The simplest way to protect it is to put a 5.1v zener diode across the input. Just
make make sure that there's a fuse between it and the power supply! If a zener won't
handle the current that the PS is capable of delivering then build a crowbar circuit
using a SCR with a zener diode controlling the gate.* You shouldn't have any trouble
finding the schematic for such a circuit. Many power supplies incorporate them for
protection in the event that the PS fails and outputs too much voltage.
You should also consider putting a diode in the input circuit to prevent trouble in
the event that someone decides to plug in an AC output wall wart or a wall wart that
has the polarity reversed. Again, a fuse between it and the power source is a good
idea.
*Actually the zener doesn't have to be capable of handling the entire PS current.
Only perhaps twice the amount of current that the load will draw. That's because the
fuse will blow if the current is excessive and the fuse rating should be based on the
expected load current. The zener should be capable of handling perhaps twice as much
current as the fuse rating. The zener should be rated higher since fuses will
frequently carry more current than they're rated for and they take several seconds to
blow even with exceesive loads.
Joe
At 07:59 AM 2/11/03 -0800, you 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]
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>E-Mail: [EMAIL PROTECTED]
>E-Mail: [EMAIL PROTECTED]
>WWW: www.QuestEngDev.com/
>WWW: www.Smart-Fly.com/
>--------------------------------------------------------------------
>
>--
>Author: Robert Ritchey
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