Electroluminescent LCD backlight

[Demon]

Hi folks, for years I've been looking for simple economical DC/AC converters for EL backlights without using specialized ICs. The main problem with EL backlights is that they run off high AC voltage. I finally hit paydirt with a schematic from Nerdkits that runs off 5V:
http://www.nerdkits.com/videos/backlight

I'm including a pic of their schematic as well as a sample parts list I got from DigiKey.

For the two 150 Hz and 150 kHz signals, I used 555 IC timers using this tutorial from Guelph University:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

I've finished the two signal generators, the 150 Hz and 150 kHz signals are spot on. Now I'm starting breadboarding the EL circuit and will report back once complete.

Some of you have gotten these LCDs cheap on eBay but never got around to figure out how to get the backlights working properly, hope this helps.

Robert

[Demon]

Hmm, this sucks, it isn't working. They used this choke:
http://tinyurl.com/593uyv

Radio Shack is now Circuit City here in Quebec (La Source), and that part number doesn't come up on their web site. I used this choke:
http://tinyurl.com/9xzkv2

I didn't think of checking the current rating, they mention measuring "roughly 100mA of peak inductor current". But the part I picked at DigiKey is only rated for 44mA, that probably is not a good thing. The choke I have is about 1/2 the size of an ordinary 1/4 watt resistor, theirs appears the size of a 4.7uF electrolytic capacitor. I have a feeling I need a larger inductor with larger windings.

They discuss theory and a high voltage test. "After a current is allowed to build up in the inductor, the switch is opened." And then they check the voltage on the lead on the GND side, they measured "a 300V peak". If I try that test, my power supply shuts down, as if I shorted the lines, maybe if I put a LED in there to act as a load...


(keep in mind that this is my very first foray into inductors and frequency generators - kinda happy I got the 2 timers generating the exact frequencies on my first day)

[Ron Marcus]

Look on Microchip's website for a high voltage generator using a PIC. It is informative and pretty cool.
Basically, you are creating an instantaneous high voltage at a minute current. Anything you use to test it must be high impedance, or it will drain the charge that the inductor is producing. A small cap able to withstand the voltage can be charged so you may test the voltage easier. Keep in mind though, the cap can store enough energy to kill or severely hurt.
Try to pick an inductor rated for at least 50% over the peak current (150 mA). Below this point, you will give up alot of efficiency, as the inductor saturates and the inductive field collapses. For testing, you can get a neon bulb from RS. Attach a 100K resistor in series, and put it across the EL terminals. It should glow with anything over 75 volts or so. Increasing the frequency of the inverter, enables you to reduce the inductance and the inductor size. Watch the maximum voltage on the inductor also. If the load gets disconnected, you may get a way higher voltage than expected. Good luck!

[Demon]

(I PMed Lester to move this thread over to General Discussion, I'll post final schematics here once I know what I'm doing)

I was trying to stay away from using a PIC for this exercise and finally learn to use the 555 and mosfets. The 555 is running just fine, I just realized that the pulses are not at 5V, so I have to put schmitt triggers to clean things up.

I just got back from the local electronic store with 100uH canisters that look just like what NerdKits used. I have to solder leads to make it fit the breadboard first. I also took the opportunity to take 5 of every inductor size they had in stock, I like having a good selection when I don't know what I'm doing.

Yeah, I knew about the high voltage thingy, that's why so few adventure this way. I think I'll cut the leads on the caps so they're bare minimum and put some electric tape to cover them, same over that 100K resistor and output leads. I put one multimeter on 600V AC across the leads so I'll know if I have something coming out through there.

I'm kinda confused with the mosfet. I identified the gate, drain and source leads from the datasheet, but the arrows don't look anything like those on the Nerdkits schematic. I used the same mosfet, IRF730A.

[Ron Marcus]

If you have a multimeter with a diode test function,put the leads across the various transistor leads. When you get a diode reading, the positive supply connection will be the negative lead. In otherwords, the diode should be reverse biased. This is if the gate is not diode protected.
Ron

[Demon]

If you have a multimeter with a diode test function,put the leads across the various transistor leads. When you get a diode reading, the positive supply connection will be the negative lead. In otherwords, the diode should be reverse biased. This is if the gate is not diode protected.
Ron

I didn't remember that particularity with diodes, thanks.

