fusible resistor substitute

How did you calculate that? P=I^2R --> I=sqrt(P/R) -->

sqrt(7W/1.5ohm)=2.16A

2 amps, rather :-) and I wouldn't use one with any higher rated current than "the original one (the resistor)" had...

It depends on that did the resistance of the resistor have any purpose, or was it just to heat it up so that it will work as a fuse. That is of course highly possible in case of such low resistance...if you can tell it, your idea might work, although it sounds a little crazy :-)

On 25 Jun 2006 09:41:06 -0700, " snipped-for-privacy@yahoo.com" put finger to keyboard and composed:

I've always wondered about how fusible resistors were rated.

Here is the datasheet for Vishay's CMF fusible 0.25W/0.5W metal film flameproof fusible resistors:

The average fusing time for the 0.25W versions is 1 sec at 15W, 16s at

5W, and 24s at ~4W. That's a lot slower than a typical glass fuse. The fusing specs also state that lower resistance parts take longer to fuse than higher resistances.

Here is the datasheet for the LCA...SI carbon fusible resistor series:

The LCA0207SI has a power rating of 0.25W but requires a 4A overload before it will fuse. At this current the maximum time to fuse is 15 secs. So it seems that the equivalent circuit is ...

o---- R ---- F ----o 1/4W 3A ???

I'm conFUSEd ...

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:

Use a SLOW BLOW fuse. They have the same characteristic you're describing, blowing faster for bigger currents.

Lenny: If it were me and I was able to pinpoint the exact fault that caused the 7 watt resistor (like a shorted semiconductor or capacitor,etc.) to burn out I would without hesistation use a 10 watt replacement (forget the fuse substitute) and let the set run for an hour or so and test for overheated components and excessive current through the resistor. Usually this type and value of low ohm resistor is used for current limiting and/or inrush current control and in light of another drastic fault condition, the 10 watt resistor would perform it's task and open up just as the original value did. electricitym

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snipped: snipped-for-privacy@yahoo.com wrote:

On Mon, 26 Jun 2006 08:03:00 +1000, Franc Zabkar put finger to keyboard and composed:

Oops, that should be 4W, not 4A, so the fusing current is 4x rated current.

- Franc Zabkar

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On Mon, 26 Jun 2006 02:30:58 -0000, Jim Land put finger to keyboard and composed:

True, but it would have to be a *very* slow fuse.

For instance, these slo-blo fuses require only 0.6 sec before they rupture at 4x rated current:

OTOH, the fusible carbon resistors require 15s to rupture at 4x rated current.

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.

I think I'd look for a 1 ohm 5 watt fusible resistor. The lower resistance will roughly accout for the lower wattage rating - yet still give the protection.

Mark Z.

The explanation to that confusing thing might be that the fusibility of fusible resistors is intended just for preventing fire hazard if the res is overloaded, rather than to protect other components on the circuit. So the only thing that matters is that the "fusing point" is below the resistor's ignition point :-)