[sdiy] Diode reverse polarity protection revisited

[Harry Bissell]

I'd be wary of the 10 ohms unless you have onboard regulation, or use really big onboard caps.
Significant voltage drop is possible with things like LEDs etc.  This also couples the active components
together at the rails.

No problem if you consider it first, just a warning to the less initiate (can you say newbie?) that there
are potential errors introduced. They might be insignificant, or maybe not...

H^) harry


----- Original Message -----
From: Joe Grisso <jgrisso at det3.net>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 11 Jan 2011 14:37:52 -0500 (EST)
Subject: Re: [sdiy] Diode reverse polarity protection revisited

Every module I've designed has a standard power input section that
includes a polyfuse for each rail, a reverse-biased shunt diode after
the polyfuse, a 10 ohm resistor after the polyfuse and filter
capacitors for local supply in case my module does something naughty
like induce ripple. The cost is so negligible compared to the pain and
suffering induced from improper hookup that I think every manufacturer
should be doing it.

If it's plugged in backwards, the polyfuse trips because the diode is
conducting.
If something happens downstream of the protection circuit, the polyfuse trips.

The 10 ohm resistor acts as a secondary fuse if the polyfuse fails to
a short (which has happened to me once), and the R in the RC network
for power filtering.

Also, Maxim has a really good application note on reverse voltage
protection. You can find it here:

http://pdfserv.maxim-ic.com/en/an/AN636.pdf

-- 
Joe Grisso
Detachment 3, Ltd.
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-- 
Harry Bissell & Nora Abdullah 4eva