How do I turn a DC current on and off very quickly?

[Kiara]

I want to make a DC current (from a battery) oscillate between full voltage and zero voltage (not negative) really quickly, at least 30 Hz.

I tried using an electromechanical switch, but the current from the battery is too low to produce any appreciable electromagnetism, so the switch didn't work.

I tried using a transistor circuit, but it didn't work either. Again, I think it's because the current is too low to use with the transformer that's used to regulate the transistors.

Anyone else have a solution? I'd rather have it be solid-state, but if the solution needs moving parts that's fine, as long as it works.

 

[davenn]

a square wave oscillator :)

Dave

 

[Born2bwire]

Or if you want to be a bit fancier, a PWM controller.

EDIT: This is pretty low frequency. I mean, the easiest thing to do is by a pulse wave modulation chip. But I guess you could also build this easily enough with a relay, some transistors, a rectifying diode and an AC voltage source of 30 Hz.

 

[cjl]

A pretty simple circuit with a 555 timer (or similar) should do it.

 

[Born2bwire]

Doh! I forgot about the 555.

 

[Kiara]

huh! never heard of that before. it looks promising. do you know if it can handle 30 Hz?

 

[cjl]

Easily. I've used it a couple of times recently - it'll go up to at least several hundred hertz, and I suspect substantially higher than that (I haven't tested it above that though).

 

[larny]

If you do a Google search for "555 timer", you will find plenty.

Here are a few

http://www.kpsec.freeuk.com/555timer.htm

http://home.cogeco.ca/~rpaisley4/LM555.html

http://en.wikipedia.org/wiki/555_timer_IC

How much current do you want it to drive?

 

[Kiara]

a lot. what I have so far is a 90 V battery pack (ten 9Vs in series), which I'm going to use a transformer to ratchet up to around 9000V once I figure out how to quickly turn it on and off. a 320 Ah battery pack connects to that and then goes to the load.

so the IC itself wouldn't drive very much current, I would use transistors to amplify the waveform before it went to the transformer.

 

[cjl]

Ahh, but that voltage will be a problem. The 555 timer is not meant to operate on anywhere near that much voltage. What are you trying to do, out of curiosity?

 

[larny]

a lot. what I have so far is a 90 V battery pack (ten 9Vs in series), which I'm going to use a transformer to ratchet up to around 9000V once I figure out how to quickly turn it on and off. a 320 Ah battery pack connects to that and then goes to the load.

so the IC itself wouldn't drive very much current, I would use transistors to amplify the waveform before it went to the transformer.

Do you have a transformer?

Or are you intending to buy one?

You will need a properly designed transformer for a 9 kV output.

You don't necessarliy need transistors to drive the transformer.

The 555 could possibly drive it directly.

It depends upon how much current will be drawn from the 9 kV side and the turns ratio of the transformer.

Why did you think you need a 90 Volt battery?

 

[Kiara]

Ahh, but that voltage will be a problem. The 555 timer is not meant to operate on anywhere near that much voltage. What are you trying to do, out of curiosity?

yeah, around 5-15 V, right? what I outlined was what I was going to do before I learned of the 555 chip, so I'm not going to try to run 90V through it.
what I need is a sustained arc across about 3 mm of air. I read that the breakdown of air is about 30kV per cm, so that comes out to about 9kV for 3 mm.

Do you have a transformer?

Or are you intending to buy one?

You will need a properly designed transformer for a 9 kV output.

You don't necessarliy need transistors to drive the transformer.

The 555 could possibly drive it directly.

It depends upon how much current will be drawn from the 9 kV side and the turns ratio of the transformer.

Why did you think you need a 90 Volt battery?

I wound my own transformer, although I'm almost certain it's not going to work. I took 3/4" diameter PVC pipe, and wound 12-gauge wire around it. it's actually three transformers wired together, each with a ratio of 9:1, so the total ratio should be 729:1. that's why I wanted to amplify the current coming out of the 555 chip, because it would be reduced so much that the last transformer probably wouldn't even work.

I won't be pulling a lot of current from the transformers, because I have a large current source wired up on the other side.

I thought I needed a 90V battery because I didn't want to wind too large of a transformer. that's actually the reason I split the transformer into three parts, because I don't have the means to wind a large one.

After learning about the 555 chip I redrew the circuit. Now I have the ten 9V batteries hooked up in two parallel packs connected in series, giving an output of about 18V. From the battery the current splits. One branch passes through a resistor, reducing the voltage to about 12-15V, then enters the 555 chip. The other branch passes through the collector of a TIP31C transistor whose base is connected to the output of the 555 chip. The emitter of the transistor then hooks up to the transformer and returns to ground.

 

[larny]

I have attached a data sheet of a 555. Note that the SE 555 has a max suppy voltage of 18 Volt, but the others have a max of 16 Volt.

So about 15 Volt would be a good choice.

I suggest that you connect 3 silicon diodes in series rather than a resistor to reduce the voltage from 18 to 15 Volt. Remember that a 9 Volt battery has a bit more than 9 Volt EMF when fully charged.

I'm very dubious about your transformer.

I suggest that you take the EHT transformer from an old TV set (CRT not LED, LCD or plasma). You may need to wind a new primary on it.

If you can arrange the correct turns ratio, you will be able to connect the output of the 555 directly to the transformer primary.

HOWEVER, if you look at the 555 data sheet, it has a max current source/sink capability of 200 mA.

This means thast you will be able to drive a max of 0.2 * 15 = 3 Watt into the load.

This is rough calculation and it may be a bit optimistic, but I just want to give you an idea.

Now 3 Watt, 9000 Volt implies a max output current of 3/9000 = 0.3 mA.

This is also optimistic as it assumes a perfect transformer, etc.

I don't know what current the arc will draw.

I suggest that you would be better off using a H Bridge to drive the transformer.

This will give you a much better result.

You are taking on a fairly complex design for your limited level of electronics knowledge.

It is also potentially lethal, so be careful

 

[Kiara]

thanks for the info!

 

[larny]

thanks for the info!

You're welcome.

I remembered that the Australian electronics magazine Silicon Chip published a circuit for a "Jacob's Ladder" that uses a car ignition coil as the transformer.

So it is essentially what you need.

I did not read the article, so I don't know what the output voltage is, but it certainly provides an arc.

Note that they use a lead acid 12 Volt battery.

I have attached a scan of one page, but I can scan the rest for you if you wish.

Also, you can buy the printed circuit board for the project (and other projects) from a company that is based in Sydney.

RCS Radio/Design

www.rcsradio.com.au

enquiries at

sales@rcsradio.com.au

 

[Fairweather]

You could use an old auto radio vibrator if you can find one. It is a simple make and break relay.