RE: headlight relays...

[rawaudi@frugal.com (Robert Wheeldon)]

>Is there any cheap and simple way to make it "soft on"?? Someone suggested
>tube diode. I got the foggiest idea what tube diode is...

Well, I guess that depends upon what your defnition of "cheap and simple" is
and what the market for such a device is.

The problem with incandesent (that includes halogens) lamps is that the
resistance of the lamp when cold is several times lower than when hot.  Thus
when you turn on an incandesent lamp, there is a huge rush of current thru
the bulb.  Before the filament warms up and reduces the current thru the
bulb, the smallest cross  section of the filament gets VERY hot and
eventually acts like a fuse.  So, a soft start circuit needs to limit the
current thru the bulb until the filament heats up then apply full voltage to
the lamp.

The simplest thing I can think of is placing a resistor (or another lamp) is
series with the bulb with a time delay relay to short out the resistor after
a second or two.  Of course, you have to add this to EACH lamp you want to
soft start.  Unfortunately, most time delay relays are designed to only
switch a fraction of an amp or so and you need to switch something like 5 amps.

Now, if you were selling a million of these devices a year, you could design
a custom part that would cost a buck to manfacture.  Unforutnately, your
fixed costs would be $$$$$$.

I'm sure a simple soft start circuit could be made.  Would it be anywhere
near as cost effective as replacing a $5.00 headlight - NO.

Regards,


Robert Wheeldon
'90 V8Q
Principal Engineer
Boeing Commercial Airplane