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I'm checking out some EL wire through the simplest means I've found: using the EL Escudo shield. I have connected it to an EtherTen microcontroller, and following the instructions in the EL Escudo manual, it all goes swimmingly. That's until I don't use a power adapter directly.

I'm assuming that using USB to power the EL wire doesn't give the inverter enough juice on its own: the inverter makes a tiny little squealing noise (normal for the inverted I'm told), but no light in the wire.

What I'm a bit confused about is trying to run the shield with power over ethernet (the EtherTen has PoE). If I plug the ethernet lead into the device, I get the same result as trying to power with USB: not much. But if I plug the device directly into the power adapter which was running the PoE (I'm using this midspan injector), it seems to run without any problem. I've been using PoE for other small projects and haven't had a problem yet.

So I'm wondering if there's something specific about direct power source (as opposed to PoE) that an inverter or EL wire needs?

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Very likely, the PoE injector has a current limit circuit to deal with a cable or device fault. For example, this Wikipedia article claims 350mA / 600mA limits for 802.3af and 802.3at Moreover, it seems that the limits are negotiated, so you could be set to an even lower limit.

The limiter would be there to protect the power supply and to ensure the continued operation of other devices that are powered by the same injector. And, more importantly, to protect against unsafe short circuit currents in the cabling which could, in the extreme, lead to a fire.

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  • \$\begingroup\$ Thanks Toybuilder, I think you're probably spot on. I took a quick look at the midspan injector manual and it mentions a max current of 1.5A total across all (4) ports. I'm only using one port at the moment though, so I'd assume it was able to take the maximum 1.5A. I'm certainly not interested in bypassing a current limit: protecting against shorts etc is important as I'm building a toy-like device which will be used by a child. \$\endgroup\$ – kodamapixel Jun 3 '11 at 3:22
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Apparently there is a bad interaction between the mains power adapter, the POE midspan injector, the POE power extractor, the wires, and the EL inverter. You've told us which midspan injector and POE power extractor you are using, so that's already 40% of the information we need to solve this problem -- 40% better than many Stack Exchange questions. :-)

I'm going to assume you're using the EL Inverter - 12v mentioned at the bottom of the EL Escudo page you linked.

I'm also going to assume that, with the inverter disconnected, everything else you're trying to power off the same PoE link (the EtherTen and anything else you have connected to it) uses less than 0.25 A at 12 V.

You are going to check these assumptions, right?

According to that EL inverter datasheet, that EL driver is only specified to run over a fairly narrow range of 11 V to 13 V, and it pulls a normal operating power of 2 W to 5 W (typical) to 9 W.

That implies that at 12 V, the normal operating current is 0.166 A to 0.417 (typical) to 0.750 A.

power

Some forms of PoE can send more than 13 W of power to the powered device, which is more than enough to supply the worst-case 8 W steady-state power required by your EL driver.

Your "plug the device directly into the power adapter which was running the PoE" test indicates that you have plenty of power coming out of the mains adapter.

current

As Toybuilder mentioned, some midspan injectors have a current limiter. It is possible your EL driver simply pulls more power (steady-state) than your particular POE setup can handle -- in that case, you are forced to use a power supply that can give your EL the power they need.

However, the particular midspan injector you linked has no such current limiter. Yes, the datasheet says "maximum current: 1.5 A total across all ports". But that is the absolute maximum that the rectifier diode bridge is guaranteed to work before it overheats and dies; it's not the point where some current limiter cuts the power. (It's more like a car that can go 100 miles per hour, but at 101 mph the wheels fall off; and less like a car that can go 100 miles per hour, but at 101 mph the governor kicks in and smoothly reduces the speed back to 100 mph).

Since your current is less than 1.0 A, you should be good here.

voltage

I think this is most likely your problem.

With your current setup, there is 4 diode drops between the power supply and the EL inverter (2 in the PoE midspan injector, 2 in the PoE power extractor on the EtherTen). Typically a diode drop is around 0.6 V, so these four have a total voltage drop of roughly 2.4 V.

If your power supply happens to supply exactly 12 V DC, then the voltage at the EL inverter will be roughly 9.6 V -- less than the "11 V" minimum spec. (The voltage drop over a long Ethernet cable will only make this worse).

If you have a bench power supply, it would be pretty simple to use it to power your midspan injector at 12 V and then ramp it up until your multimeter shows you are actually getting 12 V at the EL driver input -- you'll have about 14.4 V at the bench power supply output.

If that bench test successfully lights up the EL light, perhaps the simplest way to fix this "under voltage" problem (and free up the bench supply for other things) is to use one of the alternatives mentioned on the EtherTen page:

  • first: install the "Power-over-Ethernet Regulator 14-24V" on the EtherTen. Second: plug a mains power adapter that puts out 24 V (really, anything in the 15 V to 24 V range, with at least 1 A) into your midspan injector. OR
  • first: install the "Freetronics Power-over-Ethernet Regulator 802.3af" on the EtherTen. Second: use some standard 802.3af power sourcing equipment. (It will supply at least 44 V to each device).

startup power

An EL inverter has some similarities to a switching voltage regulator. Many switching power supplies have difficulty starting up with a current-limited source.

Jerrold Foutz describes this problem in more detail in "Latchup of Constant-Power Load With Current-Limited Source".

Some possible things to try:

  • Hook the EL inverter directly to the EL wire (without the EL Escudo shield between them). Does the same problem happen? If so, the main problem is somewhere other than the EL Escudo shield.

  • Try connecting a very short cable (~6 inches?) between the midspan injector and your EtherTen controller. If it starts working, you know one problem is the inductance and resistance of the ethernet cable.

  • Add a big capacitor directly across the power wires just before they go into the EL lamp driver. This might be enough to supply that start-up burst of energy.

  • Add that capacitor, then somehow let the capacitor charge up first, and then later turn on the EL lamp driver.

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  • \$\begingroup\$ Wow. First up, thanks for taking the time to write such a thorough response. You'll have to bear with me though - my electronics knowledge is very limited... Inverter. Actually, I'm using the 3V EL Inverter, following the instructions found in the EL Escudo User Guide. Power. The power adapter is 12V 2A - obviously more than enough to power the 3V inverter, which makes me think I'm overlooking very simple. \$\endgroup\$ – kodamapixel Jun 12 '11 at 8:23
  • \$\begingroup\$ I have been trying some of the things you suggested and noticed something which might be of interest: when I plug the Arduino+EL Escudo into the power adapter, the EL Wire lights up and stays lit. However, I'm using some example code provided with the EL Escudo library which should either blink the EL Wire, or fade it up and down. It's as if the power is going directly to the EL Wire. On the other hand, powering via USB or PoE isn't quite getting there (there's still a slight hum from the inverter though). \$\endgroup\$ – kodamapixel Jun 12 '11 at 11:24

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