My Electric Fence Energise query - the answer and further questions

[Chris Green]

Well, I have just got round to taking my electric fence energiser apart and now I understand the basic workings.

The the stored energy for the high voltage pulse comes from a pair of

20 microfarad 900v capacitors. These are charged using a switch mode power supply and then are discharged through a step-up transformer to the fence. There is what I presume is an SCR doing the switching (of the capacitors to the transformer) and an electronic timing circuit of some sort triggering the SCR once a second or so.

Thus it's somewhat more sophisticated than I thought with active electronic components in it.

I think the most likely components to be feeling their age are the two

20 microfarad capacitors so I'm looking for replacement(s). I can find zillions of motor capacitors with similar sounding ratings but they're all 450 volts AC so I doubt they're quite up to 900 volts DC.

They are PlesseyDucon, made in Australia, a company long gone out of business.

Can anyone point me at somewhere I might source replacements?

[John Rumm]

How about:

[SteveW]

You can double the voltage rating, but halve the capacitance, by placing two in series. 450V AC (assuming a sine wave) has a peak voltage of

636V, so a pair should be good for 1272V DC.

In this case though, any difference between the capacitors could mean one seeing a higher voltage than the other, so I'd be tempted to use three 67uF in series to be certain.

[Chris Green]

Rather expensive and anyway "Doesn't post to United Kingdom".

Why is it that all these capacitors for electric fences come from Australia!

[John Rumm]

The firm that makes them is Italian... might be worth searching there...

[Chris Green]

"Following the sad news that Icar capacitors entered administration in November 2020", they've gone!

However there are several firms offering alternatives to ICAR capacitors when you search so that has given me a few places to try.

Thanks.

[nothanks]

A word to the wise: caps like this can retain charge for a long time so make sure there's a bleed resistor across them and measure the voltage before getting your hands near. Also, once apparently discharged the voltage can rise again (called dielectric absorption, I think - hazy memory!) so keep something suitable connected across the terminals if handling.

[Chris Green]

Yes, thank you, I'm well aware of the dangers. However I think the design of the fence energiser is such that when you remove mains power the SCR is triggered to discharge the capacitors. There is always a 'tick' after you turn the power off, and when I have checked with a meter the capacitors are discharged.

[Animal]

you've tested the caps?

[Fredxx]

They are likely to be polypropylene capacitors that are meant to be self-healing, which actually means any internal shorts blows the material away to eliminate the short. The value tend to degrade over time but rarely down to zero.

4 of these in parallel might do the job: will be others.

Have you checked the voltage across these capacitor with an appropriate meter? If at full voltage, I guess 800V or so, does it drop every second of so?

Something tells me either you will see a permanent 800V or 0V where the failure is going to be something else? BICBW

[Rod Speed]

Because there are far more electric fences here.

[Chris Green]

Yes, as far as I am able. Both show as rather less than their nominal

20uF value (though just about within tolerance still) but one shows a leakage resistance of only 12 meghohms. That leakage is just measuring with a multimeter so I suspect it might be a lot worse if tested nearer the working voltage.

[Chris Green]

Yes, I'm pretty certain they are polypropylene self-healing. The major application for these capacitors is motor start and I've been wondering if these would be suitable. However, are they robust enough to cope with repeated almost dead-short discharges? That's what the 'pulse grade' are characterised for.

The energiser is still working, just not as powerful as it used to be, so I'm looking for components that might have degraded over 25 years. The pulse storage caapacitors seemed an obvious candidate. The other possibility I guess is the transformer, though that looks pretty sound still and I expect it to be more go/no-go than to degrade.

I will check the voltage across the pulse capacitors though (carefully!) as it could be that the power supply side is not giving the voltage that it should. The only question is what *is* the right voltage, presumably (as you say) some significant fraction of the 900 volt maximum voltage of the capacitors.

[Harry Bloomfield Esq]

Wire two in series, and then you would have the equivalent of a 900volts rating.

[alan_m]

Could it be the fence material itself becoming higher resistance or surface oxidising or perhaps a fence insulator breaking down or dirt etc. causing tracking?

[Fredxx]

<snip>

You can run a polypropylene capacitor close to it's rated voltage, so anywhere between 50% to 80+%; 450v to 720+V would be my expectation.

Less than 450v and I would say there's an issue. If 0V check for a short.