Electric Fence Charger Repair

Electric Fencer

My parents keep some goats. An electric fence is the best way they've found to contain them. Basically you run multiple strands of metal wire around the field supported by insulators, then hook it up to a "fencer" that pulses a high voltage, pulse through it once or twice a second.

Never Do This (rant)

I've heard of people just hooking the (fence) wire up to a 110/220 volt line. That's a dangerously bad idea! Grab a line hooked up to a fencer and you'll get a jolt, but it's intermittent, and it won't kill you (pacemakers / various health conditions excluded). Grab a 110VAC line in wet boots and you're going to have a very bad day, you could DIE! or your kids, or whoever, it's a really really bad idea! DON"T DO IT!

Back to the topic

Lightning is a potential problem with these as they have electronics in them and they are hooked to (potentially) several miles of wire. A strike within a few miles may well induce a serious spike in it. They are pretty rugged though, as they expect to have thousands of volts present during normal operation (20KV insulation is specified for the hookup wires).

Anyway, we've been using this one for years (10+?) before it just stopped working. We've had to replace the fuses several times, and they were blown, but replacing them didn't fix it this time, so I took it apart. We swapped in a new unit while this one was broken, as we had no way to tell when or if I could fix it.

Operation (seems to go like this, I could be wrong)

• 120VAC is run through a transformer that (I think) both steps up and down the voltage (I didn't test this, and couldn't see any markings) There are fuses and MOV's on the a/c input for protection / filtering. 
• The high voltage feed is rectified and charges the big capacitor. (there could be some flyback or something going on here too, idk) The low voltage is rectified and powers most of the circuit board. (probably some regulation in here)
• The large cap is discharged by the SCR through a second transformer that steps the capacitor voltage from hundreds to thousands of volts. The output is connected directly to the case (ground) and a heavily insulated terminal (fence). The case is grounded by multiple grounding rods with the first no more than 20ft away.
• The whole unit is built into a grounded metal box (Faraday cage?), there should be no arcing inside it (or anywhere actually), but there is a very audible "Tick" when it's working. A wire, grounded to the case, placed near the insulated output will jump a bright blue spark over 3/4 of an inch!

Diagnosis

• This isn't something I want to test on my bench, as the voltages / EM pulses could easily damage my test equipment. 
• Fortunately it's a pretty simple circuit, so it's not that hard to figure out. 
• A visual inspection didn't turn up anything obvious. 
• A few minutes tracing circuits indicated that the TO-220 SCR (2n6509GOS) would be the place to start (this would take the brunt of a surge on the fence loop). 
• I tried to test the SCR in-circuit. It failed, but I couldn't be sure the other components weren't interfering so I removed and tested it. It still failed. So for ~$1.50 I ordered a new one and now it works!

If you mess around in one of these be very, very careful! High voltages all over the place! That cap looks like it could kill a horse!

Top PCB

The main (active) components were the 2n6509, a 2n6028, and a LM324. I ordered all of these, but only needed the SCR.

Bottom PCB

The SCR is a 2n6509GOS 800V 25A

Failed SCR, Doesn't look damaged, but it tested as failed, and replacing it restored the unit to operation.

Links

http://www.circuitstoday.com/how-to-test-an-scr