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Re: Driving/Fog Lights


I added some individual relays to improve the voltage reaching the headlamps on my 1986
5000CS Turbo when I added some high wattage 9004 bulbs.  My nice new high wattage bulbs
started burning out only after 3 -6 months of use so I went back to the standard bulbs wattage
55/65W. Even with the stock bulbs I still was replacing the bulbs every 12-18 months.

Most bulbs burn out when they are first turned on (cold) due to low bulb resistance and the
fact that the inrush current is something like 5-10 times the normal operating current. I have
measured this using a current probe and an oscilloscope. This is also true for most household
light bulbs.

Previously I had gone several years without replacing any bulbs. I theorized that the shorter
bulb life was caused by 1. (lousy bulb manufacturer) or 2. The relays I added had lowered the 
voltage source resistance enough that the inrush current was being increased. Eventually
 this burned out  the bulb sooner than when I had the stock wiring with all of the added resistance 
from the fuse box through the headlamp switch and out to the lights.

Any one else had this sort of problem with lights burning out? Just one nerds opinion.

One of my PIAA 959  (85W) bulbs burned out after around 2 years of use.

Scott M.