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Spark through air

by adoado
Tags: spark
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Nov29-10, 06:41 AM
P: 72
Hello all,

I am studying physics at university, and I read something interesting in my book: If the electric field strength increases beyond 30,000 V/m in air, a spark will be created between the two potential differences.

This was really interesting, and being a more practical person I wanted to see if I could accomplish such an experiment at home to achieve this myself.

Here are my thoughts: I learnt from by book that V = Ed (that is, the voltage across a capacitor is equal to electric field strength multiplied by the distance between the electrodes). So, I found out with a 4mm gap I need the voltage 1200 volts.

Here is the issue, electronics not being my strongest side of physics; is there any way to step up a 9 volt DC battery to the required voltage?

Again, trying to use some physics I thought of two solutions: Increase the resistance in the circuit (V=IR, so as R increases, V increases) or use a transformer to step DOWN the current, so V increases..

Any ideas or comments would be greatly appreciated!

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Nov29-10, 11:08 AM
P: 644
Don't you mean 30kV/cm ? And therefore 3kV/mm.

It is possible but probably quite difficult for an "amateur" to convert 9VDC to several thousand volts.

The 9V battery is a voltage source, not a current source. Therefore you cannot "generate" higher voltages by putting it in a circuit with a high resistance. The voltage across the battery will still only be 9V. If you increase the resistance R, the current I will decrease so as to keep V=RI constantly equal to 9V.

To use a transformer, you have to convert the 9VDC to AC first...
Nov29-10, 11:32 AM
P: 595
I seem to remember 30Kv/INCH, but could be wrong...

How about an automobile ignition coil and breaker points, if such still exist?

Bob S
Nov29-10, 12:30 PM
P: 4,663
Spark through air

Hello Adrian-
Here in thumbnail is a simulation of a pre-1970 automobile ignition circuit showing a 25-kV voltage pulse. Resistor R1 (which includes coil primary resistance) should be chosen to limit dc current to about 1 or 2 amps. You should be able to find an ignition coil and condenser in a local junkyard.

Note in simulation that the voltage pulse occurs when the points open, and not when they close. (The very small pulses at 0 ms, 4 ms, and 8 ms are the points closing). Why is there a large voltage pulse only when the points open?

edit The above simulation is of course for charging the primary coil inductance to 1 amp using a 12 volt car battery. You could build a small dc-dc converter (9 volt battery to 300 volts), like in this stun gun circuit below, charge a capacitor to ≈300 volts, and discharge it into an automobile ignition coil or similar transformer. What you will need is the charging circuit like in

In this circuit below (circuit3), a 9 volt dc dc converter charges C2

to about 300 volts, and T2 is an ignition coil.

These are all dangerous circuits. be careful.

Bob S
Attached Thumbnails
Nov29-10, 04:34 PM
P: 644
Quote Quote by Bob S View Post
Why is there a large voltage pulse only when the points open?
After opening, the current through L1 changes very quickly due to the LC component values in the resulting oscillatory circuit.

After closing, the current through L1 is built up quite slowly due to the series resistance.

Only a quick change in the current through L1 produces a high voltage output for the spark plugs.
Bob S
Nov29-10, 11:14 PM
P: 4,663
Here in thumbnail is the basic configuration for a capacitor discharge ignition circuit. The thumbnail shows a 30 kV spark from a capacitor charged to 300 volts. The capacitor charging resistor R1 can be increased to 1 meg to limit charging current to about 0.3 milliamps. The RC time constant would then be 20 milliseconds. The output voltage pulse is about 100 times the capacitor voltage. For example, 50 volts on the capacitor will give a 5000 volt output pulse.

Bob S
Attached Thumbnails
CD_­ignition ckt.jpg  
Nov30-10, 02:17 AM
P: 96
If you want to get high voltage w/o too much work you can try an ionizer, these days you find them a bit everywhere, e.g. air filters, hair driers. Just make sure you know what you are doing before messing around with them.

If you want to go big and do not mind the work, van der Graaf is your choice. It is a lot easier than one would imagine. E.g. some karts at grocery stores that have rubber wheels rolling close to sharp metal features will pick up charge from a synthetic floor a charge the metal frame; they will give you a 'nice' spark when you touch them.

Another nice option Tesla coils

If you just want to see high voltage sparks, pull off your sweater in a dark room, better if you are wearing a synthetic fabric t-shirt

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