Electric Fence Zapper

[David HK]

Ross,

This is what I followed:-

I can easily change things around as you suggest:-

Dave

[rossw]

Ross,
This is what I followed:-

For a number of years, I specialised in servicing, repairing, calibrating and training people in the correct use of engine analysers (amongst other things). Accredited service agent, with all the circuits of all their equipment - I can only say that regardless the source of that diagram... it's WRONG.

It is working purely as a capacitive pickup. It would do just as well with a piece of insulated wire of similar length.

Properly constructed, putting the plug lead through the U core but NOT CLOSING THE CORE will result in barely enough signal to detect. Closing the core and it produces highly repeatable, low-noise output proportional to the current flowing in the lead.

Indeed, it is my experience with these devices that lead me to suggesting the capacitive divider to measure volts, and the inductive loop to measure current - because both are tried and true methods of doing so.

Just did a quick google for a similar circuit - this is "fairly typical" of a crude hand-held device.
http://www.tradeofic.com/Circuit/4558-IGNITION_TIMING_LIGHT.html

[David HK]

Ross,

Did you view the You Tube URL mentioned in my post ....

That's where the circuit came from.

Here are some other pick ups .......... 


Any views?

I am not actually out to measure voltage at the moment. All I want to sense is whether or not the fence line is active. Am I up the creek?


Dave

[David HK]

Here is a capacitive version

Dave

[rossw]

Ross,

Did you view the You Tube URL mentioned in my post ....

Yeah, and he still got it wrong

I watched the video frame-by-frame everywhere I could see the board, but there wasn't enough detail to see where BOTH the conductors went. I bet he built it right, just documented it wrong.

Here are some other pick ups .......... 
Any views?

Both of those should work for their intended purpose.

I am not actually out to measure voltage at the moment. All I want to sense is whether or not the fence line is active. Am I up the creek?

My view is that you want to DETECT A THRESHOLD VOLTAGE even if you don't want to actually quantify it.
The coil system you are currently working on is simply not suitable for this purpose.

It's great to measure when there is a spark.
It's great to measure the current actually flowing during the pulse.
BUT.... if you have 10 metres of wire around your vege patch to keep the yetties out and it's all nicely insulated, this thing will generate virtually no output UNTIL something contacts the wire and ground causing current to flow.

Your original circuit is close to the mark for your purpose - but ditch the inductor. Instead, put a capacitor from ground to the diodes, lets say 100nF. Now you want to sense the voltage from the hot wire. Something like a 47pf cap would give you around 2000:1 divider ratio (so a 20KV pulse should give you about 10V input, which should be clamped by the zener). This cap could be:
 * a 47pF HIGH VOLTAGE CAP RATED TO AT LEAST WHAT YOU THINK YOUR OUTPUT IS connected to the bare fence wire itself
or
 * a high-voltage insulation wire, suitably capped at the end to prevent flashover, wrapped several times around the bare fence wire. How many times will depend on the insulation type, thickness and wire conductor size. If you have a DMM that measures capacitance, just wind on enough turns to get around 50pF
 * A metal clip that you can clip over the INSULATED wire section of your energizer output.

Hope this helps....

[David HK]

For clarity I am not trying to measure volts or amperage, I am aware of your earlier recommendations and and am working to obtain the various parts.

Just now I rolled out the entire length of my extension power cable along my street and then connected it up to the main transformer output.  I reconnected the capacitive circuit you noted to be  wrong to a battery and walked down the street testing for a pulse at undetermined intervals. Sure enough it picks up the pulse which is exactly what I want it to do.

I don't understand why you write that its wrong. I do not need to know how much current there is in the line - just that the line has an active pulse in it.

Dave

[David HK]

Our notes crossed over - will do what I can to follow your update.

Dave

[rossw]

Here is a capacitive version

This is a lousy design. Honestly, where do people get off with this sort of stuff??
I'm not going to pick on several particularly poor choices they've made, but what I WILL point out is the reason I really hate it, and why my suggestions (above) were as they were.

