Lights: A no frills soft start

[bigwheel]

This is in reference to the Bike Lights - Part 2 article, discussed in another thread.

If you are building a homebrew light and plan to use a Li-Ion battery and a 20 watt bulb, you are going to need a soft start circuit. The problem is that while the bulb is heating up, there is a current surge which causes the battery's built-in safety circuit to shut down. Soft start circuits are not very complicated to build, but still might be a problem for people who are not very handy with a soldering iron.

Here's an idea, which I have not tested, but I'm almost certain it will work. The basic idea is a manual soft start. This is somewhat of a neanderthal approach, which might appeal to single speed riders.

It uses two switches: Switch1 is the on-off switch. Switch2 is the soft-start switch, which bypasses a resistor, which is wired in series with the circuit for soft start.

For the resistor, go to radio shack and get an 8-ohm power resistor Part# 271-120.


Batt + ------------------------ switch1 --------------------switch2------------------------------> To light
.................................................. ..........|............................ |
.................................................. ..........------ Resistor-----------

Batt - -----------------------------------------------------------------------------------------------------> To light


Edit: The ascii art doesn't seem to be working well. The resistor is supposed to be connected to both sides of switch2. Ignore the dots.

For lights off: Switch1=off, Switch2=off
When initially turning on the lights: Switch1=on, Switch2=off
After a second or two (normal on): Switch1=on, Switch2=on

Warning: Do not run the lights with switch2=off for more than a couple seconds. The resistor will get really hot.

I'm not planning to build this, but I'm almost certain it will work. If you try it, please post your results.

[bigwheel]

Good News:

Rob Campbell and I both observed that *IF* you are using the 6 amp-hour Li-Ion battery (from eBay) and a 20-watt (or less) halogen bulb, you will NOT need a soft start circuit. I guess that the 6AH battery is more powerful and comes with a different circuit than the 4AH one that comes in the water bottle.

This will make it a lot more straightforward to build your homebrew lights using Lithium batteries. (You will get 3+ hours of lighting pleasure with the 6AH battery pack and a 20 watt bulb.)

However, I tried hooking a 35 watt bulb to the battery, and it didn't work without soft-start. The bulb would flicker, but not light up. However, it works fine with the soft-start or my high-low beam circuit.

Also, the safety circuit resets itself immediately without destroying itself.

[robc]

Just an FYI : These are no longer listed on eBay for the moment, but I emailed him and he has about 8 more for sale right now. They go for $25 plus $7.50 shipping which is a great deal for these batteries. I just ordered one more for me and a 2 for a couple friends, so he should still have a few more after that. The pack is flat and fits nicely in the camelbak, which will make it perfrect for helmet lighting.
In case you want to get one, his email is jason@traderhill.com

-RobC