Soft start: the geekiness continues.

Not long after I geeked up my commuter with LEDs, I had my 10W halogen headlamp blow out when I powered it on with a fresh battery for the ride home. Shortly afterwards, my roomie's 10W halogen also blew at power-on with a fresh battery, which got me thinking: he bought the same headlights at the same time as I did, and we each got less than 100 hours of runtime out of them, far less than the 2000 hour average bulb life that bulb makers advertise.

I figured -- without any data to back it up -- that the stress of the sudden heating at power-on, particularly with the higher voltage of a fresh battery was shortening bulb life. So, I cooked up a "soft start" circuit which fades the light in over about one second, to go easy on the filament. A nicer solution would be to wire up some high-flux LEDs with optics to yield blowout-proof lighting, but this was quick and easy to do.

Here's the circuit; it's a simple three-wire job which sits in series on the ground leg of the light, past the power switch. I've prototyped it, testing with 10W, 20W, and 30W light loads on my 12V SLA battery, and it worked fine on all of these. I've also tested it down to 10.5V (with the 10W only; I'm lazy). I measured total overhead at 39/92/201mW for 10/20/30W loads, nearly all of which is heat dissipated in the power MOSFET, Q1.

This one may take a bit of tweaking (beyond just getting a lower-voltage D1) to adapt from 12V to 6V operation, due to the gate-to-source threshold of Q1 (up past 4V for a solid turn-on) being closer to the battery voltage, especially as the battery runs down. Depending specifically on the MOSFET used, it could work out, but I haven't really thought about it. One could eliminate the R1/D1 regulator, and make the timing circuit track the battery voltage (perhaps with a voltage divider), though that would lead to a slower turn-on at lower voltages.

I didn't actually build this circuit and put it on my old war horse, because my days of >10mi night-time urban commutes are almost over; I'm moving soon, but I figured I'd share.

Have fun,

JAB