The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Winfield Hill
Subject: Re: A protection problem
Date: 7 Jan 2003 13:57:31 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
"Ken Taylor wrote...
> "Ken Smith" wrote ...
>> I'm currently working on a voltage regulator with a major
>> protection problem. The specs are:
>> Input voltage 23 to 28V. This is easy enough.
>> Output voltage DAC controled 0 to 25V (doesn't need to boost make
>> 25V when input is too low). This isn't too hard.
>> Output current limited at 100mA. This isn't too hard.
>> Some moron may hook a (+) or (-) 600V supply to the output. This
>> 600V supply can supply a zillion amps so MOVs aren't the answer.
>> PTC devices don't seem to be able to handle this case either.
>> I'm tinkering with various active circuits but they all seem to
>> end up with quite a few parts in them. Does anyone have an idea?
> If a fuse isn't going to do the trick, shouldn't you be looking
> at designing the system so it *can't* be hooked up to 600V?
> Prevention being worth several pieces of cure.
Properly used, a fuse should work fine in helping to create a
highly robust output. For example consider a setup like this,
. ,----||--- gnd
. | Schottky TVS
. +Vs --+---[<|----+---|<|>|--- GND
. __|__ |
. | | | inductor fuse out
. ---| AMP |--OOO--+----OOO------o/\/o---(O)
. |_____| |
. | |
. -Vs --+---|>]----'
. '----||--- gnd
where the TVS part is a bidirectional silison transient voltage
suppressor. These devices turnon in sub-nanoseconds, and the
larger ones can handle surges of hundreds of amps. For normal
output and operating voltages the TVS simply looks like a few
nF capacitor. For an event of instantaneous 600V application,
the inductors limit the instantaneous currents, the Schottky
diodes insure that the AMP doesn't experience voltages beyond
either supply rail, and the BFC limits the rate of supply-rail
increase as it absorbs external fault currents. In the event
of a monstrous application, overwhelming the BFC and increasing
the rail voltage by more than 10 to 15V, the TVS zener voltage
is finally reached and it draws enough current to insure blowing
the poor fuse, which has probably already blown by that point.
Of course critical wiring is low resistance and low inductance.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup