[Don Pearce[_3_]]

On Mon, 21 Dec 2020 10:39:57 GMT, (Don Pearce) wrote:

On Sun, 20 Dec 2020 16:44:56 -0700, Paul Dorman
wrote:

This was used for a guitar amp that was continually
blowing slow-blow fuses instantly upon turning the amp
on.

https://www.youtube.com/watch?v=k5oNQ1etN2c

In my case, the bulb would briefly light up, and then
fade out within about a second. I read this meant the
amp was normal, and didn't have a short to ground, and
when I plugged the amp into the wall normally, I was able
to trouble-shoot it normally.

But I would assume the bulb lighting up initially, is due
to the initial in-rush current, that charges up the electrolytic
filtering caps, on the outputs of the rectifiers?

Because the initial 1 second flash only happened with a
linear power supply guitar amp, and did not happen with
my switched-mode guitar amp. With the latter, the bulb
never glowed noticeably, even at turn-on.

You will find that the bulb has a rather finite lifetime.Instead use
the part designed for the job, a negative temperature coefficient
(NTC) thermistor. Any switched mode power supply is most likely to
already have one fitted, which is why your bulb seems to do nothing.

d

I should add. The cold/hot resistance change is much greater in the
NTC thermistor, which means that while it does a much better job of
limiting the initial inrush current, its resistance once it reaches
its final state will be only a few tens of milliohms, so it becomes
essentially invisible to the circuit in operation.

This would be a typical choice

https://uk.farnell.com/ametherm/sl22...tor/dp/1703898

d