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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: Resistive fuses?
Message-ID: <5170130@hplsla.HP.COM>
Date: 30 Apr 91 21:28:50 GMT
References: <1991Apr25.170500.7294@phri.nyu.edu>
Organization: HP Lake Stevens, WA
Lines: 28

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...