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Dash Idiot Light
ian.stewart - 19/10/23 at 05:03 PM

Ima touch troubled about my oil pressure light at the moment, the way I see it is the light is a switched feed, so 12v into the bulb and the bulb is then earthed via a oil pressure switch, for some reason, I wired the light thru a 7.5a fuse, while not doing anything electrical like turning things on, or fiddling, after about 5 mins the fuse blew? bulb is still good, changed the fuse for a 10a, after about 8 mins the fuse blew again, ?? this is a dedicated circuit, my limited maths says 12v feed, 2watt bulb, the ampage should be 0.66a so why would the fuse blow? a high resistance in the switch?

gremlin1234 - 19/10/23 at 05:39 PM

quote:
Originally posted by ian.stewart
Ima touch troubled about my oil pressure light at the moment, the way I see it is the light is a switched feed, so 12v into the bulb and the bulb is then earthed via a oil pressure switch, for some reason, I wired the light thru a 7.5a fuse, while not doing anything electrical like turning things on, or fiddling, after about 5 mins the fuse blew? bulb is still good, changed the fuse for a 10a, after about 8 mins the fuse blew again, ?? this is a dedicated circuit, my limited maths says 12v feed, 2watt bulb, the ampage should be 0.66a so why would the fuse blow? a high resistance in the switch?


changing a 7.5A fuse to 10A just because it blows is a VERY BAD IDEA

a 2W bulb at 12 V should take less that a quarter amp, - actually 0.166A (166mA)
so ideally change the fuse to something under 1Amp.

it won't be 'the resistance of the switch' as this is meant to conduct to ground to light the light.

I have seen bulbs, (and more usually their holders) with internal shorts.

if its not the bulb/ holder at at fault, then it's a fault in the wire between the fuse and the bulb.

ian.stewart - 19/10/23 at 06:27 PM

I know putting the bigger fuse in is a bad idea, what confuses me is if the circuit is pulling 10a, the bulb surely @0.166a will act as a fuse in its own right and why does the fuse take so long to blow, if it was a dead short the fuse should blow almost instantly.

gremlin1234 - 19/10/23 at 06:54 PM

quote:
Originally posted by ian.stewart
I know putting the bigger fuse in is a bad idea, what confuses me is if the circuit is pulling 10a, the bulb surely @0.166a will act as a fuse in its own right and why does the fuse take so long to blow, if it was a dead short the fuse should blow almost instantly.

the bulb could be considered a ~250mA fuse, but its own resistance is enough to limit the current at 12V.
if you put the same bulb across say mains voltage it would blow instantly

you could remove the bulb and replace the fuse with say a 1A, and then see what happens
[if the fuse then blows it's obviously not the bulb]

gremlin1234 - 19/10/23 at 09:00 PM

I just did the maths,
you could use a 12V,2W bulb as a fuse for a 40W mains (240V) circuit. - the 12V bulb would light brightly, but not fuse/fail.

ReMan - 20/10/23 at 12:54 AM

These should not be blowing.
You going to gave to check the components of what you rightly describe is a very simple circuit.
The ligt/fitting is a possibility, but also could there be an inadvertent feed from this simple circuit to another load or a feed to earth in some additional wiring on the fused side as well as the lamp?

coyoteboy - 20/10/23 at 08:49 AM

quote:
Originally posted by gremlin1234
I just did the maths,
you could use a 12V,2W bulb as a fuse for a 40W mains (240V) circuit. - the 12V bulb would light brightly, but not fuse/fail.


A bulb that normally runs at .16A would be running over 3.3A in a 240V circuit, I assure you that would blow - P=I2R - in the main circuit it's burning 800W. Not only would it blow, but if it didn't it would melth the holder, the wiring and the dash

Regardless, back to the original problem - you have a short across or before the bulb somewhere. Either in the bulb holder, or in the wiring along the way, this isn't a problem with the circuit, or the switch - you should be able to take the connection off the switch and short it to ground anywhere and have no problem.

[Edited on 20/10/2023 by coyoteboy]

gremlin1234 - 20/10/23 at 02:41 PM

quote:
Originally posted by coyoteboy
quote:
Originally posted by gremlin1234
I just did the maths,
you could use a 12V,2W bulb as a fuse for a 40W mains (240V) circuit. - the 12V bulb would light brightly, but not fuse/fail.


A bulb that normally runs at .16A would be running over 3.3A in a 240V circuit, I assure you that would blow - P=I2R - in the main circuit it's burning 800W. Not only would it blow, but if it didn't it would melth the holder, the wiring and the dash

I meant in series with a 40W load, but yes it would vaporise on its own.

ian.stewart - 20/10/23 at 03:31 PM

Well, perhaps I should look before I post...the Idiot has been found.... OOOPS, one answer was, you might have an earth fault, Mmmmm that might just be staring be in face, there just happens to be a bundle of black earth wires staring me in the face with a ring terminal not attached to anything but air, screw this to a earth post, fit a 5a fuse, turn on the power light on, give it 5 mins, the light is still on go in, have a cuppa, cake and a chat, and 20 mins later, the light is still on, perhaps I Might have this sorted,

coyoteboy - 23/10/23 at 04:41 PM

quote:
Originally posted by gremlin1234
quote:
Originally posted by coyoteboy
quote:
Originally posted by gremlin1234
I just did the maths,
you could use a 12V,2W bulb as a fuse for a 40W mains (240V) circuit. - the 12V bulb would light brightly, but not fuse/fail.


A bulb that normally runs at .16A would be running over 3.3A in a 240V circuit, I assure you that would blow - P=I2R - in the main circuit it's burning 800W. Not only would it blow, but if it didn't it would melth the holder, the wiring and the dash

I meant in series with a 40W load, but yes it would vaporise on its own.


Ah I misinterpreted that, sorry!

Good job Ian, always nice to find a simple answer but I'm surprised that a missing ground did it?