HV Capacitor for fence ctrl. 2

[GonzaloEE]

Hi all,

I'm designing an electric fence controller, and currently working on the HV stage. I'm leaning for a standard appproach with a switch-mode charger for a big HV cap, and a SCR to dump the cap charge in the HV transformer, connected to the fence.

I'm looking for a 1200Vdc, 20uF cap for the HV pulse generator, but those are difficult to find. I appreciate any cap supplier contact info for this application. Any ideas or hints from more experienced designers are welcome, as well.

 

[zappedagain]

Kemet makes polypropylene film capacitors with those ratings. Why do you want to store that much energy?

Z

 

[hacksaw]

most fence chargers need to consider harm to the animal and you also need to consider your local ordinances,

 

[ScottyUK]

Evox Rifa have some big high voltage polypropylene film capacitors designed for pulse and power applications.

Have you considered a power factor correction capacitor? Not sure on the continuous DC rating but likely to be 1200V or higher for service on a 400V system. They won't be as good as the polypropylene types in terms of pulse performance but for an electric fence is it that critical?

You are storing a large amount of energy - follow hacksaw's advice and check the permissible limits.


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If we learn from our mistakes I'm getting a great education!

 

[OperaHouse]

Is there some reason you are not taking the same approach as everyone else in your design? 1200V seems the hard and expensive way to do it. The stored capacity only seems suitable for a prison electric fence in a third world country.

 

[GonzaloEE]

Thanks for the guidance and safety warnings. I'm new to these kind of devices. We got an old fence charger from Gallagher (NZ), using a big pulse transformer for the fence. The cap is a Plessey-Ducon P124, 1200VDC/20uF, about 3" tall. The manufacturer folded in the 80's, so I know this is not a state-of-the-art circuit.

We need 6-13KV for a 4000mts cable length. The pulse must be short enough not to harm animals/people or burning dry grass. Not sure, but any pulse <1ms or 2-3J energy would be OK with our local regulations.

A starting point may be a very simple switching inverter, to charge the cap through a diode bridge. A cheap SCR would discharge it through a HV transformer primary, with the 2ary connected to the fence.
A microcontroller would drive the inverter PWM and provide voltage control and misc. funcions (alarms, line/Earth monitor, etc.)

As said, I'm new to this so any hints, better ideas or sample circuits are highly appreciated.

Thanks.

 

[OperaHouse]

The ones I've seen are a slight variation of a automobile CD ignition system. Charge up a cap to 300-400V and discharge it into a 12V ignition coil. Still need a good cap that can handle current, but these are still reasonably priced compared to +1000 volt units.

 

[itsmoked]

Yes, I agree with Opera: "You can run the cap at a low voltage".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[GonzaloEE]

Thanks for the lead. Looks like it's all about a trade-off between the cap and HV coil ratings.

Is correct to say lower voltage means higher capacitance, to keep the same pulse energy (roughly 1/2CV^2)?

Thanks.

 

[alehman]

only seems suitable for a prison electric fence in a third world country.

Interestingly enough, "lethal" electric fences are being installed at prisons in the U.S. as well. My firm was involved in the installation engineering for a few of them a number of years ago. They used a current-limiting step-up transformer with some fairly elaborate fault detection circuits.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[itsmoked]

GonzaloEE; Most certainly! You have it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[GonzaloEE]

Thanks itsmoked. I understand the point is using low voltage (and easier to find) parts in all the circuit but the HV coil, leaving the big voltage 'jump' between primary and secondary windings.

Some of our customers would like to check if the gardener cut the lines, without harming him. Is there any way to check for line continuity while the fence is not armed?
something like power line carrier?

BTW, we don't want to decorate our fences with fried people or cattle, just some warning pulses and then switch to an audible alarm if earth got shunted or lines were cut.

Thanks to all.

 

[itsmoked]

You must be a masochist. Have you looked at what you can buy for under a hundred bucks? A 30 mile fence charger.

http://www.patriotchargers.com/

http://www.acehardware.com/sm-electric-fence-chargers--fi-1260226_cp-1254884.1255142.html

http://www.enasco.com/farmandranch/Fencing+Equipment/Fence+Chargers/

http://www.taylorfence.net/

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

Back to my first comment, that's a lot of stored energy! CV does remain constant, but that is over an order of magnitude more than I would expect. 1200Vdc, 20uF is too exotic for a barnyard application. Could a decimal point have worn off? Compare the physical volume with your replacement. Pulse also seems like it would be too long. My best advice is to step back and look at your requirements again.

 

[GonzaloEE]

itsmoked: Thanks for the links and price references. We were hired as OEM manufacturers by a local supplier, so we can't bypass the design part. Now I see why our target cost is around $40 (x 1000 pcs)

Operahouse: Local regulations here allow us using some 7KV, 2J pulses shorter than 1ms and up to 1Hz rate, measured on a 500 ohm load (rough impedance model for a child or dog, I guess).

We got this sample fence energizer with alarm features, locally available and approved:

http://www.powerfence.com/default.aspx?pageid=459

The label says 6KV/2.9J pulses, 715us wide, each 1.4s. Looks like a good starting point.

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Do you know any transformer supplier for this kind of energizers?

Thanks for your help!