# Limiting output voltage of an electric fence energiser

## Main Question or Discussion Point

My home-made energiser runs off a car battery and uses a 555 timer to pulse a Darlington-style pair of transistors. These pulse the primary of an ignition coil.

Since an electric fence is an open circuit, I'm concerned about excessive voltages developing in the coil secondary causing degradation of the insulation.

I'm looking for device that can limit the output to, say, 10 kV. Is there such a thing? A spark gap would be the simplest, but would be noisy and perhaps cause local RF interference. A chain of MOVs? I gather these wear out over time.

I've tried to find schematics of commercial units (yes, I know I could just buy one) with no luck. I assume they use transformers able to cope with high OCVs.

Any ideas?

## Answers and Replies

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tech99
Gold Member
My home-made energiser runs off a car battery and uses a 555 timer to pulse a Darlington-style pair of transistors. These pulse the primary of an ignition coil.

Since an electric fence is an open circuit, I'm concerned about excessive voltages developing in the coil secondary causing degradation of the insulation.

I'm looking for device that can limit the output to, say, 10 kV. Is there such a thing? A spark gap would be the simplest, but would be noisy and perhaps cause local RF interference. A chain of MOVs? I gather these wear out over time.

I've tried to find schematics of commercial units (yes, I know I could just buy one) with no luck. I assume they use transformers able to cope with high OCVs.

Any ideas?
How about a resistor?

anorlunda
Staff Emeritus
Google 10 KV surge arrester. I found several starting at $13. How about a resistor? Is there such a thing as a high voltage-rated resistor? If so, that would be ideal, and very nice and simple. The output of the unit would be able to jump across a standard carbon-film type, so it would have to be several in series, or just one with a large body. I need to balance the damping of high winding voltages with maintaining output power and voltage. Even a 1 M resistor would pass, briefly, 0.1A at 10 kV which would be an effective short on the fence. Google 10 KV surge arrester. I found several starting at$13.
Found some, thanks.

Problem is, as with the gas discharge tube http://docs-europe.electrocomponents.com/webdocs/115d/0900766b8115d18a.pdf

That they aren't rated for continous (well, 1Hz in my case) operation. The surge arresters seem to be based on MOVs.

Tom.G
Limit the voltage on the Primary side, it's much easier.

The High voltage spike (pulse) on the secondary is due to the rapid collapse of the magnetic field upon current interruption. You could put a resistor across the primary to increase the L/R time constant, put a Zener across the primary, put a capacitor across the primary to reduce the ringing frequency, or limit the primary current to reduce the amount of flux that collapses.

Asymptotic and davenn
davenn
Gold Member
2019 Award
Limit the voltage on the Primary side, it's much easier.
what he said

Tom.G
Baluncore
2019 Award
Limit the voltage on the Primary side, it's much easier.
That is a good move. If you reduce the duty cycle of the NE555 timer the primary current will have less time to build up. That reduces voltage and improves economy.

The HV pulse will travel along the transmission line of the fence until it gets to the open circuit end. At that point the pulse voltage will double as it is reflected. If you want to prevent that voltage doubling, use a spark gap. Connect a non-inductive 180 ohm resistor in series with a spark plug to ground at the far end of the fence. That will terminate the line.

Limiting the primary duty cycle seems a popular choice, but below is a schematic with a sketch of the output waveform from the 555. When I built the circuit last year I set the duty cycle so that the resultant spark would jump about 10mm - roughly 10kV, as far as I could find out.

Is the duty cycle not low enough already?

Terminating the fence with a spark plug and, I'm guessing, a non wire-wound 180 ohm resistor could work. It is a car ignition coil, after all. Mechanics will always tell you to never crank the engine with the plugs disconnected for fear of damaging the ignition coil.

Still open to ideas on the primary side, though.
Thanks to all for the answers so far.

Also itching to find a real electric fencer to tear down. They seem to whine at ever increasing pitch into the inaudible, before emitting the pulse. Some form of 1 or 2 transistor oscillator, running a transformer whose high frequency output is rectified into a high voltage capacitor, which then, via spark gap or similar, dumps its charge periodically into the primary of a second transformer? Or is that a stun gun?!

@Guineafowl, it seems to me your circuit already limits the output voltage with the resistor and capacitor you have across the primary. I think that freewheeling diode on the primary shouldn't be there.

Guineafowl
@Guineafowl, it seems to me your circuit already limits the output voltage with the resistor and capacitor you have across the primary. I think that freewheeling diode on the primary shouldn't be there.
That was an attempt at an RC snubber.

In fact, in the final build, I DID remove the freewheeing diode, as the circuit wouldn't work with it there. Well spotted. But why? It's an inductive component being switched on and off, and I was hoping to clamp any transients on the low voltage side. Was the diode too slow in recovery?

That was an attempt at an RC snubber.

