Dimming an EL panel is harder than it looks

Performance

Visual Performance

Now the final payoff, see it in action!

Electrical performance

I was curious to see how stable my 120V rail was, so I wanted to take a look at it with my oscilloscope while it was under load.  My O-scope can’t take 120V inputs, so I chopped down the voltage to something manageable using a 1k$$\Omega$$ and 100k$$\Omega$$ resistor divider.  The resulting cut should be $$\frac{1}{101}$$.  Here’s the waveform I measured:

It might not seem like too much sag until you realize that I’m reading the severely attenuated voltage.  Looking at this graph, it looks like my voltage rail sags from 1.148V to 0.4565V.  Scaling that up to full size, it’s going from 114.8V to 45.65V.  That’s almost 70V of ripple!

So how did this happen? My guess is that it’s a result of my calculations depending on a constant or nearly steady current draw.  As the high voltage H-bridge switches back and forth, my EL panel is anything but constant.

Hold on a minute, if I sized my components appropriately, shouldn’t I be able to handle the current draw?  I sized all of my components assuming the worst case current draw from the EL panel!  How much current is being drawn anyway?  We know that my output capacitance is .03$$\mu$$F, so let’s see if we can estimate the output current based on the rate of voltage drop across this capacitance.  This assumes that the transformer contributes nothing, but you’ll see why that’s okay.

The initial voltage drop is approximately $$119.4\frac{mV}{\mu s}$$ which works out to about $$12\frac{V}{\mu s}$$ of the actual rail.  Let’s calculate the current:

$$\Large I=C\frac{dv}{dt}$$

$$\Large I=.03\mu F\times 12\frac{V}{\mu s}=360mA$$

Update:

There are likely a number of factors contributing to my huge spike in current draw.  One of the reasons is that my model of EL materials was incorrect at the time of writing this post.  More details here.

Remember what I said before about regulating flyback converters and fluctuating current draw?  I believe that my flyback regulator IC is having trouble with this rapidly changing current draw.  After all, once the EL panel is fully charged, I’m basically presenting an open circuit load to the flyback converter.  To prevent the output voltage from rising, the converter will have to severely drop its duty cycle.

Once my EL panel polarity is switched, there is a huge current draw, and my driver IC isn’t ready for it.  It can’t react immediately (because the flyback is not a first order system), so there is a delayed response.  This is why you can see a short delay before the output voltage starts to rise again and why it’s okay to ignore the transformer’s contribution to the output current during the initial slope.

You can actually see the system overshoot and oscillate a little in the following trace.  This was my voltage ripple with a piece of EL wire as the load rather than my panel.

Cool huh?

This problem could be solved a few different ways.  I could add more output capacitance to my flyback, but this would be hard considering that all my caps need to be rated for 120+ volts.  I could also build a better flyback converter that I could continuously adjust in firmware so that it can prepare for upcoming current loads.

Adding more bypass capacitance “worked” as you can see in this post.

I measured the current draw of my circuit and found that it never gets over 200mA at full brightness, so that means that my batteries should last 10-12 hours on a single charge! Can you say “over engineered”?  If I were to do it again, I would probably try to stick with some more conventional alkaline batteries.

Audio Performance

One problem I somehow completely failed to predict was the sound the power supply produces.  You can very clearly hear a buzzing tone at the frequency that the panel is being driven.  This tone is caused by quickly oscillating magnetic fields that cause components to vibrate (you might remember hearing a 60Hz hum when standing under a power line; same thing).  For a sample of the sound, check the end of the video.

This isn’t too much of a concern for me because this device is clearly meant to be used in loud environments where the buzzing sound is drowned out.  My bigger concern is that this thing produces a lot of stray alternating magnetic fields that will be picked up by an electric guitar and highly amplified.

Considering I made this for a guitarist, this is a really bad thing.  I might investigate this problem and try to find a way to reduce the radiation, but honestly, with all the crap going on in my power supply, I doubt I’ll be able to improve it much.

Instead, I’ll probably end up keeping this for myself and making the guitarist something else.

I actually improved this buzzing problem in this post.

Conclusion

I am a little disappointed to find how dimly my EL panel actually lights up.  It seems to just be the nature of the material that it can only get so bright.  All that means is that you have to be in a fairly dark room to see anything.

