Electric fence

[camoz]

Hey all,

I have been asked by my brother-in-law to look at an electric fence that he bought at an auction (I know big mistake, but he paid $20 so he was happy to take the risk), and of cause it doesn't work. Initially, I said "That's why someone put it in the auction", but then he admitted that he had initially tried using a 12v battery to test it before realising it was running on a 6v battery (it's a solar charged unit). So we are unsure if it was broken to start with, or as a result of using the wrong battery. I was hoping I could get some advice, but firstly let me clarify, I only have a basic understanding of electronics (listened a bit to my father as a teenager, as this was his profession, but unfortunately he has passed away), so please forgive me if I ask some silly questions or make some silly assumptions.

I decided that perhaps as he had doubled the voltage, the manufacturers may have planned for this in the circuit and protected it in some way, so after giving it a few weeks without power to ensure everything had discharged, I had a look at the circuit. From what I can tell (understand), the circuit uses a 1N5342B, 6.8V, 5W Zener Diode to regulate the voltage, and the excess bleeds to ground (this may be one of those silly assumptions I was referring to). Testing the diode with a multimeter, it is open circuit, so excitedly I thought great, just replace the diode and he is back in business. Then the brain started working, and I thought if it had short circuited then yes, but open circuit means the full 12 volts was allowed to flow into the circuit, so this is not good news?

Below is a picture of the front and back of the circuit board (with the diode removed, I have put an arrow to show the location of the diode)

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With the diode in position and running on a 6 volt battery the led was flashing as you would expect, but no ticking or and measuring the output with an electric fence tester, no readable voltage is being produced (the 5 light type electric fence testers).

So I guess my questions would be:

* Would him using the 12V battery have resulted in causing the diode to go open circuit, and if so would it have resulted in damage to other parts on the circuit board (I think that's called a cascade failure).

* I need to order the diode online (can't get it at Jaycar), so is it worth replacing the diode (cost about $4 with postage), or should he stop at the $20 he has already spent and bin it.

I realise that it is probably a bit hard without having the circuit board in hand or a circuit diagram, but any help would be greatly appreciated.

Cheers,

Camo

[camoz]

Hi all,

After some thought, I think I already have my answer on if further components have been damaged:doh: (should have worked this out earlier). If my assumption is correct, that the diode just acts to bleed off excess voltage, then given we were not getting any output when the 6 volt was applied, then obviously something else is damaged.

Given we were getting a pulsing LED, anyone care to suggest the likely damaged item/items?

Cheers,

Camo

[malb]

The images I am getting through the forum don't have a lot of detail but here goes with some basic analysis.

The big orange coloured block is the output transformer that generates the high voltage pulse to energise the line. Adjacent to that are a switching transistor or SCR to trigger the pulse and a fairly large capacitor. I suspect that the output system is a Capacitive Discharge system akin to CDI ignition in cars and bikes.

There are two IC's on the board, do they have part numbers printed on them? I suspect that one is the control circuit for the CD charging system (appears to be a smaller transistor and an iron cored transformer or choke to form an inverter). This system would run continuously while the unit is switched on and powered, and convert the 6V input to a higher voltage (could be up to 200V or more) stored in the large capacitor near the orange output transformer.

The other IC could well be a simple CMOS chip that works over a supply range of 3- 15V. I would envisage that this would be configured as a pulse generator and LED driver system, generating a brief pulse once every couple of seconds or so to trigger the output transistor/SCR, dumping the stored charge in the large capacitor into the output transformer and producing the high voltage output pulse. Other parts of the IC would be configured as a pulse stretcher and LED driver, to produce a longer pulse for the LED so that it becomes more visible to the human eye.

I suspect that the overvoltage may have taken out some of the elements of the CD inverter system (IC, transistor, diodes), preventing it from operating and providing the dump voltge to the large capacitor. Trying to fix the unit would require access to the IC part numbers to establish chip parameters and pin configuration and access to an oscilliscope and high voltage 10:1 probe to analyse the operation without unduly loading it and affecting it's operation.

As mentioned the pulse generator section would probably survive the limited (to it) overvoltage and continue to operate, trying to trigger the output transistor/SCR and flashing the LED.