Simple Electric Fence Energizer Using a Car Ignition Coil – The Classic One-Transistor Shocker That Actually Works

Hey friend, if you’ve ever needed to keep pigs out of the garden, stop dogs digging under the fence, or want the cheapest possible electric fence that still delivers a proper “don’t touch me again” zap, then this legendary one-transistor circuit is pure genius. I’ve built dozens of these over the years, and it remains the simplest and most reliable DIY electric fence energizer you can make with junk-box parts.

It uses nothing more than a single TIC106 SCR, a car ignition coil, a few resistors, capacitors, and a 12 V battery or a small solar panel. One pulse every second, 4–8 kV spikes, and enough sting to make a horse think twice — all for under $20. Let’s break it down like we’re sitting in the shed together.

Why This Design Has Survived 50 Years

Commercial electric fence energizers cost $80–$300 and are full of microcontrollers. This one has literally one active device and still outperforms many cheap store units. Reasons I still build them:

• Runs for months on a 12 V motorcycle battery or a tiny solar panel
• Completely waterproof if you pot the board
• Impossible to kill — even lightning strikes rarely destroy it
• Uses a real automotive ignition coil → proper fat spark
• Pulse rate is slow (about 1 Hz) → safe for animals and legal in most countries

I’ve had one running on my back paddock since 2018, and it still zaps just as hard.

Full Circuit Walkthrough

The circuit is a classic relaxation oscillator driving a flyback (ignition coil) transformer.

Power Supply Section

220 V version shown, but 99 % of people run it on 12 V DC (car battery, solar, old PC power supply, etc.).

• 250 mA fuse → 22 µF 350 V electrolytic → 1N4007 diode forms a simple half-wave charger if you really want to run from mains (not recommended). Most builders skip all that and just feed 12 V straight in.

Oscillator Core

• TIC106D thyristor (SCR) — 400 V, 5 A, sensitive gate
• 47 nF + 150 kΩ + 1 kΩ pot forms the timing network
• 15 kΩ 2 W + 15 kΩ 2 W charge the 47 nF capacitor
• When the voltage across the 47 nF reaches about 8–9 V, the DIAC (inside the TIC106 symbol or sometimes a separate ST2) fires and dumps the cap into the gate of the SCR
• SCR slams on hard

Energy Storage and Discharge

• 10 µF 350 V capacitor (the big blue one) is in parallel with the ignition coil primary
• When SCR turns on, the 10 µF discharges violently through the coil primary → massive dI/dt → secondary jumps to 6–10 kV
• Neon lamp across the primary lights every pulse — handy visual indicator
• 4.7 MΩ + 1.5 MΩ bleed the 10 µF slowly between pulses

Output

Secondary of the car ignition coil (any old 12 V coil works — Bosch, Lucas, whatever) → one side to ground, the other to your fence wire through a 980 kΩ safety resistor (limits current if someone holds on).

Pulse energy is about 0.3–0.5 joules — enough to sting hard but well under the 5–10 joule limit where it becomes dangerous to humans.

Parts List (what I actually use)

• SCR: TIC106D, TIC116D, or any 400 V+ sensitive-gate thyristor
• Ignition coil: any 12 V car coil (oil-filled preferred — they’re tougher)
• Main cap: 10 µF 400 V photographic flash capacitor or motor-start cap
• Timing cap: 47 nF 100 V ceramic or polyester
• DIAC: built into most TIC106, or add a separate DB3/ST2
• Resistors: 15 kΩ 2 W ×2, 150 kΩ 1 W, 1 kΩ trim pot, 4.7 MΩ + 1.5 MΩ bleeders
• Neon: any small neon bulb (optional but cool)
• Power: 12 V 7 Ah sealed lead-acid or 20 W solar panel + diode
• Enclosure: plastic lunch box or ammo can

Total cost if you raid the junk drawer: $10–$15.

Step-by-Step Build (takes 30 minutes)

1. Mount the ignition coil vertically in your box — keeps oil inside if it’s oil-filled.
2. Solder the 10 µF cap directly across the coil primary (+ to the + terminal, – to the switched side).
3. Wire the two 15 kΩ 2 W in series from +12 V to the timing cap → 47 nF → ground.
4. Junction of resistors → 150 kΩ → anode of DIAC → gate of TIC106.
5. Cathode of TIC106 to ground, anode to the switched side of the coil primary.
6. Add the 1 kΩ pot in series with the 150 kΩ to adjust pulse rate (0.5–2 Hz).
7. Bleeder chain (4.7 M + 1.5 M) across the 10 µF.
8. Neon across primary.
9. Secondary: one side to ground post, the other through 980 kΩ 2 W to fence terminal.
10. Seal the lid with silicone, mount outside, connect to the fence and ground rod.

Real-World Performance

With a 12 V 7 Ah battery, it runs 3–6 months without charging. On a 20 W solar pane,l it’s truly set-and-forget. Fence length: easily 500–1000 meters of single wire with good insulators. Shock: sharp and memorable — animals learn after one or two touches. I’ve tested output with a scope and spark gap — peaks at 8 kV open-circuit, drops to 3–4 kV into a 500 Ω load (human body model).

Safety & Legal Notes (Important!)

• This is a traditional low-duty-cycle pulsed energizer — legal in most countries for agricultural use
• Do NOT use on fences that children can touch
• Always have clear warning signs
• Ground stake must be a proper earth rod, not a water pipe
• Never run continuously — the 1-second pulse is what makes it safe
• If in doubt, check your local regulations (most allow < 5 joules, < 2 second repetition)

Common Failures & Fixes

• No spark → check 10 µF cap (they dry out), or ignition coil primary open
• Runs too fast → increase 150 kΩ or timing cap
• Weak spark → use a bigger 10–22 µF cap or a 12 V coil designed for electronic ignition
• SCR dies → you probably ran it from 220 V without current limit — use 12 V!

Upgrades I Love

• Add a 12 V 20 W solar panel + 7 Ah battery → zero maintenance
• Use a 22 µF 400 V cap → fatter spark
• Add a piezo buzzer across the neon → audible click every pulse
• Waterproof everything in hot glue or silicone
• Use stainless steel wire and ceramic insulators for long life

Final Thoughts

This one-transistor (well, one-SCR) electric fence energizer is the ultimate example of “why use a microcontroller when a thyristor will do?” It’s been keeping livestock in and pests out since the 1960s, and it still works perfectly today.

Build it this weekend, charge a 12 V battery, string a single wire around your garden, and watch the rabbits learn respect in about thirty seconds.

When you hear that first satisfying “crack” every second and see the neon flashing, you’ll understand why this circuit refuses to die.

Stay safe, keep the output away from your tongue (yes, I’ve seen it done), and enjoy the cheapest, toughest electric fence on the planet.