Electric Fence Repair

[glee]

I have a 7kV electric fence unit which stopped working.



I replaced the blown 1A 250V fuse, no joy.

Closer inspection revealed slight scorching around the leads of R1 and R2 (centre bottom) and D1 (not visible in image):



Questions:

1. I can see that R1 & R2 are 100k resistors but how do I determine the power rating I need for replacements? The spec sheet for D1 (BYV26E, attached) shows ratings of 1A and 700V, suggesting a 700W power rating. Although I'm sure that's in keeping with the circuit requirements, it still sounds huge to me (and gives me pause when probing around in here). The fuse is 20mm x 5 mm, to give a sense of scale.

2. Do I need to remove the diode D1 from the circuit to test it?

3. Should I just assume that any lead burn implies replacement necessary? That is, from many conversations i get the impression of binary state components - either they are working or not - but I assume there is a gradient of damage between these extremes? Whether some degree of damage makes a component more likely to fail in the near future, seems like an open question.  But from a practical standpoint, are there times when pre-emptive replacement is sensible?

4. How does one calculate how long it takes for the caps to discharge (when a working unit like this is taken offline)? Is there a safe way to manually discharge caps?

Thanks.

[ConnorGames]

First off, for the diode - E-Fences are pulsed, so you need to consider that power ratings are often for average, not peak, power. This is also REVERSE voltage, so you have almost no current flowing at these voltage levels - power would be forward voltage(for 10A surge, 4v for this device)*current. Resistor wattages are usually something you have to guess by size; if in doubt; get the next size up. Resistors can be tested with a multimeter, out of circuit. Those caps should be discharged by now IF the designers were sensible, but I'm not the one to deliver a final word on safety. If I were you, I would check the SCR and transformers/inductors. If the resistors test OK on a DMM, you can leave them, just redo the solder joints. Look up "capacitor discharge stick" for ideas on how to discharge those caps. Diodes should be tested out-of-circuit.

[glee]

Thanks for the reply. That's very helpful.

A bit of search suggests that the resistors appear similar in size to some 2W parts. Does that sound like a reasonable value?

If I were you, I would check the SCR and transformers/inductors.

OK. Any advice for how to go about this?

[SeanB]

If the resistors are OK, I would look at replacing the capacitors C7,8 first. They lead a very hard life, fast high current charging and a really fast discharge into the energiser transformer. They are the most likely components to fail, other than the external transformer.

With the energy level the SCR is easy to check. If it is cracked it is broken, or if it is short circuit ( under 10ohms both ways) between any of the pins it is dead. Otherwise it is likely to be working.

[G7PSK]

The usual thing to go on fence energizers is the transformer shorts on the HT side, the other thing is the capacitors, also check for a neon tube, as many units fit one as an indicator that the unit is on, they will break down with age, on some of the older units the neon was also part of the timer circuit as well.