Angel Vilaseca wrote:
> Commercially available HV fuses are quite expensive.
>
> I thought of using a small spring instead, like the ones in ball-point pens.
>
> They are about one inch long and could be spread between two isolated
> standoffs about 2 1/2 inches apart.
>
> The ball-point pen springs are made with thin wire, but could easily
> carry several hundred mA.
> In case of a flashover current through the tube can reach tens of
> Amperes. In that case the wire would melt, the spring would split and
> the two halves would quickly retract, in less than a millisecond and
> stay about one inch apart, so the arc would quickly extinguish.
>
>
This sounds good, but "I think" (not having tried it) that the bronze
spring would lose its temper before the wire burned through leaving a
rather limp wick. Most fuses have a much thinner portion for the rated
current with the heavier portion acting as a heat sink. High voltage
fuses (the ones I've seen and worked with) are usually long, on the
order of 3 to 6 inches and consist of a thin conductor packed in sand
inside a ceramic tube about a 1/4" in diameter.
It would be interesting to run some tests to see what the failure
current of the standard spring would be against both abrupt shorts as in
a shorted tube or flash over and one where the current is brought up
slowly. I think the spring would lose its tension due to the constant
conductor size, but this would sure be worth the time experimenting.
I think the problem could be circumvented using two sizes of springs
with the smaller just being soldered to the larger. The larger along
with the mass of the solder would serve as a heat sink to prevent
premature failure at the joints. The springs out of the larger pens may
have a conductor as much as 3 to 4 times the wore size of the smaller ones.
OTOH We could probably build a fuse using a single wire, the ceramic
tube, and sand Leads would need to be heavier and the ends of the tube
could be sealed with plain old epoxy.
73
Roger (K8RI)
> I wonder if this would work. Has someone tried it?
>
> Old Philips TV sets had a similar system on some power resistors. They
> were soldered on two small springs. If the resistor became too hot, the
> solder would melt and the springs would retract, thereby interrupting
> the circuit.
>
> Angel Vilaseca HB9SLV
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