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Spotlight fuse keeps blowing.


reb78

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I have a fused relay that feeds my spotlights. It's switched from the feed to the full beam headlights. It's a 15 amp fuse and lately it's been blowing occasionally. It doesn't go straight away and the fuse casing looks like it gets hot as it's melted a little.

I've had a quick look at the wiring tonight. Nothing appears to be shorting. I'll check again in the light at the weekend and also clean up the earth connection. What else should I be looking at?

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how many watts are the spots?, just thinking if you have a bad earth and the voltage is lowered the current will try and increase to keep up. Hence the fuse getting warm, especially if you are near the limit.

If the cable is up to it between the fuse and the spotlights you could use a 15A fuse to each. how thick are the wires feeding the lamps

55w/12v= 4.6A~ each lamp

if they are on with lower voltage

55w/9v= 6.1A ~each lamp

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Err, no it won't.

V = IR, therefore I = V/R, with an increase in resistance or a drop in voltage, current will fall.

As P = I2R we see that power is relative to the current and resistance, so your bulb will operate at a lower power.

Also, with P = V2/R , you can see that as voltage falls, the power falls with the SQUARE of the fall in voltage, assuming fixed impedance of 2.6 Ohms we get:

14V = 75W

12V = 55W

10V = 38W

Resistance changes as a bulb heats up, but for this demonstration it can be safely ignored.

Reb, what bulbs are you running? If 100W you are right on the limit, in fact on 55W, and potentially 15V from the alternator you are still quite close.

Have you tried using a different brand of fuse? Some are complete pap...

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Thanks chaps. Its been a long time since I looked at the bulbs but i think they are 55W in each lamp. These lamps are generally really reliable and have never needed a lot doing with them. I didn't install this setup - mum had it put in when she owned the 110 in about 1995 - its worked perfectly until this year when I noticed the fuse had gone a couple of months ago (same as above - a bit melty). I just replaced it and its been ok for the last couple of months but last night i noticed it had gone again. I imagine the fuse came from Halfords, but i will try another, check the wiring (including inside the housing) carefully in the daylight and clean the earth.

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Well...

P = IV

and...

I = P/V = 4.6A

With V at 12V, and P at 55W we can use....

V = IR

...or...

R = V/I = 2.6 Ohms.

You can also use P = I2/R to get there directly, which goes to...

R = P/I2 = 55/4.62 = 2.6 Ohms.

If you can't remember the power formulae, then you can get there from just P = VI and Ohm's Law quite quickly.

Think I may have just broken the internet with that last post.

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Just wondering where you got 2.6 Ohms from Bowie?

agreed with the rest of it but I=P/V i.e watts/volts ^_^

Bowie69 is correct. The power is a result of the voltage and the current - it doesn't create either of them.

So as he says, if the voltage across the bulb is reduced because of a high resistance elsewhere and the available volts from the battery can't change then then the current has to reduce because I=V/R

I would check the contacts for the fuse, I often find they get slack and cause resistance that ends up melting stuff.

and now that same lower current passing through the bulb and another resistance, heats-up the additional resistance at a place that isn't designed to cope with heat (as it shouldn't have resistance) and it melts.

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I had similar symptoms with the relay kit I used for my headlights. Cheap 'waterproof' fuse holder was letting in moisture and corroding the terminals leading to more resistance and a melted fuse. All replaced now and seems much better!

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