From: "Christopher R. Carlen"
Subject: Re: simple over voltage protection?
Date: Wed, 18 Sep 2002 17:15:08 -0700
Organization: Sandia National Laboratories, Albuquerque, NM USA
NNTP-Posting-Date: Wed, 18 Sep 2002 19:20:15 +0000 (UTC)
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.0) Gecko/20020529
X-Accept-Language: en-us, en
Jim Thompson wrote:
> On 18 Sep 2002 11:24:41 -0700,
> firstname.lastname@example.org (Gary Lecomte),
> In Newsgroup: sci.electronics.design,
> Article: ,
> Entitled: "Re: simple over voltage protection?",
> Wrote the following:
> |"Brill Pappin" wrote in message news:...
> |> I'd like to provide some very basic over voltage protection for a device I
> |> am designing... there are obviously several ways to do it, but I wondering
> |> what simple methods others have had good experience with.
> |> Any comments or schematics I can take a look at?
> |> - Brill Pappin
> |It somewhat depends on the application and the voltages.
> |Why not put in a voltage regulator? Or use an MOV and fuse.
> If you're trying to provide a back-up safety against a failed
> regulator, use a crow bar to blow a fuse.
> ...Jim Thompson
In case you need more detail: a crowbar is an SCR. Stick a fuse in
series with the input to the device. After the fuse, an SCR anode to
(+), and the SCR cathode to GND. The gate goes to the midpoint of a
network composed of a zener diode cathode from (+), anode to the SCR
gate, then from SCR gate through a 1k (for typical low voltage stuff)
resistor to GND. Choose the zener voltage just a notch above your
running voltage, or just below your maximum tolerable voltage. I use a
5.6V zener for protecting 5V circuitry, for instance. Oh, a 0.1uF
capacitor would be wise from SCR gate to ground also, for noise avoidance.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA