Use an ignition coil to charge an electric fence.

Discussion in 'General Electronics Discussion' started by Roger VanCouwenberghe, Feb 28, 2018.

1. Roger VanCouwenberghe

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Feb 28, 2018

I hope to switch the primary terminal of an automotive ignition coil twice/second. Is there a device that can do that with 30,000 volts? Are there solid state devices that can do this?

I'm thinking of lowering the voltage to the primary, maybe try nine, six, or four volts to make this thing non-dangerous while still giving a strong enough shock to repel deer and cattle.

2. Bluejets

4,291
911
Oct 5, 2014
Doubt you'll get 30,000 v from a kettering coil.

Any circuit like a 555 set up as astable with the right switching element should do ok.

Why not just buy an approved electric fence unit?

3. duke37

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Jan 9, 2011
Where are you going to get your 30,000V from?
Does your hair stand on end?

4. ChosunOne

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Jun 20, 2010
I'm out of touch with what modern cars use, but last I checked, automotive ignition coils aren't powered by 12VDC on the primary side--they use an already-reduced voltage: 6VDC or less, I think. You might check up on that before you start.

CTP4500 likes this.
5. kellys_eye

4,275
1,146
Jun 25, 2010
You don't switch the high voltage side - only the low voltage.

Whilst a car ignition coil can easily be used for electric fence purposes a lot depends on the length of fence as it is the energy content of the pulse that makes it effective or not.

Here's a link to a schematic that I posted for someone else earlier: the energy content is selectable

6. duke37

5,364
769
Jan 9, 2011
Electric fencers have an output of about 5kV, enough to punch through a thick fur coat. Energies of 0.1J(battery) to 20J (mains) are used.
A transformer of about 1:20 is used to get this voltage so 250V needs to be generated and switched.
This can be done by providing the voltage directly from a capacitor or using the transformer to store inductive energy.
A simple timer can be made with a resistor, capacitor and a neon lamp. When the neon strikes, the current is used to trigger a SCR to discharge the main capacitor through the transformer.

Cars have had various designs of ingnition systems, I have recently made an inductance for a low voltage system for a Lohner car. I do not know what the current fashion is, Rich has given me a faulty array of four coils, one for each plug. I have not yet tackled it with a hack saw to see what is inside.

7. Bluejets

4,291
911
Oct 5, 2014
I'd be surprised if the "modern" coils are not cdi. Hacking them open will not prove much as the difference between kettering and cdi would not be visible.

8. duke37

5,364
769
Jan 9, 2011
But I have a compulsion disorder that makes me curious. The transformers are much smaller than the old type coil used for four cylinders.
The Kettering system should be quite good if the primary current is controlled. If the normal points are kept closed with the engine stopped, the coil may burn out. If the engine is running fast, there is little time for the current to rise to a sufficient level.

9. Roger VanCouwenberghe

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Feb 28, 2018
I'm pretty cheap, plus I like to do projects.

10. kellys_eye

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Jun 25, 2010
Me too! That's why I posted that schematic (post #5) - it works too.

11. CTP4500

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Mar 3, 2018
Thanks for this tech info. The car battery is 12V.

12. kellys_eye

4,275
1,146
Jun 25, 2010
IIRC the ignition coil is actually fed via a ballast resistor which itself is shorted out during starting.

13. dvvv

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Mar 1, 2019
How much area does ignition coil based fence cover

14. duke37

5,364
769
Jan 9, 2011
That depends on the energy aching the fence. 0.1J is sufficient to fence off a silage pit. 20J will charge several km and will start fires if the sparks ignite dry vegetation.

The area will depend on the shape of the fenced area. Once the animals are trained, you do not necessarily need a fence. A piece of string across drives seem to be sufficient to keep cows to the road particularly if there is fresh grass at the end of the walk.