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Help with transformer spark (probably simple if you are knowledgeable on subj.)

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mrpsychotic:
I am building this poster that is for a school project that is supposed to be about me so I decided to do it on electronics. One of the parts is a small spark that I am trying to make.

The difficult thing is that I am trying to make it from a battery supply (whilst using a transformer) and that I am presenting it so the spark needs to be pretty visible and noisy.

I had a wall power supply that took 120VAC and output 4VDC. I removed the output smoothing and rectifying circuitry so all I was left with was a transformer.

I connected a 9v battery to what used to be the output through a pushbutton switch. Then I placed what used to be the 2 input wires (there is a center tap as well I think) close together. Upon clicking the pushbutton switch I get a spark of maybe 1mm. (the spark seems to jump when I open a previously closed switch rather than when I close it.

Is there a way I can use a capacitor to get a more sustained charge? My results were faint in sound and visually. I need to present this to a room.

Is there a way I can automate the switching process? I tried to use a ceramic resonator but I have little understanding of how those work since I have only used them in embedded electronics and would just align the proper pins. I am thinking about sing a 555 timer and tying the output to a transistor which would supply the transformer with 9VDC.

Is there a way to get a spark from DC battery supply without a transformer? It seems like the transformer adds complication since it is made for AC.

I am thinking about tying the output of my current setup (which produces a small spark) to the input of another transformer. Will this work? Will the spark become even more faint if I do this?

Thanks, and I understand that I am working with HV at my own risk.

Fraser:
I built such an item many years ago whilst at school.... it was used to demonstrate a car engines EHT spark ignition system

To create the spark that you desire in the simplest manner you can use an old car ignition coil commonly available cheaply from a car scrap yard or even a garage. The creation of a spark in air requires enough voltage to break down the insulating effect of an air gap.... in the case of a car ignition coil this will be around 12 to 30 kV depending on the model used. There is a safety warning at the end of this text.... please read it !

The ignition coil is usually driven off of a mechanical contact in the cars distributor or an electronic switching module. You will need a 555 timer IC running in astable mode at around 10 Hz driving a 2N3055 power transistor via a resistor to it's base. (~10K should do) The 2N3055 emitter is at 0V and the Collector connects to the ignition coil -Ve terminal. The +ve coil terminal is connected to the +12V supply. A diode should also be connected in reverse polarity across the coil to prevent back emf damage to the 2N3055. The output from the coil EHT terminal should be connected to the spark gap using a car EHT lead as that has appropriate insulation. The other side of the spark gap connects to 0v

Such an arrangement creates an EHT voltage cable of jumping a gap of 10mm or so with a good 'crack' and lots of ionisation odour.

HEALTH AND SAFETY WARNING !

Car ignition coil output voltages can be dangerous if contact is made with them. The circuit detailed above is capable of inflicting serious pain or injury. Appropriate precautions are required to prevent anyone coming in to contact with the coil EHT output. Your school should be consulted regarding the dangers of such a device before it is used. If you have any doubts regarding the safety of your construction or your ability to build it, you should not proceed with this design. I'm not kidding when I say that this circuit is hazardous and must be built and used responsibly.


An alternative method of creating an EHT voltage that will jump an air gap is the use of an AC power source connected to a voltage multiplier. Someone else on this forum can probably advise you on those designs or you can Google it. You would basically use the 555 circuit described above to drive the low voltage side of a mains transformer at it's rated voltage. The high voltage side would then create the 120V AC to drive the voltage multiplier that consists of diodes and capacitors. Very high voltages can be created this way and they will jump a spark gap OK. The building of such a device is more involved than the car ignition coil method but may be safer as lower currents are available.

mrpsychotic:
Is there a way I can charge a row of capacitors in parallel (say 10) and then Switch them to series so that there voltages are added when they discharge?

Zero999:

--- Quote from: mrpsychotic on November 27, 2010, 05:24:58 am ---Is there a way I can charge a row of capacitors in parallel (say 10) and then Switch them to series so that there voltages are added when they discharge?

--- End quote ---

Yes, it's called a charge pump converter. An oscillator and a diode + capacitor multiplier will do this. The trouble is, to get 1kV from 9V you'll need 111 capacitors and as many diodes so it's not practical.

With the transformer, you get a spark when the battery is disconnected because of the magnetic field suddenly collapsing around the coil. Voltage is generated in a coil when the magnetic field surrounding it changes, the greater the rate of change, the greater the voltage. When the battery is disconnected, the magnetic field suddenly disappears so the induced voltage is very high.

If you want more voltage, use a 230V transformer (order from Europe) or get a transformer with two 120V primaries and connect them in series. Yes, a 555 timer oscillator is a good way to go but I'd advise using a MOSFET such as the IRL540 rather than a BJT because it's easier to drive and will have lower switching losses.

Mechatrommer:

--- Quote from: mrpsychotic on November 26, 2010, 10:34:54 pm ---I had a wall power supply that took 120VAC and output 4VDC. I removed the output smoothing and rectifying circuitry so all I was left with was a transformer.
I connected a 9v battery to what used to be the output through a pushbutton switch. Then I placed what used to be the 2 input wires (there is a center tap as well I think) close together. Upon clicking the pushbutton switch I get a spark of maybe 1mm. (the spark seems to jump when I open a previously closed switch rather than when I close it.

--- End quote ---
how on earth a 30:1 transf being "backed/inversed-voltage" at 9V and producing a 1mm spark on the 1st winding? according my calculation, the out volt should be just around 270V? is it possible? ???

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