# How to protect low voltage components from very high voltage pulses?

Thread Starter

#### icecats

Joined Feb 20, 2023
20
Hi,
I have a high voltage electric livestock fence that runs from my house to a barn about 1/2 mile away. I want to build a communication device that makes use of this existing wired connection to transmit a low voltage signal (ideally ~12 volts) along the wire with ground return to complete the circuit. I have a working prototype of this part of the circuit.
The technical challenge I am now trying to solve is how to transmit this signal on a live fence without destroying my low voltage electronics. It is a design requirement that the fence remains powered during communication (transmission will occur from both the house and barn and the fence energizer can only be turned off from one of these places).

Here are a few options that I have considered:
1) use a diode or series of diodes with a high breakdown voltage to block the high voltage fence pulses
- HV diodes have a large forward voltage drop​
- HV diodes are expensive​
2) strategically disconnect/isolate the low voltage circuitry from the fence before each high voltage pulse
- this seems technically challenging because it requires precise timing​
- this seems technically challenging because it requires quickly switching high voltages​
3) use a voltage-divider-esque circuit that limits the high voltage down to a safe level and then send the low voltage signal the opposite direction through the divider
- this is just a thought I had of something that could be possible with the right circuit elements, but I'm not sure how​
Here are the specifications:
- high voltage energizer pulses: max 10 kV, ~100 mA, <10 ms pulses at ~1 Hz​
- low voltage signal: 12 volts, very small current​
- low voltage components include transistor, DC power supply or battery, microcontroller​
Thanks!

#### MisterBill2

Joined Jan 23, 2018
16,621
What sort of signal would be sent through the wire? If it is simply an analog voltage signal using the earth connection as a reference then there is no practical way to use the same wire as is connected to the high AC voltage source, either pulsed or constant .
If the communication signal was some sort of modulated carrier frequency then a coupling method to block the DC voltage and pass the carrier frequency is quite possible.

So what the TS must provide is an adequate description of the " low voltage signal: 12 volts, very small current " signal. Is it audio frequency? Or a digital signal of some kind?

Thread Starter

#### icecats

Joined Feb 20, 2023
20
What sort of signal would be sent through the wire? If it is simply an analog voltage signal using the earth connection as a reference then there is no practical way to use the same wire as is connected to the high AC voltage source, either pulsed or constant .
If the communication signal was some sort of modulated carrier frequency then a coupling method to block the DC voltage and pass the carrier frequency is quite possible.

So what the TS must provide is an adequate description of the " low voltage signal: 12 volts, very small current " signal. Is it audio frequency? Or a digital signal of some kind?
Currently the communication signal is 5-10 kHz square wave, but this can be adjusted as required.
When you say that there is no practical way to use the same wire, do you mean that there is no way to keep the communication circuitry from getting destroyed or no way to distinguish the signal from the fence pulse?

#### crutschow

Joined Mar 14, 2008
33,365
If the communication signal was some sort of modulated carrier frequency then a coupling method to block the DC voltage and pass the carrier frequency is quite possible.
What DC voltage?
The TS needs to block a 10kV pulse.

A big question is how much pulse attenuation can be tolerated as caused the suppression circuit, since the pulse energy is likely in the neighborhood of 1 joule.

Thread Starter

#### icecats

Joined Feb 20, 2023
20
A big question is how much pulse attenuation can be tolerated as caused the suppression circuit, since the pulse energy is likely in the neighborhood of 1 joule.
I think this is a question about whether it is acceptable in my application to attenuate the pulse. It is acceptable to partially or fully attenuate the electric fence pulse when the communication system is connected to the circuit.

#### crutschow

Joined Mar 14, 2008
33,365
Okay here's a thought:
Couple the signal through a 100pF, 20kV disc capacitor to a 1megohm resistor to ground.
Add a 10V Zener to ground to suppress the HV spike.
That should allow the transmission of the 12V 10kHz square-wave without draining too much power from the HV pulse.

Sim of example circuit below:
The 10kHz signal (red trace) is seen at the signal out (yellow trace) except when interrupted during the pulse rise and fall times (green trace).
Edit: In the real circuit the signal will likely be blanked for the duration of the HV pulse.
What type of signal modulation will you use?

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Thread Starter

#### icecats

Joined Feb 20, 2023
20
Okay here's a thought:
Couple the signal through a 100pF, 20kV disc capacitor to a 1megohm resistor to ground.
Add a 10V Zener to ground to suppress the HV spike.
That should allow the transmission of the 12V 10kHz square-wave without draining too much power from the HV pulse.

