[sdiy] circuit protection question

[David]

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?
-- Say...+15V header is now connected to PSU ground and PSU ground and
-- -15V are connected to the header ground and the -15V header pin is
-- unconnected (the power connector is shifted one over where it should
-- be).
-- 
-- Your article convinced me to use your technique years ago. I'm just a
-- bit embarrassed that I don't know everything that is going on with
-- this.
-- 
-- p.
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