On 12/21/2020 3:39 AM, 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.
It's not clear to me that a thermistor would prevent a fuse from
blowing in a guitar amp, or any other circuit, that has a short
to ground. Because while the resistance may be high initially, while
the thermistor is cold, the resistance will go low as it heats up,
allowing the fuse to blow.
A thermistor also doesn't give you a visual indication that a
short-to-ground is in the circuit under test, as a light bulb does.
Remember a light bulb in series with the AC line, is mainly a
trouble-shooting tool.