Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ub!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Resistive fuses?
Summary: Slow-blow fuses & an important point about using fuses
Keywords: fuses
Message-ID: <4906@kitty.UUCP>
Date: 26 Apr 91 14:17:05 GMT
References: <1991Apr25.170500.7294@phri.nyu.edu>
Followup-To: sci.electronics
Organization: Recognition Research Corp., Clarence, NY
Lines: 45

In article <1991Apr25.170500.7294@phri.nyu.edu> roy@alanine.phri.nyu.edu (Roy Smith) writes:
>	Yesterday, somebody brought me a fuse (looking for a replacement)
>from a piece of laboratory equipment which has me puzzled.  It looks on the
>outside like an ordinary fuse (the small glass cylinder type, about an inch
>long, with metal ends), but inside had a small (1/8 W?) 22-ohm resistor in
>series with the fuse wire.  It was an 1/8A fuse.  Didn't say slow-blow on
>it, so I assume it wasn't.
>	Anybody have any idea why it contains an internal resistor?  Why
>not simply have the same resistor mounted externally in series with the
>fuse holder?

	I have never seen a fuse containing an internal resistor with an
explicitly identifiable value as you describe.

	However, what you describe sounds *exactly* like a Littlefuse-brand
slow-blow fuse rated for 1/8 ampere.  If you were to measure the resistance
of a Littlefuse 3AG-type slow-blow fuse (P/N 313.125) you will most likely
measure a nominal 22 ohms.  Many slow-blow cartridge fuses are designed with
two elements.  If you closely examine a typical 1/8 ampere Littlefuse slow-blow
fuse, you will most likely see a black series element which is in fact a
resistor.  However, it is not marked as a resistor.

	I would venture that you simply have a 1/8 ampere slow-blow fuse.

	Many engineers do not realize that sensitive fuses of 1 ampere and
less - whether slow-blow or not - may present a significant amount of series
resistance.  Look at the above example: a 1/8 ampere slow-blow fuse has a
resistance of 22 ohms.  This is usually not a problem when the fuse protects
a power supply (at most, we might have a voltge drop of 2 volts), but it can
be a serious problem if the fuse is used to protect an instrumentation circuit.
Furthermore, the resistance of a given fuse will be very dependent upon its
ambient temperature.

	Manufacturers of sensitive cartridge fuses make little attempt to
maintain any tolerance over the resistance presented by their fuses.  They
also don't tell you that their fuses have such resistance.  Look in a
typical Bussmann or Littlefuse catalog, and I bet you will find no mention
of series resistance, let alone find an actual specification.  As another
example, a regular (not slow-blow) 3AG-type 1/8 ampere fuse has a typical
resistance of 15 ohms - which is simply the resistance of less than one
inch of fuse wire.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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