Electroluminescent LCD backlight

[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

[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.