Your requirements are not a great deal unlike modern automotive power source
protection requirements.
"Handle anything, anyone could possibly imagine before or after the fact"
Fred makes some great points. This approached can be enhanced by using an
automotive type "load dump tolerant" regulator. They simply detach from the
power source when te input voltage exceeds some (usually pretty high) level.
No problems with burning fuses, wires, etc or with pissing off UL types.
See TI for these, they have been doing a lot of work in this area.
If you feel the need for a fuse, I also suggest the Raychem PolyFuse
devices. I have had excellent luck with them in several applications. Just
remember that their guaranteed hold/release ratings cover about a 2:1 range.
Their RXE-250 devices, for example, are guaranteed (At 20 deg C) to hold at
2.5 A and trip at 5.0 A. And this is also a function of temperature. The
trip/hold points vary as a function of the ambient temperature.
To protect a 5.0 V bus, I prefer 6.2 V Zeners. Within tolerance they will
clamp at 5.89 V (comfortably above 5.0 V + 10%) to 6.51 V (comfotably below
the usual 7.0 V Absolute Maximum rating for 5.0 V parts).
As to power rating, just get out your slide-rule and data sheets. A 6.2
Zener will dissipate about 15.5 W at 2.0 Amps, but will have to handle up to
32 W at 5.0 A. Common axial lead zeners from On-Semi are rated 5 W
continous but handle 180 W surges up to 8.3 ms. This is ample time for a
PolyFuse to trip. See RamChem for some of their (very good) applications
data on these characteristics of PolyFuses.
I think DeClan's concern about sucking power near the zener voltage is not a
concern. I think he is thinking more about MOV type devices which do have a
very sloppy breakdown curve. Ok for TV sets, but not for this application.
The Zener "knee" is very sharp by comparison, leakage is in the uA level up
to the point of conduction.
Best wishes, Bob Smith
--- Avoid computer viruses, Practice safe hex ---
-- Specializing in small, cost effective
embedded control systems --
Robert L. (Bob) Smith
Smith Machine Works, Inc.
9900 Lumlay Road
Richmond, VA 23236 804/745-1065
[EMAIL PROTECTED]
----- Original Message -----
To: "Multiple recipients of list CHIPDIR-L" <[EMAIL PROTECTED]>
Sent: Tuesday, February 11, 2003 1:05 PM
> Funny thing happens when you try and protect against idiots. The world
comes up
> with a better idiot.
>
> Bob, there are many crowbar devices on the market that will limit the
voltage to
> some definable level. However I think there are several issues this
approach
> surfaces. What will limit or interrupt the current? A crowbar circuit
will
> keep drawing current until something shuts it off. Wall warts are
designed to
> be current limiting but not usually current interrupting. I can foresee a
> scenario whereby both the wall wart and the crowbar sit there and cook. I
have
> seen units with crowbars that still burned up.
>
> An approach that has worked for me is to use an AC wall wart and board
mounted
> bridge rectifier, capacitor, and an onboard regulator. This allowed me to
define
> the regulation and noise on my 5 volts and allowed the customer to use any
wall
> wart that would fit the funny little coaxial power plug.
>
> Fred Townsend
>
> Robert Ritchey wrote:
>
> > Hi,
> > I have a question about protecting a circuit from an overvoltage
> > condition. We
> > have a new device that we are planning to use a regulated 5V wall-wart
type
> > device for power. They have also requested overvoltage protection in
case
> > someone plugs in a 6V or 8V unregulated supply by accident. My
expertise
> > is mostly digital so I am not sure what the best and cheapest way to do
this
> > would be. Thanks in advance for any help or pointers.
> >
> > -Bob
> > --------------------------------------------------------------------
> > Robert Ritchey
> > Quest Engineering & Development
> > 1328 East Cottonwood Lane
> > Phoenix, AZ 85048-4765
> > Tel: (480) 460-2652
> > FAX: (480) 460-2653
> > E-Mail: [EMAIL PROTECTED]
> > E-Mail: [EMAIL PROTECTED]
> > E-Mail: [EMAIL PROTECTED]
> > E-Mail: [EMAIL PROTECTED]
> > WWW: www.QuestEngDev.com/
> > WWW: www.Smart-Fly.com/
> > --------------------------------------------------------------------
> >
> > --
> > Author: Robert Ritchey
> > INET: [EMAIL PROTECTED]
> >
> > Fat City Network Services -- 858-538-5051 http://www.fatcity.com
> > San Diego, California -- Mailing list and web hosting services
> > ---------------------------------------------------------------------
> > To REMOVE yourself from this mailing list, send an E-Mail message
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> > also send the HELP command for other information (like subscribing).
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>
> --
> Author: Fred Townsend
> INET: [EMAIL PROTECTED]
>
> Fat City Network Services -- 858-538-5051 http://www.fatcity.com
> San Diego, California -- Mailing list and web hosting services
> ---------------------------------------------------------------------
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> also send the HELP command for other information (like subscribing).
>
>
--
Author: Robert Smith
INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
San Diego, California -- Mailing list and web hosting services
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