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Wednesday, November 8th, 2006

Time Event
On powering EL wire
A while back I became enamored with EL wire. The stuff is pretty neat: you apply an alternating current through it and it glows a color depending on the material. It's wire, and about 3/16" diameter for the most part. It's bright enough to be seen in the day, and more than bright enough for night. It's artful, can be functional (I've used it as a reading light in a van during overnight puzzlehunt games), and has a "ooo, neat" factor I rather like.

EL wire glows when you apply a small AC current, say 30mA, with a potential of at least 80V (p-p) or so. How does one make such a supply? Well, one neat trick is that EL wire will glow when you plug it directly into a wall outlet (well, with a resistor). But how about mobile applications, say wearables? A couple of AA cells can be tucked away nicely. 3VDC to 150VAC? Tricky.

The 555 IC is one of the oldest integrated circuits, and it's still manufactured today. There are entire books devoted to circuit designs using the 555 in both traditional and innovative ways. Apply some power to +V, add a few external resistors and capacitors, and the 555 produces an oscillating output. Send this pulse through a capacitor and you have a smoothly-changing alternating potential. Better yet, send that signal through a transformer and you can step up your 3v to something possibly much greater. With enough windings on the transformer you can step it up to 100V. Great!

But a 555, resistors, capacitor, and transformer are bulky. I would really like a circuit that's much smaller for driving the wire. Enter, the world of EL _lamp_ drivers. EL lamps are like EL wire, apparently, but are lamps (okay, duh): they're glass and filled with gas. They're used for LCD backlights, eg, in a cell phone. And, because _that_ market is pretty big, there's a lot of different EL lamp driver IC manufacturers. Seems like it's only an issue of finding one that's available to work for the EL wire.

I ordered a handful of Micrel's MIC4826 EL lamp drivers to try some experiments. Now, to begin with, these chips are surface mount components. Itty bitty doesn't even begin to describe them. I also ordered some special prototyping boards that make it much easier to experiment with SMT components. And, in the end, soldering the small part wasn't nearly so bad as I'd expected; solder paste is great stuff. The IC still requires some external components, though, but these are generally pretty svelte, and could even be surface mount items as well, if I really wanted to be masocistic. I wire it up to the EL wire, turn on the power and -- a very dim glow. Hm. Looking at the waveform, the MIC4826 isn't quite producing what it's supposed to -- the voltage is a bit low, and the curves aren't very sinusoidal. I experiment with the external components and correct both of these things, but the wire is still much more dim than when powered directly from the wall. Certainly, it's not bright enough to be especially visible in daylight. Hm. And, unfortunately, this is about where this story ends. I believe that the EL wire is pretty particular about the waveform it's being driven with. If that waveform is more square than sinusoidal then it's just not going to be as bright. Given this revelation, I now wonder if it wouldn't be better to go back to the 555 circuit, possibly modifying it to use smaller components, although sticking with a transformer to do the voltage step-up. I believe that the output of the EL lamp driver IC may simply be doomed to inadequacy.

I also need to buy a resistor substitution box. And some high voltage capacitors.

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