I’m honestly just super excited to have this project (and this blog post) finished.  My last real post was over a month ago because I’ve been spending so much time on this thing.  If I had any idea how complicated it was going to be from the start, I would have split it up into multiple blog posts during development.  Oh well.

Anywho, it was a cool project and I really feel like I contributed something to the web in the field of controlling EL materials.  I hope that someone takes what I learned and makes something even cooler.


Continue this story here.

28 thoughts on “Dimming an EL panel is harder than it looks

  1. yea there is a MUCH MUCH easier way to do this..
    you take the standard inverter and the el panel and put a CDS photocell in series then take a standatd lm328 amplifier and hook the output to a bicolor LED and shine that on the photocell
    DONE! i can give you schematics and stuff if you want

    • So you’re just talking about putting an adjustable resistance in series with the EL panel? I suppose that would work, but it’s not the most efficient way to do it.

      Also, looking at a few datasheets, the maximum power dissipation though a CdS cell is around 100mW. If I were to use one on a panel, it might need to dissipate upwards of 7-800mW. It couldn’t take the heat. I suppose you could put a few in series, but that’s going to get pretty messy.

      This solution might work in smaller loads like short EL wires though. Thanks for the tip!

    • When I first read your comment above I thought you were using the photocell to read the light from the EL panel and then reverse the voltage polarity when it detected maximum brightness, which should generate AC that would self-regulate to the right frequency to sustain that brightness level. But then when you mentioned LEDs you lost me! But would a feedback control mechanism like what I thought you meant actually work?

      • Matthew was recommending using a optoisolator as a variable resistance. The idea is that you could control current from a low-voltage circuit and not have to worry about the 120V blowing anything up.

        You present an interesting idea, but I think it would end up being more trouble than it’s worth. The EL panel is a fairly “slow” material. It takes a few milliseconds for it to react to changes in current, so using its brightness in some kind of feedback system would prove difficult. To say nothing of the wiring/mechanical complications this would create.

  2. >> I chose the 1N4148 which has a gate capacitance of just 4pF.

    Not gate. Junction.

    Since you were ordering from Digi-Key anyway, you could have purchased some BAT-41 Schottky diodes (19,000 in stock) . The BAT-41’s capacitance is 2pF, half of the 1N4148’s capacitance, and its forward voltage at 20mA forward current is 0.30 volts per diode, significantly less than the 1N4148’s 0.95 forward volts per diode.

    • Whoops, typo. Thanks.

      I figured out my diode problem after I already made my order, so I had to use what I had on hand. Next time I’ll be sure to choose more wisely. Thanks for the tip!

  3. while pouring over various patents on zinc sulfide phosphors i discovered reference to the color shift in el wire, my 555 based driver was a quick and dirty means of observing it myself and as an excuse to learn something about driving el wire… more of a first attempt using spare parts before devoting more time to designing a proper circuit rather than basing it off some simple dc to ac inverter circuit i found in the internet

    along with the brightness being a function of frequency i found voltage had a similar effect but would cause the el wire to age faster than driving at a higher frequency would, and yes driving at higher voltages also caused my salvaged transformers to overheat and leak paraffin wax everywhere

    • Oh by all means, I think it’s great what you did, and it really helped me in my initial research! I was just making a point that in general, there aren’t too many resources available on EL wire and driving it properly.

      • there really arent, i had to resort to reading dozens of white papers and patents to figure everything out, my result was a working driver that was extremely inefficient, occasionally overheated and administered ~130 volt shocks when i wasn’t paying attention

        ive also been working out some ideas on mixed frequency driving el wire to increase lifespan while making it brighter by rapidly switching between low and high frequencies at a rate where it would be brighter while impacting lifespan al little as possible

  4. I also noticed the frequency shift.
    Wonder if this effect can be used to make a variable spectrum microscope light?
    For certain applications this would be handy indeed as if the wavelength shift is linear then you could image a sample at a range of multispectral frequencies rather than green or blue.

    I like the Supertex HV852 for this as it draws under 5mA with small wires allowing a PIC to directly drive it and adjust frequency with a single drive lead and a capacitor.
    So one 12F683 can run six separate EL wires, with Charlieplexing even more.

    • as far as i have observed there is only a single color shift at ~2000 hertz and i have not found any information to indicate there any more

      it is far from linear and im not sure the amount of light would be sufficient for a microscope anyways

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