redundant when used in pedals.  But if it makes you happy and you really just want to add an additional layer of protection (and have the room to space them out) then by all means go for it! 

[raulduke]

You don't see them much in commercial fx pedals as far as I can see, but at the right rating (eg. 100ma or even lower if available) they could be another usefull protection component for shorts, damaged IC's pulling excess current etc, and also from preventing any damage to external PSU's etc. in the case of a pedal pulling too much current under fault.

This.  Polyfuses have a voltage drop and when they come back online (presumably after they cooled down) they can take a while to get back up to full voltage.  This may or may not work for the original intended purpose.  In addition, they do not like to be stuffed very close to other components (traps the heat) - they need space to cool off to run properly.  IMHO think they are best suited for devices which draw in excess of 250mA...places where you would want to put a fuse but a "auto-reset" type instead.

Since most modern 9v power supplies (one spot, pedal power, etc) have some sort of protection already built in...they are redundant when used in pedals.  But if it makes you happy and you really just want to add an additional layer of protection (and have the room to space them out) then by all means go for it! 

Thanks for the info guys; I must say I didn't consider the heat dissipation side of it. In a closed enclosure with no air outlets they may not be the best idea.

I might buy a couple just to play around with anyhow.