# Voltage Divider/EL Wire Circuit Help

Discussion in 'General Electronics Discussion' started by tony48, Jun 12, 2013.

1. ### tony48

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Jun 12, 2013
Hello all,

This is my first post to these forums and I'm requesting help with a circuit I am working on for an EL Wire dimmer. It's pretty basic and I am a novice so I may be making simple mistakes. Please bare with me .

Here is the circuit I am trying to create. It takes a 12v DC input and powers 2 LED's. A voltage divider is then used (with a potentiometer) in an attempt to vary the voltage from 8-12v (lower input voltage to the EL Wire inverter yields a dimmer glow from the EL Wire).

I got this together on a breadboard and ran voltage through it using a power supply. I used a multimeter to measure output voltage as I varied the resistance using the potentiometer. Sure enough, it worked and the LED's remained constant brightness. Output voltage varied from 8-12v.

The varying output voltage powers an EL Wire inverter. This is where I ran into problems. When the output voltage of the circuit was used as the input voltage for the EL wire inverter the smooth variance from 8-12v was no longer present. Instead, voltage stayed at about 0.6v for most of the time and then, at a certain point of rotation of the potentiommeter, jumped up to 11-12v. Clearly the EL wire inverter is interacting in some way with my circuit that I am overlooking. Also, I've done a lot of soldering on the inverter board and ruined some things. To fix it I soldered around them and bypassed the "sound" part of the inverter so it is only constant on. Doing all of this may have messed something up with the inverter.

Extra info about the EL Wire inverter: Here is a link to the one I am using. It's pretty common and available from a variety of online sites
More importantly, I know that is basically works by taking a DC input and making it an AC output which excites the phosphorous in the EL wire and makes it glow.

Thanks for baring with me through that and thanks very much to anyone that can help. I know it's a tall order and will probably require someone that understands EL Wire and/or EL Wire drivers to help answer my questions.

Last edited: Jun 12, 2013
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Your 500k resistor will cause the voltage to fall to almost zero when you connect the inverter.

I would look for a better means of adjusting the output voltage than this. Is changing the input voltage really the recommended way of varying the brightness?

3. ### tony48

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Jun 12, 2013
Thanks for your help, I'll try to clear things up.

It's a 500k potentiometer, not resistor. This way I can vary the resistance in order to change voltage. Can you explain why this will cause the voltage to drop to almost zero?

Changing the input voltage is regarded as a decent way to vary brightness of EL wire. It's pretty tricky to do and pulse width modulation is the only other way I know of. This requires a micro controller which I don't want to use.

I am not familiar with any other methods of adjusting output voltage. Any help here would be greatly appreciated!

EDIT: I have done some more research on varying voltage and want to know if a voltage regulator like the lm317 would work well? Link to lm 317 and data sheet at sparkfun

I'm using an online calculator (Click here) and have been playing with the values. I have a 500k pot on hand. If I put that in for the value of R2 and ~60k for R1 then I get my desired max output of 12v (Are these resistances too high?). I do need some more help however. Below is the schematic. I've read some places that the capacitors are not necessary but in the case they are, I'm not sure what to buy for the 2 capacitors. I don't really know the difference between polarized and non polarized, etc. If someone could help me out with picking those from Radioshack it would be greatly appreciated!

Thanks again for your help! I think this voltage regulator is a much better choice than the setup I was going for previously, but feel free to weigh in and let me know if this will work for my circumstances.

Last edited: Jun 12, 2013
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The problem with the regulator that you have shown is that the maximum output voltage is about 2V less than the input voltage.

The 500k resistor causes a large drop in voltage because the inverter draws current.

You measure the voltage without the inverter and there is almost no current drawn.

But let's assume the inverter draws 1mA (it will draw a lot more than that.

The voltage drop across 500k is given by I * R where I is 0.001 and R is 500,000. So the voltage drop would be 500 volts! But you only have 12V.

Actually, the greatest current you can draw from 12V is V/R = 12/500,000 = 24uA.

As the pot reaches 0 ohms, the circuit will suddenly come to life with the full 12V.

As the pot wiper moves onto the resistance track, even if it only moves 0.5% into it, the resistance is 2.5k, and the max current would be under 5mA.

I would recommend (if PWM of the input voltage is allowable) is to set up a 555 as an oscillator with variable duty cycle, driving a mosfet to turn the load on and off.

Something like this:

(from here)

5. ### tony48

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Jun 12, 2013
Thank you very much for explaining that so clearly for me and please excuse my ignorance. I'm trying to learn but am, unfortunately, in a little over my head.

The reason I have not considered PWM is because it is a little more complicated than I would like. I have no experience with it, or building a circuit as complicated as the one you pictured. However, I am not totally opposed to it, especially with the excellent link you provided. Excuse me for asking/begging, but do you know of any slightly more simple circuits that will get the job done for me? The EL wire inverter will only be pulling about 250 mA.

When looking for the NE555 online I noticed that there are a lot of premade circuits available for fairly low prices. Would I be able to purchase a circuit that performs like the one you pictured? What would I be looking for? Perhaps this??? My main concern with it is, is 13kHz fast enough? I've heard faster recomended but I would assume 13kHz should be fast enough.

Also thinking about making this circuit: PPretty much the same thing, different format

Last edited: Jun 12, 2013
6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Yeah, that's fine. Well over-specced, but fine.