In fact, in the final build, I DID remove the freewheeing diode, as the circuit wouldn't work with it there. Well spotted. But why? It's an inductive component being switched on and off, and I was hoping to clamp any transients on the low voltage side. Was the diode too slow in recovery?
It's because the primary and secondary share the same magnetic core (energy store), you clamp one you clamp the other too. The diode allows the current to "freewheel" and not drop quickly enough, hence no voltage spike. But otherwise your circuit is all you need. If your output voltage is still too high you just need to lower the resistor value. You may need to play with the capacitor value a bit too but that one is not very important.

Guineafowl
Baluncore
2019 Award
Is the duty cycle not low enough already?
The title of the thread "Limiting output voltage of an electric fence energiser" suggests that it is too high.
Did you measure 10kV with no fence, with a long fence or with a long terminated fence?
That certainly evolved into an interesting circuit.

Guineafowl
Tom.G
Here are the measurements of an Ignition coil from a high performance V8 engine. Note that it fires two plugs at once; therefore your turns ratio, firing one plug at a time, is probably closer to 1 to 47. And your secondary inductance would be around 10.5H

If you choose, you could use a Zener in series-opposed to the (non-existent) diode shown across your Ignition coil. Select the Zener voltage to be about 1/50 of your desired Secondary voltage.

Or someone on the forum could play with the numbers to use a modified pulse width or snubber circuit.

Last edited:
Guineafowl
Tom.G
It's an inductive component being switched on and off, and I was hoping to clamp any transients on the low voltage side. Was the diode too slow in recovery?
No. It was clamping the voltage to about 1V on the primary. With a 1 to 47 turns ratio the secondary was clamped to about 47V; enough to get your attention if you grabbed onto it, but not enough to get through your shoes at the fence.

Guineafowl
Another issue might be the voltage in the primary circuit. As Tom.G mentioned above the primary and secondary voltages are related by the winding ratio of your coil. Working backward from the 10kV output, assuming your coil is 1:100 ratio you get 100V on the primary. Adding to this the 12V battery voltage this shows up as a 112V direct voltage across your power transistor as it's trying to switch off. You need to check that the transistor can take this voltage repeatedly. It gets worse if the coil ratio is only 1:50, then you get 212V on the transistor for the same 10kV output.

Guineafowl
The title of the thread "Limiting output voltage of an electric fence energiser" suggests that it is too high.
Did you measure 10kV with no fence, with a long fence or with a long terminated fence?
That certainly evolved into an interesting circuit.
No fence - the spark jumped 10mm in free air so I guessed 10 kV. It powers the fence just fine like this - no foxes have raided the chickens since, and I can get a good shock at the far end of the (unterminated) fence through a blade of grass - standard 'country' way of testing.

I didn't know the RC snubber would have an output-clamping effect - it was really just an add-on I'd seen for RF supression. Before this, I could hear the 'ticks' in the car radio speaker, and when testing on the bench the computer mouse would freeze, needing a reboot to start it up again.

I did lose a 555 or two before the opto was added. Also, before the diode across the 2n3055, I'd get shocks from its case.

What would we do without breadboard?!

Here are the measurements of an Ignition coil from a high performance V8 engine. Note that it fires two plugs at once; therefore your turns ratio, firing one plug at a time, is probably closer to 1 to 47. And your secondary inductance would be around 10.5H

If you choose, you could use a Zener in series-opposed to the (non-existent) diode shown across your Ignition coil. Select the Zener voltage to be about 1/50 of your desired Secondary voltage.

Or someone on the forum could play with the numbers to use a modified pulse width or snubber circuit.

View attachment 211340
So as the 2n3055 turns off, the zener limits the max primary voltage spike, and thereby limits the secondary voltage?

200V zener for 10kV output - so if I want the voltage across the primary not to exceed 200V, just to be clear, I reinstate the freewheel diode and place the zener cathode-to-cathode?

This will certainly be considered for the next revision of the circuit. There have been many.

Vitro - The 2n3055 can only take a Vce of 70V, so I need a higher rated one. Perhaps the next circuit could do away with the Darlington array and just use a MOSFET?

Tom.G
What would we do without breadboard?!

So as the 2n3055 turns off, the zener limits the max primary voltage spike, and thereby limits the secondary voltage?

200V zener for 10kV output - so if I want the voltage across the primary not to exceed 200V, just to be clear, I reinstate the freewheel diode and place the zener cathode-to-cathode?
Yes and Yes.

Guineafowl
OR.. from some research, I could use an automotive IGBT, such as the IRGB14C40LPBF, rated for 430V and with its own protection clamps.

200V zener for 10kV output - so if I want the voltage across the primary not to exceed 200V, just to be clear, I reinstate the freewheel diode and place the zener cathode-to-cathode?
I like this solution proposed by Tom.G, it should give a nice rectangular pulse. But figure out your coil ratio before choosing the Zener. If it's indeed 1:50 then use 200V but if it's 1:100 then you need a 100V. Also, in this case get rid of the resistor and capacitor.
Vitro - The 2n3055 can only take a Vce of 70V, so I need a higher rated one. Perhaps the next circuit could do away with the Darlington array and just use a MOSFET?
Whatever you can find that's good for 200V+.

Guineafowl
Tom.G