Snap, crackle and pop!

[Mike Arman <armanmik@n-jcenter.com>]

>>
>>On my 93 100CSQ i have foglights in the bumper...  I just took out the fog
>>light relay and put in a 15 amp fuse, and jumpered two of the connections
>>together...  But now that i have 100w h3 bulbs in there, the fuse and wire
>>melted so i had to go with a 30 amp fuse now the wire doesn't even get
hot...
>>
>>Levent Cur
>>'93 100CS Quattro 5spd (Mine) 93,500 Miles (Mint)
>>'87 5000CS Turbo Auto (Sister's) 199,946 Miles (Very Near Mint)
>
>Hi Levent,
>
>Putting in a larger fuse will not make the wire feel any cooler. Power =
>Current squared x Resistance. Which component of that formula do you think
>was affected by a larger fuse? A fuse is a passive device which is nothing
>more than a piece of wire which is designed to melt when a given amount of
>current at a given voltage flows through it. In other words, you want the
>fuse to melt *before* your wiring does or your insurance company is going
>to own a very charred Audi. I suggest you have someone who knows what they
>are doing take a look at what you have done just to be on the safe side.
>
>Regards,
>
>Clifford Ilkay


Levent, Clifford is correct. It would be a shame for your car to burn down
over something like this - 

2 x 100 watt bulbs = 200 watts, which at 12 volts draws 16.6 amps (volts
times amps = watts: Ohm's law)

If the 15 amp fuse blew (which it did because you asked it to carry 16+
amps), going to a larger fuse will mean that the NEXT WEAKEST link will
burn out - in this case, it will be either wiring or a relay or the light
switch, and whatever it is, it will be much more expensive than the fuse!

If the fuse is rated 15 amps, that is usually 150% to 200% of the maximum
current expected in the circuit - so the wiring (which on Audi is not noted
for being especially over-engineered) is rated at 7 to 10 amps. - something
is going to give, and yes, there is a significant chance you'll have a
fire, or at the very least, will burn up an expensive and
expensive-to-replace wiring harness.

Put some relays in instead - use the wire that now goes to the bulbs to
trigger a pair of relays - run power to the relays from the alternator or
jump-start terminal, and then direct to the 100W bulbs.

Best Regards,

Mike Arman (52 years old, and fairly close to mint - hair long gone on top,
multi-focal glasses, bad attitude, but I love my Audi!)