Powering Electroluminescent tape

RichardO

Joined May 4, 2013
2,270
wayneh:
Don't forget to assign real parts to your NPN and PNP transistors. LTspice seems to default to some kind of low power transistor.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
I've finally done it, I have glowing EL tape! There are still questions, though, see below.

First the history. I started this project back in 2014 and went through a few strategies at the paper stage. For instance, here. I finally decided to try using a wall wart transformer in reverse by driving it with a 12V variable-frequency square wave and stepping up the voltage from 12V to 120V to power the tape. I got started on my breadboard but then it mostly sat in the basement.

An unknown attached to that strategy was whether the wall wart would work at the higher frequency needed for EL tape, 400-2,000Hz. A recent thread asked and answered this question. Responders @OBW0549 and @LesJones were kind enough to run the experiment with signal generators and oscilloscopes and verify that it should work. So I went ahead and finished my build.

I also had trouble getting the voltage doubler (U1b oscillator, C4, C5, D1 and D2) working and finally traced it back to ordering the wrong capacitors for C3. Forehead slap.

My simulation file is attached. Everything to the right of the wall wart transformer is an attempt to model the EL tape itself. You can ignore it. Here is the schematic:
Screen Shot 2019-01-24 at 1.40.11 PM.png
If you run this simulation, you might want to increase the run time to 0.5 or 1 second, so that the voltage settles down.

This is working on breadboard EXCEPT that I'm only seeing 6VAC on L1 and 60VAC across L2 instead of 120V. I need some help fixing that. I'm seeing 12VDC on C6 and it should only be ~6V. But grounding the gate pin on either M1 or M2 kills the light, so they are both switching. I'm confused why there's not 12VAC on L1

As is and running at ~500Hz, [correction - it was over 110Hz. I was thinking A440 but a guitar's A string is 110Hz] this dimly lights my new EL tape and the brightness increases with the frequency set at VR1. The photos understate the on/off brightness difference.

IMG_0833.jpg IMG_0832.jpg
 

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Thread Starter

wayneh

Joined Sep 9, 2010
17,496
What voltage do you read on the (+) end of C5?
Here are several measurements while running.
Power supply 11.83
Top of C5 20.1
M1 (high side) gate 7.16
M2 (low side) gate 5.70
Top of C6 5.35
AC across C6 10.9
AC into L1 5.8
AC out L2 57.5
 
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OBW0549

Joined Mar 2, 2015
3,566
Hmmm... pretty hard to tell what's going wrong from that. However, assuming the DC readings are from a DVM which is responding to average value, I'd say the M1 gate voltage reading is a bit low; I'd have expected to see roughly 1/2 the C5 voltage reading (assuming 50% duty cycle). Also, I'd expect to see little if any AC voltage across C6-- not 10.9 volts.

I'm stumped.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
...I'd say the M1 gate voltage reading is a bit low; I'd have expected to see roughly 1/2 the C5 voltage reading (assuming 50% duty cycle).
Agreed. That must be a good clue but I don't know what to do with it yet.
Also, I'd expect to see little if any AC voltage across C6-- not 10.9 volts.
The meter is confounded by the DC there. The meter reads something like 22VAC for the power supply. There are NOT peaks to 22V there. Maybe I'll use a 0.1µF ceramic cap in series with the meter probe to see what shows up then.
 

OBW0549

Joined Mar 2, 2015
3,566
The meter is confounded by the DC there. The meter reads something like 22VAC for the power supply. There are NOT peaks to 22V there. Maybe I'll use a 0.1µF ceramic cap in series with the meter probe to see what shows up then.
That should work, though a bit more capacitance would be better.
 

Tesla23

Joined May 10, 2009
542
This is working on breadboard EXCEPT that I'm only seeing 6VAC on L1 and 60VAC across L2 instead of 120V. I need some help fixing that.
I've only had a quick look at the schematic, but isn't the voltage at the junction of the output transistors a square wave switching between 0V and 12V, so isn't this is 6V DC with 6V RMS AC imposed on it? What am I missing - how are you supposed to get 12V AC from this? To get 12V AC wouldn't you need an H-bridge?
 

OBW0549

Joined Mar 2, 2015
3,566
I've only had a quick look at the schematic, but isn't the voltage at the junction of the output transistors a square wave switching between 0V and 12V, so isn't this is 6V DC with 6V RMS AC imposed on it? What am I missing - how are you supposed to get 12V AC from this? To get 12V AC wouldn't you need an H-bridge?
Duh. I can't believe I missed this!

That's the answer, alright.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
I've only had a quick look at the schematic, but isn't the voltage at the junction of the output transistors a square wave switching between 0V and 12V...
Yes.
...so isn't this is 6V DC with 6V RMS AC imposed on it? What am I missing - how are you supposed to get 12V AC from this?
Awww crap, I've made this booboo before and now it's bit me again. The simulation shows the voltage out of L2 going from +60 to -60 and I was thinking that was 120V but of course it is not. The tests of wall warts in the linked thread show the same thing. The data has been right in my face all along and I missed it.
To get 12V AC wouldn't you need an H-bridge?
Looks like it!
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Don't feel bad; we've all had moments like that...
Sigh ... now looking up H-bridge circuits.

I've already got the high-side voltage on board, so it shouldn't be too hard and will use up two more of the old IRF540 MOSFETs I have on hand.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
Or you could take the circuit as-is and drive a 6 VAC wall-wart with it.
Found one! I had a small 4.5VAC, 120mA transformer. At ~110Hz (low A string on a guitar) and with the EL tape lit, I see about 135VAC at the EL contacts. If I crank up the frequency to 330Hz (high E string), the voltage sags to 110. Backing off a little brings the voltage back up to 125VAC. The circuit is drawing about 60mA from the 12VDC supply.

So, I think I'm seeing some limitations imposed by the transformer. My next test is to up the supply voltage to ~19V by using an old laptop power brick. That should get me to nearly 200VAC on the output unless the transformer whacks me even harder.
 

OBW0549

Joined Mar 2, 2015
3,566
You could get more voltage by using a transformer with dual primaries; these are usually used with the primary windings in parallel for 110 volt operation, or in series for 220 volts. Something like this.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
You could get more voltage by using a transformer with dual primaries; these are usually used with the primary windings in parallel for 110 volt operation, or in series for 220 volts. Something like this.
Yeah, I have a box with a few transformers pulled out of appliances over the years. I'll have to dig through. But things are looking pretty good in the meanwhile. I'll probably commit my little switcher to a protoboard.
 
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