[sdiy] circuit protection question

[blacet at blacet.com]

Just to be clear, the polyfuse does not "blow" (typically). It is a
resettable fuse that goes back to a low resistance state once the circuit
fault is corrected.


> On one of the Blacet modules I built, I had an electrolytic cap in
> backwards. It was across the power rails. The poly fuse blew and I was not
> getting +15 volts. Brilliant guy I am, I replaced the polyfuse with a
> jumper
> and promptly blew the cap. Quite exciting, but I did find the error :)
>
> I have on more than one occasion plugged a power connector in one pin off
> and have yet to blow a module. I too have incorporated the reverse diodes
> in
> the Steiner project I'm working on. I'm not using the polyfuses however.
>
> David
>
> -- -----Original Message-----
> -- From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> -- bounces at dropmix.xs4all.nl] On Behalf Of megaohm
> -- Sent: Tuesday, December 14, 2010 10:00 AM
> -- To: blacet at blacet.com
> -- Cc: synth-diy DIY
> -- Subject: Re: [sdiy] circuit protection question
> --
> -- On Tue, Dec 14, 2010 at 9:02 AM,  <blacet at blacet.com> wrote:
> -- >
> -- > Why don't any other companies use this? Too cheap? I dunno.
> -- >
> -- > People WILL plug things in backwards, it's guaranteed.
> -- >
> -- > John "the Guru" Blacet
> -- >
> --
> --
> -- John and all,
> --
> -- I use your technique on all my modules.
> -- I've come up with a few questions I'm wondering about:
> --
> -- It seems to me that the reverse diodes are doing the protective work
> -- for the module. What are the polyfuses doing? I'm figuring that they
> -- protect the PSU? I wondered if they protected the circuit in different
> -- ways. So I tried an experiment and put an op amp in backwards. It
> -- smoked.
> -- I'm also a bit foggy on the value of the polyfuse and figure it
> -- depends on the current draw of the particular module. Looking at a
> -- Bourns data sheet, generally, the lower the trip point the higher the
> -- resistance of the polyfuse in it's normal operating state. Doesn't
> -- adding resistance in series with the power lines make them less stiff?
> --
> -- Semi related:
> -- How about if someone plugs the power cable in and they are off to the
> -- side one pin?
>%