Electric Fence Tester

Max Carter

I probably could buy an electric fence tester for 15 or 20 bucks, but I had these items in my pile of electronic junk, so decided to build one instead..

An electric fence charger puts out a high voltage AC spike, not unlike that of an automobile's ignition system, every couple of seconds. It looks something like this on an oscilloscope (Fig 1). Connected to a well maintained fence, the charger can deliver a painful shock (not lethal) to any grounded critter that comes in contact with the fence.

Figure 1

Bad insulators, weed shorts, poor connections, dry ground, etc, can reduce the voltage available on the fence and degrade its ability to discourage critters. A handheld tester can help find defects.

Tester Schematic

C1 and C2 form a capacitive voltage divider that steps down the fence voltage to a range that can be handled by ordinary low-voltage components. The values shown divide the input voltage by 100. So, for example, 5000 AC volts (peak) on the input shows up as 50 AC volts (peak) at the junction of the two capacitors. Given the short duty cycle of the input pulses, the effective voltage at this point turns out to be around 4.0 volts RMS. The diode rectifies the pulses to DC. The NPN transistor, connected as a voltage follower, supplies current gain (~200) sufficient to drive the meter. C3 acts as a damper to prolong the pulses enough to allow the meter to respond. The 200k pot calibrates the meter so as to produce a full-scale reading from the charger with the fence disconnected. Subsequent measurements are relative to this reference.

Builder Notes

• A value for C1 in the 50-100 pF range will extract a useful sample without loading the fence charger. C1's voltage rating should be 20kV or higher. It's OK to combine capacitors in series or series/parallel to meet the capacitance and voltage specs. The large capacitor shown on the prototype below is a high current model meant for transmitting applications I happened to have on hand. Here's a source for a more petite (and way less expensive) model:
  
  
  
  
  
  
• Select C2 for a capacitance value 100 times the value of C1. For example, 100pF x 100 = .01µF. Voltage rating ≥50V. Note that raising the value of C2 lowers the output voltage.
  
  
• 200k CAL pot Futurlec TRIM200K
  
  
• A 50µA DC meter is available (for example) here.

As Built Prototype

(back)

Grounding rod

About 13" long (325mm), made with 7/32" steel rod, sharpened at one end. A long bolt or nail would work as well. Pushed six inches (150mm) or so into the ground, the rod makes a connection sufficient to operate the meter (≦100kΩ). Pour a cup of water on the driven rod if the soil is exceptionally dry.

Calibration

• Turn the CAL pot fully counter-clockwise (maximum resistance).
  
  
• Connect the tester to a functioning charger with the fence disconnected. Connect the tester's ground lead directly to the ground terminal on the charger, then touch the probe to the "hot" terminal on the charger.
  
  
• Gradually turn the pot clockwise until the meter pegs on the pulses (pointer goes to full scale and a bit over). Full scale becomes the reference for future voltage readings.
  
  
• Reconnect the fence. With the fence connected, the meter could read anywhere from half scale to full scale. Fences are different and different chargers act differently. Precipitation - fog, drizzle, rain - will affect the reading.

Testing Electric Fence

• Start at the charger location. Drive the ground rod and connect the ground lead. Touch the probe to the fence. Full voltage (half-scale and above) is an indication that the fence is free of shorts (weeds, etc).
  
  
• If reduced voltage is noted (below half scale), head down the fence line, away from the charger. Look for faults like weeds shorting the wire, drooping and/or twisted wire, broken insulators, bad grounds, bad connections, etc. Drive the ground rod and check the voltage after each fault is cleared. Keep looking and fixing until normal voltage is obtained.
  
  
• If you suspect the fence may be open somewhere, proceed to the far end of the fence and check the voltage there. If normal voltage is present, the fence has normal continuity (is not open). If lower than normal voltage is noted, proceed back toward the charger, testing along the line until the open or bad connection is found. Repair the break and re-check. If normal voltage is then noted, the fence is fully operational.
  
  
• Note on battery life: The circuit draws zero battery power when not receiving high voltage pulses, and only tiny currents for short durations when receiving pulses. The life of the battery will approach its shelf life. It should last for years.