[sdiy] circuit protection question

David dingebre at 3dphysics.net
Tue Dec 14 20:43:29 CET 2010


Ok, fine :) 

I was using "blow" in a over general sense referring to the fact current
stopped flowing and I was very sad the module didn't work.

David

-- -----Original Message-----
-- From: blacet at blacet.com [mailto:blacet at blacet.com]
-- Sent: Tuesday, December 14, 2010 11:41 AM
-- To: David
-- Cc: 'megaohm'; blacet at blacet.com; 'synth-diy DIY'
-- Subject: RE: [sdiy] circuit protection question
-- 
-- 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?
-- >%




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