I had reversed the leads on the mosfets after not getting results and let out some magic smoke,but I wasn't able to notice what component roasted. I started touching them and I still couldn't find it, even by smell.

So I started testing the caps, reads ok, resistor, reads ok, inductor, hmm, this one read different than the others in the bag, not ok. I tested the diode but I noticed the reversal thingy, but the others in the bag all read the same way, so, reads ok. At this point I didn't know how to test the mosfet.

I asssumed the inductor was the weakest link in the chain, but I wasn't sure if I had fried it because the circuit had worked, or that I shorted the circuit. But now that I have the larger canister inductor, I'm pretty sure the reversal of the leads on the mosfet worked and the tiny inductor died from a current overdose.

Gonna test this soon (that's the problem with not having a dedicated workspace in a family environment, always gotta put everything away).

[Lawmate]

Hi Robert
Just wondering whether you managed to get any headway with this one? I am trying to do exactly the circuit. I have been using a TS555 for the high frequency side and a NE555 for the lower frequency side. It is for a battery powered driver for EL wire so the voltage will vary between 3.9V and 4.8V. Did you find out how the inductance relates to the outputted Voltage?
Cheers

[Demon]

Hi,

You're lucky I read this, I've been away from the forum since I did this test.

Yes and no, I got the backlight working by playing with the timers. It wasn't lighting up as bright as I hoped, nor was the voltage going as high as necessary. But it did light up, so the circuit is good in principle.

I'll be getting back into this once I finish building a new batcave; hopefully by this summer.

[Lawmate]

That sounds good. I tried it again with a scope and couldn't get it working still. I think that I have bought inductors that are over- rated and I think this is the problem. I'm gonna order a range of 100uH ones with different current ratings and see how they work out. I also think maybe the 680pF capacitor is a little on the small side. Anyway, good luck on your bat cave

[Ioannis]

What is the rated current of your load?

The inductors presented in this thread are just RF chokes and not intented for SMPS circuit use.

If the inductor is saturated you may just burn the mosfet or the inductor itself.

ON time, Vcc, MOSFET On resistance, Inductance and Inductor resistor are the factors to consider for the current through the inductor.

Simple circuit but lots of math behind it.

An oscilloscope is a must for the checks.

Also, if you change places between MOSFET and Inductor and reverse polarity of the Diode, you can drop the second MOSFET. Just the polarity of Vout would be inverted.

Ioannis

[Demon]

This was my first foray into this sort of circuitry. I was just looking for something to light up these LCDs and that was the only circuit I could find.

If you can draw a better schematic to do this, I'd really appreciate it. Giving details on the parts needed would also be a huge help.

In other words, assume you are talking to a 10 year old.

[mister_e]

in other words, assume you are talking to a 10year old.

*chuckle* At least you got some experience lately

[Demon]

Look on Microchip's website for a high voltage generator using a PIC. It is informative and pretty cool.
...

Looking under Application Notes - Voltage Regulation, I found this:

TB053 - Generating High Voltage Using the PIC16C781/782
http://ww1.microchip.com/downloads/e...tes/91053b.pdf

The only spec I see for the inductor is:
"Power inductors, in the range of 330 μH @ 0.655A, are common and readily available."

When I looked for "power inductors" at Digikey, I was faced with this:
http://search.digikey.com/scripts/Dk...er%20inductors

I selected "through-hole 330uH" and 31 items popped up, then sorting by Current and this item at 850mA was the most affordable over 655mA ($0.85 ea.):
http://search.digikey.com/scripts/Dk...me=811-2103-ND

I'll order a bunch in my next order and do some testing.

[Demon]

*chuckle* At least you got some experience lately

You're only a 30 minute drive from here and don't forget I can run faster than you, even with my walker. :P lol

[rmteo]

An excellent source of information (technical as well as pricing) on power inductors etc. http://www.coilcraft.com/

[Demon]

What is the rated current of your load?
...

Googling for PC-016ATE, I made my way here via Wombat:
http://www.serialwombat.com/parts/LCD-107.pdf

mArms=2.9

Also got more information on what we should aim for:

Vrms=100
fEL=400Hz

[Demon]

This circuit is interesting:
http://www.simplecircuitdiagram.com/...mple-inverter/



I just wouldn't know what to try as transformer to go from 5vdc to 100vac, nor how to control frequency at 400Hz.

I don't have many transformers on hand to fry.

[shahidali55]

Try this and let me know how it works out.