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Friday, November 13, 2009

DIY CFL HL w/DC-DC Inverter

'Nuff acronyms? ;)

It's not pretty, and it probably won't work too well for too long, but it is just a test to see if it is worth pursuing this idea. The CFL needs a minimum of 42 volts to properly kick on, and the battery is around 41-ish at full charge, and at the end of most rides is around 38. That means about 4V needed to make up that difference to reliably start the CFL. Once it's started, it remains running down to much less than safe LVC for the 36V SLA pack. It only takes around 200mA at full 115VAC, and is about 180mA at 42V.

Basically, I opened up a cheap 59-cent CFL, soldered a pair of wires to the + and - of the DC-side ripple-filter capacitor on the input (bypassing all the 115VAC-only stuff, such as an inductor and a bridge rectifier, as well as a 1.5 ohm resistor being used as a fuse).

The + wire goes to a DC-DC converter made out of a Samsung AC-input 5VDC output celphone charger's + output. The - wire goes to the output of the breaker/ammeter, basically to pack negative.

The Samsung celphone charger (SCC) is isolated, so either wire of it's own output can be wired back to any point on the pack without a problem. It's ac input is soldered one side to pack negative (via the breaker), and the other to pack positive. It's DC output is wired + to the CFL's + input, and - to the pack positive.

This gives me a volt or so headroom, which is enough for the test trip, which will be around 24 miles, with a partial recharge at the other end.

Ok, so it's not exactly safe to just hang a CFL off the front of the bike, right?

Well, I'm tired and in a hurry to get something together for tomorrow's test trips, and so I can test it out now while it's still dark to make sure it actually works well enough to see by and be seen. (especially the latter). So it is not pretty, but it will work for a test.

I took a styrofoam Denny's cup with the bottom knocked out and fit it inside a 32-oz harder plastic QT cup. The QT cup's bottom is slit in quarters, and the base of the bulb and the two wires are pushed into it. The tips of the quarter-tabs sticking out press into the bulb threads and clamp it in place. Seems fairly tight grip, doesn't come out by shaking or vibrating it. Duct tape on the back end around threads/base/cup to ensure just a bit more security. :)

Why a styrofoam cup? Because it's pretty reflective of light, being white, and diffuse, which means light will scatter forward and to the sides a bit, making me more visible. I've got streetlights to see by myself, so I'm more worried about others not seeing me, especially when I come up to intersections where they have a stop sign and I don't, and I would like to not worry so much at each one. That's why I had the CCFL 180-degree-view headlight on there before (still there, just testing it's replacement or add-on).

It's also soft, so if the bulb does vibrate and bounce against it it's less likely to suffer any damage. It's also cheap, already being in my cabinet. :) And I am now usefully recycling it!

Neither of the cups is thick enough to block even most of the light, so I also took some black spraypaint and tried to paint the outside of the plastic cup, but the spraynozzle broke it's stem and jammed just after the first squirt, so it just kind of spurted all over the place, and I ended up just dripping and smearing what I could onto the cup as the can let go. Ugh.

For now, it's just duct taped in place under the arch of the bars, to the left of the CCFL HL. The power switch is just twisting the postive input and supply wires together. It'll get lots better once I prove it works. :)

It is VERY bright. It's at least 20W equivalent, probably a lot more than that, at this voltage. At 115VAC it's a 60W equivalent, drawing 13W actual. It's most definitely brighter than my CCFLs.

This is with the CFL only, from front

and from rear

then with my taillights, from rear

without any bike lights, front and rear. It's not totally dark out there, it's actually pretty bright from overcast clouds reflecting city lights back down, plus streetlights and whatnot--the camera just doesn't pick any of that up. :)


  1. This may be the best ever post on your blog- some really surprising (re)use of items to fabricate a pretty good headlight. My favorite is novel wiring of the cellphone charger output in series with the battery pack to get the necessary voltage to the CFL.

  2. I wasn't certain it would work, but the only other thing I had handy was a 3-D-cell flashlight. I actually had tested it using that, by screwing the battery cap down with the neg wire in it, and wiring from the spring that put power to the bulb (with the entire light head taken off) to the neg of the pack at the breaker. Then from pack pos to the CFL, and it did work, even with a switch. :)

    But it was very heavy, bulky, and requires more batteries. I already have more power on there than I need for most trips; why carry more?!

    So...pondering a Roman Black variation for a minute led me to the AHA! of the celphone charger, as I was working with them only days ago, and were the whole reason I had even thought of trying the CFL. :)

    In theory, I could just stack a bunch of chargers this way, if they're all isolated, and get a fairly high output voltage including the pack, and get at least 3/4 brightness out of the CFL.

    Then I just have to be very safe with the wiring and insulation, so I don't kill myself by accident. ;)


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