roy@alanine.phri.nyu.edu (Roy Smith) (04/26/91)
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?
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"larry@kitty.UUCP (Larry Lippman) (04/26/91)
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?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry
tomb@hplsla.HP.COM (Tom Bruhns) (05/01/91)
larry@kitty.UUCP (Larry Lippman) writes: ... > 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. etc. I have some low-current fuses that also have standard-looking resistors complete with color code. My impression (don't have them handy right now to check if this is always true) is that the slow-blow part depends on the resistor heating enough to melt a junction which is pulled apart by a spring force. I don't believe they depend on the destruction of the resistance element itself to provide the fusing action. Note that 1/16 amp slow-blow 3AG type is about 85 ohms; looks like constant I^2*R... so expect the voltage drop at rated current to go up at low currents, to keep V*I(max) approximately constant. Might want to keep this in mind when fusing low-current circuits...