# Need high voltage/current DC with adjustable frequency

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Hey everyone. What would be the easiest way to make an device which will spark high voltage and with relatively high current thought spark gap and still allow me to adjust the frequency of the spark between say 50Hz and up to at least 2000Hz?

I would want this device to have some power behind it, at least 20 watts. Voltage isn't very important but to have a spark I know I probably need at least 2Kv+. I can run this on 120v or 12v, its just an experiment so whatever is easier.

I was thinking maybe using a neon sign transformer to charge a cap, but high voltage rectifier isn't easy nor cheap. Than I though maybe get a car 12v neon light transformer since it works on DC, and output is DC pulsed which I think can directly charge my HV cap? And I'm guessing I could adjust the frequency by the size of the cap and/or by spark gap distance right?

Are there any other easier cheaper ways to do this?

#### davenn

Moderator
Sep 5, 2009
14,150
was thinking maybe using a neon sign transformer to charge a cap, but high voltage rectifier isn't easy nor cheap. Than I though maybe get a car 12v neon light transformer since it works on DC

transformers work on AC not DC

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Yea I can run it on main AC I just need the output to be DC.

And automotive 12v transformers for neon lighting do work on DC. But I believe they might still put out AC which is hard to rectify. If they put out pulsed DC I think I could use it

#### davenn

Moderator
Sep 5, 2009
14,150
automotive 12v transformers

what are those ... give a link

you are not going to get continuous sparking with DC, that's why it has to be AC

Apr 29, 2012
340
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#### Bluejets

Oct 5, 2014
6,197
Car ignition coil, 12v battery, say 555 timer to give required frequency pulse and drive the coil with a MJ10012 darlington transistor via a bc639 . Will need a base resistor on the MJ10012 of about 40R 5W and a base and collector resistor on the bc639. On the phone presently but will get exact figures when I get home. ....cheers

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Thanks bluejets. I though about using a car coil as I have a few at the house but I don't think there is much power behind a car coil. Even at 2000Hz I don't think the coil would draw more than 5 watts but I could be wrong. I would like 20 watts+. I'm not even actually sure 20 watts would be sufficient for what I'm trying so I was hoping to have more so I can start at low power and go up.

And

I though pulsed DC and AC an quite different? I specifically need pulsed DC, I need the spark to only spark in one direction. Would a pure sine AC spark in both?

.
PS I did some searching around and I guess an automotive coil can easily require 3-8 amps. And from my calculations an average single coil distributor style ignition system fires about 70 times per second max at 8000 RPMs. Not sure what will happen if I kick it up to 2000Hz, but I now see people making jacob's ladders with them and they seem fine

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#### Bluejets

Oct 5, 2014
6,197
The number of times per second that an ignition coil fires depends on how many cylinders.
In most things, energy out is equal to energy in minus any losses.
Even at 3 amp in on a 12v coil, one could say at even 50% efficiency, one would expect to see 18W out.
Don't know how you found 5 watts.
Coil won't respond to 2000hz and maintain good spark as there is not enough dwell time to charge the coil.
CDI will do better but you need quite a hefty charge system for the cap.
Maybe rip the complete ignition system out of a four pot screamer.(motorbike)

#### davenn

Moderator
Sep 5, 2009
14,150
I thought pulsed DC and AC an quite different? I specifically need pulsed DC, I need the spark to only spark in one direction. Would a pure sine AC spark in both?

They are
so you need to learn the difference
when you use Hz as in your 50 to 2000 Hz that indicates AC because Hz (Hertz) = cycles per second

if you are really talking about pulsed DC, then you refer to the pulse count in pulses per second and you can then also state the pulse width requirement

a pulse has 4 main descriptive parts ....
rise time, pulse width, amplitude and decay time

so you really need to stop using Hz and start using pps

Dave

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Ahh ok thank you both for your help.

Ok so at 2000 ppm the coil spark will not be as hot but it coil should still be putting out around 20 watts right?

I was thinking about using my phone with a signal generator app, connecting it to a car or home audio amplifier and connecting that to a automotive coil. I think if this works I wouldn't have to buy any equipment for now.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
I'd opto-isolate the output of my phone (because I don't want to accidentally fry it).

Then I'd use a proper circuit to drive the ignition coil.

But apart from that you can probably get away with what you have described.

This page (http://www.sw-em.com/Ignition_Additional.htm) has some graphs of primary current and primary vs secondary voltage. Note that the primary will see a large voltage spike. Also note the fast falling edge required on the input.

An audio amplifier may not be able to correctly generate the falling edge, has an AC output, and may be destroyed by the voltage spike of the first discharge.

#### supak111

Apr 29, 2012
340
Phone I'm using is an old android phone not worth $10 now days and the audio amp is cheap crappy 100watt no name amp so no big deal if they die. Or maybe just do what BlueJet say: drive the coil with a MJ10012 darlington transistor via a bc639 but drive the bc639 directly with the signal generator app from the headphone jack on the phone? I like the app because it will let me precisely control frequency, waveform, amplitude, duty cycle... It can even do frequency sweep which might help me figure out the optimal frequency for this little experiment. . #### supak111 ##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★ Apr 29, 2012 340 Ok guys I found a 30amp PWM thats very cheap like$15. It has adjustable frequency: 240HZ, 2.2KHZ, 22 KHz by adjusting the little micro switches on the board, red in the picture below.

#1&2 OFF=22khz, #1 ON=2.2khz, and 1&2 ON=240hz

From what I can tell all the switches do is redirect one of the wires to the controller through a resistor. Do you guys think adding a veritable resistor in the place of the switches would allow me to control the frequency anywhere between 240hz and 22khz?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Do you guys think adding a veritable resistor in the place of the switches would allow me to control the frequency anywhere between 240hz and 22khz?

The problem will be in finding such an emphatic resistor.

On the other hand, a variable resistor might do the trick. You would need to measure (or read the value of) the three resistors and connect the variable resistor in series with the smallest of them (probably the one for 22kHz).

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
I wish I could tell what size the 3 resistors are on the board so I can order the variable resistor. 3 little silver/beige resistors just to the right of the red switch are the once that the switch uses and than it goes not the chip which I'm assuming is just a 555 chip?

Here is a better view of the resistor and micro switches

#### davenn

Moderator
Sep 5, 2009
14,150
. 3 little silver/beige resistors

those are capacitors, not resistors

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
Oh too bad. So that means that this sets frequency by changing the size of a cap and my method above will not work to change frequency with a veriable resistor

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#### davenn

Moderator
Sep 5, 2009
14,150
So that means that this sets frequency by changing the size of a cap and my method above will not work to change frequency with a veriable resistor

well without a circuit of that unit, it's pretty difficult to tell how they are doing the control
have you looked for circuit info ?

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
340
I can't find the circuit diagram anywhere, all I have is a picture of the board from top and bottom but its difficult to tell whats going where.

From what I can see they are using those micro switches to control the frequency now. And after the switches circuit goes though those 3 what you say are caps and into the pin 6 threshold of a 555ic.

Are you sure all 3 of those are caps? To me top 2 which go to 2 switches looks a bit different than the bottom one which is always connected to pin 6 on 555.

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