Sim of example circuit below:
The 10kHz signal (red trace) is seen at the signal out (yellow trace) except when interrupted during the pulse rise and fall times (green trace).
Edit: In the real circuit the signal will likely be blanked for the duration of the HV pulse.
What type of signal modulation will you use?
Thanks for the detailed response! For now I am simply changing the entire signal frequency for different "messages" but might implement Manchester style frequency modulation.
I am not sure I fully understand the circuit you sketched. I have sketched a circuit diagram showing my interpretation of the entire fence circuit. The 10 kV supply (fence energizer) is connected between the insulated fence wire to the ground. Initially, this forms an open circuit. The 12v signal generator is also placed between the wire and ground. It also forms an open circuit until a receiver device (not shown) is connected OR the 10 kV supply sends a pulse. When the pulse is sent, the 10 kV potential will be applied to the signal generator and all of its components. Lastly, I sketched in the suppression circuit in parallel with the signal generator. Is this the correct interpretation?
Thanks!

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#### crutschow

Joined Mar 14, 2008
33,365
Is this the correct interpretation?
Yes, that looks correct.

How does the signal generator generate the signal without being damaged by the HV pulses?
My suppression circuit is for the receiver.

Thread Starter

#### icecats

Joined Feb 20, 2023
20
How does the signal generator generate the signal without being damaged by the HV pulses?
My suppression circuit is for the receiver.
That's the problem I'm trying to figure out! My signal generator prototype so far has only been applied to a fence that does not have the high voltage energizer connected. Sorry for the misunderstanding.

#### crutschow

Joined Mar 14, 2008
33,365
That's the problem I'm trying to figure out! My signal generator prototype so far has only been applied to a fence that does not have the high voltage energizer connected.
Okay.
So the only solution I see offhand is to use a high voltage diode in series to block the HV pulse.
Edit: You could make you own HV diode by connecting twenty 1N4007 1kV diodes in series. The are only about U\$0.20 ea.
Of course you would have to increase the signal voltage to overcome the high forward voltage drop of the diode, but I don't see that as a significant problem.
You just add a DC bias to the signal which is near the forward drop of the diode, so the square-voltage could stay at 12V.

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#### Ian0

Joined Aug 7, 2020
8,949
It should be possible to use the same techniques as data-over-the-mains, by coupling in with a pulse transformer (with all the HV suppression etc.)
When the pulse has gone, the environment should be quiet. Perhaps you could wait for the HV pulse, and use it to sync the data?
But is it practical? Farmers seems to spend rather a lot of time mending electric fences!

#### crutschow

Joined Mar 14, 2008
33,365
coupling in with a pulse transformer (with all the HV suppression etc.)
What etc. suppression would protect the transformer from 10kV?

#### Pyrex

Joined Feb 16, 2022
214
Hi,
imho, it's better to use a higher carrier frequency- 100 kHz or so. And a HF transformer , with capacitor to form a tuned to 100 kHz LC tank. The unwanted HV pulse will be significantly attenuated. Decoupling capacitor 20 pF or so.
Ian0 did mentioned- " It should be possible to use the same techniques as data-over-the-mains ".
Those communications worked quite well on 35kV-330kV power lines. Carrier frequency was 17kHz-500 Khz, as i remember .Probably, still work in some countries or regions all over the world

#### Ian0

Joined Aug 7, 2020
8,949
What etc. suppression would protect the transformer from 10kV?
Same as you suggested - I didn't see the need to repeat it.

#### MisterBill2

Joined Jan 23, 2018
16,621
Post #13 gets closer to what I would suggest.
As for "what DC?", that is the low level communications signal, DC at some voltage modulated in some manner.
Consider that most electric fence chargers use some form of transformer to produce the high voltage spike pulse, with a current pulse in the primary section. I have not seen a fence charger that simply connects and disconnects a high voltage source. So the high voltage pulse probably has a bit of ringing from the inductive pulse driving the transformer.
Using 100KHZ or higher as a carrier frequency, with a series resonant LC network, followed by diode clamping, will allow the communications signal to flow while limiting the pulse to the clamp level.
It is important to not reduce the high voltage pulse being used to charge the electric fence.

#### crutschow

Joined Mar 14, 2008
33,365
Consider that most electric fence chargers use some form of transformer to produce the high voltage spike pulse
That's a good point.
The impedance of the transformer at the signal frequency may generate sufficient attenuation of the signal.
Using a higher signal frequency as you suggested would help minimize that attenuation, although the capacitance of the electric fence wire is also a factor in the attenuation.