Capacitors, as you know, have an "equivalent" resistance at a particular frequency. So a 1uF cap for example, if given a 1KHz sinewave will behave much like a 159 ohm resistor. Change the frequency to 10KHz and it looks like a 15.9 ohm resistor.
The circuit, as shown, is a capacitor (formed by the wire in the plug lead as one plate, the insulation of the plug lead as a dielectric, and the metal clip as the other plate). That's fed to a resistor (and the SCR gate), with absolutely no current limiting). Lets assume the clip forms a capacitor of 100pf (just for convenience).
Lets say the sparkplug is firing at just 20 times a second. Some would say 20Hz, 100pF, 79 megohms.
So we have a divider with a 20KV spark, 79M resistor to a 3K3 resistor as the divider.
We should get about 835mV out. Might just trigger the SCR.
The reality is, a spark has a VERY fast risetime. To the capacitor, it's going to behave more like 100KHz
At that frequency it's looking more like 16K. 20KV and you're going to get something like 3.4KV at the divider (because the resistor is still 3K3). The 1N4148 is facing the wrong way to clamp it, the SCR gate will need to. While the pulse is short, there's a lot of stress on things.

Using a capacitor+capacitor divider, both caps will see the same "signal frequency". Regardless of the pulse risetime, the divider RATIO will remain the same. This is true for R+R and C+C but not R+C or C+R dividers.

[David HK]

Ross,
I think there has been a misunderstanding here.

I am not intending to plonk this capacitance sensor on a ‘live’ wire non insulated electric fence. It struck me that the pulse from an electric fence machine was similar to that in a car ignition system and there may be some occasions where a simple piece of test equipment was required to test the output albeit in the form of a pulse. One could equally test for voltage with other equipment.

I am fully intending to follow your recommendations to test for amps and volts, but the first problem I have is to obtain the parts. 20kV capacitors are almost impossible to find locally except perhaps in a scrapyard but 2 and 3 kV capacitors are around in various shops.

For the 1,000 turn coil action is in hand to make a coil bobbin, but if you could indicate a wire size that part of the project could be accelerated.

Dave

[rossw]

Ross,
I think there has been a misunderstanding here.

I am not intending to plonk this capacitance sensor on a ‘live’ wire non insulated electric fence.

Yes, but when you "make" a capacitor from a wire wrapped around your (insulated) wire - you're making a capacitor. If the wire insulation is good for sparkplug voltages guess what? You've just made a 20KV+ capacitor!

It doesn't NEED to be a device manufactured as a 47pf/20KV cap if you can MAKE one

[David HK]

Reference you last note.

I will make one.

Give me a while to get things organised, but before I commence construction I will show you a proposed circuit layout so that you can advise accordingly.

Dave

[madlabs]

Whew, a lot to catch up on in this thread. A quick question or two...

So when Dave's zapper arrived, it didn't work. I tested the output transformer by putting 3VAC into one side and seeing what came out the other side, which was 4.75VAC. So I unwrapped the transformer but didn't find any obvious damage.

Anyway, to the question: Think I can get away with using .55mm wire to rewind it? I have a bunch of triple coated polyimide wire in that size from my HV days,

Jonathan

[David HK]

Ross,

I am about to start making up the 1,000 turn wire coil.

You mentioned previously:- 

2. an inductive loop CURRENT sensor. Basically, make a CT. If you wound say 1000 turns of fine wire around a core, passed your high voltage output (via suitable high voltage cable (sparkplug lead?)), and put a burden resistor across its output, you should see 1mA for every 1A in the "primary". Choosing a suitable resistor (10R lets say) would give you 10mV out per amp in the primary.

Could you please offer me an idea of wire size # so that I can finalise bobbin width and height.

Dave

[rossw]

Could you please offer me an idea of wire size # so that I can finalise bobbin width and height.

Well, I mentioned 1000 turns only because it makes the maths easier.
Given the current in the (secondary) is quite low you can use wire thats pretty much as thin as you are comfortable using. That stuff thats thinner than human hair can be pretty hard to work with! I really don't think it's anything to be too hung up on. I wouldn't use anything heavier than 28 gauge, I'd probably use 30 or 32 gauge if I were making it myself, but even finer should be ok.

[madlabs]

Dave,

Well, drat. I rewound the transformer today and while I am getting a little more out of it than before it is still not right. I should have replaced the primary as well. Now the input voltage drops to 1V and I am getting 6v out. So better than before but I am guessing that I damaged the primary windings, as I had a higher primary volatge last time. And I thought of replacing the primary but didn't have quite enough appropriate wire.

Oh, and the transformer that I am using to feed the transformer is an old filiment transformer and rated for 15 amps. It only dropped from 3.3VAC to 3VAC last time, so the primary must be toasted.

Back to the drawing board,

Jonathan