Re: Hmm!


"Phil Allison" <philallison@xxxxxxxxxx> wrote in message
news:67kgbfF2p1nigU1@xxxxxxxxxxxxxxxxxxxxx

"Trevor Wilson"

I wonder what the DC resistance of a 250mA fuse might
be?

**Depends on the fuse. I just measured half a dozen 0.25 Amp 3AG fuses.
They ranged from 12 Ohms up to 15 Ohms. COLD.


** That is an unbelievably high resistance value - not at all
consistent with a 250mA fuse.

**Nevertheless, it is correct. I zero'd out the test lead resistance. I
double checked the figures with my handy dandy Bob Parker ESR meter. The
results were within acceptable limits.


Some no brand, 1/4 amp 3AG fuses I checked ranged from 3.5 to 4.5 ohms.

These 20mm "T" or delay fuses from Bussman are speced at 0.66 ohms for a
250mA size:

http://www.farnell.com/datasheets/81115.pdf

The specs show that R rises to 1 ohm at rated current.

**I checked the resistance rise of a 12 Ohm measured fuse. At 200ma, it rose
to 15 Ohms. I probably over-stated the reality when I said: "several times".
When I get some time, I'll take my measurements at levels closer to the fuse
ratings. Based on what I found, I'd guess that the resistance of the fuse
will almost double at it's rated blow current (500ma).




** Fuses act as non-linear resistors. They get hot and change resistance.


** Speaker voice coils do that too.

**They do indeed.

Trevor Wilson



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