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1. 06:33 AM - Re: Fuses and shorts (Robert L. Nuckolls, III)
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| Subject: | Re: Fuses and shorts |
At 03:35 AM 1/26/2024, you wrote:
>Hello Joe, hello Bob.
>
>Apologies for e-mailing you both off forum but I have difficulties
>when trying to post to the AeroElectric forum.
No problem.
>I'd like to ask a couple of questions and if it is possible, could I
>ask you to also post this request to the forum please.
>
>
>General question; what are the events that will actually cause a fuse to blow?
Fuses are, in general, fabricated from materials
with low melting temperatures compared to preferred
conductors like copper. Fusible elements
are not zero ohms conductors. Current passing
through them creates a voltage drop. Volts x Amps
equals Watts. Watts manifest in HEAT.
The thermal dynamics for a fuse are 'tuned'
to allow melting of the fusible element when
subjected to a particular current over time
event. Fuses come with an 'Amps' rating that
only generally defines its dynamic behavior.
Fuses have a 'fusing constant' mathematically
depicted as Amps(squared) times Seconds.
Consider the plots that describe characteristics
of the plug-in, plastic ATC style fuses:
https://tinyurl.com/yw8kttde
These curves tell us that at ATC fuse subjected
to 3X overload can be expected to open on the
order of 100 milliseconds. The 3A fuse can
be expected to carry 4A indefinitely.
>Also, the word "short" is mentioned together with blown fuses, but
>what exactly is meant by a short in our amateur-built aircraft?
Shorts are 'hard faults'; severe overload which
will cause a fuse to operate in a few milliseconds.
Shorts are generally caused by mechanical
compromise of the conductors that bring
them in contact with 'ground'.
>I have yet to find a really good source for advice on fault-finding.
>If you know of one, I'd appreciate hearing of it please.
You study the schematic . . . where does the
power go after the fuse? What components are
most likely to develop faults that overload
the circuit protection.
>Specifically I have a nav/com unit that has blown the fuse of its nav board.
>
>The unit has separate power supplies for nav and com and also for
>light dimming capability.
>
>This nav/com is connected to a cdi that has its own separate power
>supply and again a supply for light dimming.
>
>In the past the cdi vertical needle fluctuated, after having been
>steady, and if I
>remember correctly shortly afterwards the nav/com display showed nav receiver
>and gs receiver failure.
>
>It was on checking that I found the nav board power supply 5amp fuse
>had blown.
>
>I have replaced it and the fault messages have disappeared and the
>cdi needle is currently steady.
>
>But I am concerned that the fuse may blow again.
>
>If it doesn't, the issue that caused it to blow in the first place
>may damage the nav side of the unit with ensuing expensive repair charges!
It seems unlikely . . . internal fuses are
installed to prevent faults from 'spreading'.
>So you can see that I am trying to fault-find and hence my initial
>general questions.
>
>Any direction/guidance/information you and the forum can provide
>will be extremely helpful, thanking you both in advance.
If a replaced fuse does not blow immediately, then
you're looking for a soft fault . . . an overload
as opposed to a short . . . or an intermittent
short (unlikely inside a radio . . . but not
impossible.
How old is this radio? Do you have service documents
on it . . . i.e. schematics? Electrolytic filter
capacitors are early suspects for developing leakage
currents with age. If you have schematics, you
could consider replacing any such candidate
capacitors there won't be very many. Can you
scan or photograph the circuitry
downstream of the fuse and share it with us?
I'm sorry there's no short, turn-key answer to
your problem. Some grey haired technician with
years of experience with your radio might well
say, "Oh yeah . . . seen that before . . . no
problem."
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
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