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From: Ian DuRieu
X-Mailer: Mozilla 4.77 [en] (Win95; U)
Subject: Current limiter
Organization: Ye 'Ol Disorganized NNTPCache groupie
X-Cache: nntpcache 3.0.1 (see http://www.nntpcache.org/)
Date: Tue, 14 Jan 2003 01:12:35 +1030
NNTP-Posting-Date: Tue, 14 Jan 2003 01:41:18 EST
I have an application which feeds +DC (typ 12-30V up to 1 amp) to a
connector for external use.
DC volts are derived from a 20 to 100W transformer, depending on
application. It's just a bridge & filter cap.
I need to protect against external cable faults. Don't want to melt the
cable or cook the power supply.
I need a self resetting option.
A polyswitch would be ideal except that I wish to limit the fault
current so that I don't blow the transformer's mains fuse. 2-3A max
fault current (load side) would be good. The mains fuse is faster than a
A current limiter like a 317 would work except that it's forward losses
are a bit high. A 317 drops just over 3 volts min when used as a current
regulator. 1.25V across the sense resistor and a couple of volts across
It also needs a sink when running a 1A load.
It does have one nice feature in that it has a foldback current limiter
for when the I/O volts are big. And its thermal shutdown is handy if you
size the heatsink such that it goes over temp under short conditions.
OK, I know that's a bit rough, but it is used in commercial equipment.
A current source using discrete parts works fine. i.e. low running
losses, but it lacks thermal shutdown or foldback limiting.
Sure I could add all that stuff, but it's starting to move away from an
elegantly simple solution.
Any thoughts or ideas?
Ian Du Rieu
The Leon Audio Co
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