Here is another alternative that is more nicely packaged.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
That alternative circuit you linked to has a number of problems but should work.

I would advise a mosfet rather than a BJT, and some small series resistance from pin 3 (maybe around 120 ohms) so you don't over-current anything.

8. ### tony48

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Jun 12, 2013
Thanks very much for all of your help. I've decided to go with the LED dimmer you posted a link to. It's simple and cheap, plus I get free 2 day shipping from amazon prime. I would love to get the experience from building a PWM circuit but I'm already running late on this project and don't want to dedicate much more time to the dimming.

Thanks again for all of your patience in walking me through this. I've leaned a great deal that can be applied in the future.

9. ### tony48

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Jun 12, 2013
I'm afraid that I need to reopen this thread for another small question about the PWM unit that I purchased Here.

I need to know where in my circuit to place it because it is not producing the desired results. I can either place it:

1) Before the inverter so that it is varying the DC voltage that is being input to the inverter. Trying this produced a very limited range of motion (~10 degrees) that actually did any dimming. The rest of the range of motion was either completely on or off.

2) After the inverter so that the PWM unit is varying the output AC voltage that is going directly to the EL wire. Trying this method out led to no light from my EL wire.

Remember that the inverter takes a DC voltage input and creates an AC voltage output that travels into the EL wire and excites a layer of phosphorous that becomes excited and produces light (the EL wire essentially acts like a capacitor).

Thanks for any help!

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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OK, I expected that the full range of motion was unlikely to be useable, but I also expected it to be more than 10 degrees.

If you can note the extremes of the 10 degrees of movement then take the unit apart, it may be possible to expand that amount of adjustment significantly.

Once you take it apart, you'll need to measure the resistance of the pot at the two points which represent on and off (and its total resistance).

11. ### tony48

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Jun 12, 2013

Ok, i'll give you hypothetical numbers for now just for the sake of it. Say the pot is 20k, it first turns on at 10k, and achieves full brightness at 11k. What will I be looking for to expand the amount of adjustment?

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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In that case, it might be possible to replace the pot with a 1k pot in series with a 10k resistor, but it depends on how it is wired. There may be other ways.

I have one like this but it is in use. If I have time I'll pull it apart also and see if it's going to be easy to modify.

13. ### tony48

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Jun 12, 2013
Thanks very much, I'd really appreciate that.

14. ### BobK

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Jan 5, 2010
Given that EL wire draws very little current, I think a potentiometer across the output of the inverter might be the better way to dim them.

Bob

15. ### tony48

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Jun 12, 2013
Do you mean simply a potentiometer in parallel with the two output wires? What resistance do you think would work well? I'm at work now and can swing by radio shack and grab a potentiometer on my lunch break to try out when I get home.

FYI the current draw of EL wire is something really low like 10mA/ft and I'll be using around 20' of this stuff

Last edited: Jun 22, 2013
16. ### gorgon

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Jun 6, 2011
I would think that the 12v-(10%) is the allowable supply range for the inverter electronics, before the oscillator stops working, or the output voltage is too low for the El-wire.

I think you need to keep the voltage but reduce the energy applied to the EL-wire to reduce the light output. To do that you'll need to PWM the output voltage from the inverter. First you need to see what the voltage look like. Is it AC you'll need to use a bridge rectifier to make the switch operation DC, and if it's DC you can control it directly With a oscillator and a MOSFET for instance, and maybe add an optocoupler to make it safer.

17. ### tony48

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Jun 12, 2013
I've had the inverter hooked up to a variable voltage power supply and it works as low as 6v. This is why my original idea was to reduce voltage input to the inverter; when input voltage was lowered, the EL wire became more dim.

18. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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I've taken my controller apart and it seems to use a 555 to generate a triangular waveform and an LM358 to compare this with the voltage from a 1k pot that is connected via 1k resistors to gnd and +5v (there is an 78L05 regulator in there too). The output of the op=amp is used to drive a mosfet.

If you can confirm the total resistance of your pot and the resistance from one end to the wiper in the two extremes of just fully on to just fully off of your EL wire, I can calculate the value of some resistors to place in series with the pot to expand the control to close to the fill range of its movement.

19. ### BobK

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Jan 5, 2010
I hope you are wrong about the 10mA / foot. I think you must be, because that would be 1 W / foot at 100V and 20 feet would take 20 Watts. As I understand it EL wire uses much less power than that.

According to Wikipedia, the EL wire acts like a capacitance of 1nF / foot, and is driven by AC at about 1KHz. Given those specs, 20ft of EL wire would have a reactance of 8K Ohms, and draw about 12.5 mA.

I don't know how the light output varies with current though. I would start with a 100K pot and just use it as a variable resistor in series with the EL wire, and note what part of it's range gives you the light variation you want. Then reduce the pot if necessary.

Bob

20. ### gorgon

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Jun 6, 2011
In the link for the 26' driver, the recommended power supply was 12V @400mA, a total of 4.8W. I suppose the loss in the driver reduce this, and the frequency of the inverter will have a large impact on the power used.

From this, a 10k or 20k pot should be in the ballpark for regulating the output current/voltage for the 26' wire, Shorter wires need higher values.

Last edited: Jun 23, 2013