Re: Driving/Fog Lights

[steveb@newkla.kla.com (Steven Buchholz)]

Scott M. with the most outrageous e-mail address said:
> 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.
> 
Your analysis makes perfect sense [to this nerd :].  I have seen poor quality
bulbs that have a short life, but you have to expect the improved wiring to
increase the stress to the filament at turn-on.  Perhaps the thing to do is
to wire the new stuff in parallel with the original wiring to provide a poor
man's "soft-start" circuit.  The original wiring supplies current to the bulb
until the voltage gets high enough for the relays to kick in.  I was origin-
ally saying this in a joking fashion, but it does make some sense ...

Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)