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Stock feeder HV generator

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BrendanP

Wed Aug 05 2009, 09:20AM

Registered Member #2256 Joined: Fri Jul 31 2009, 08:16PM
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Posts: 1

Im developing a automated stock feeder. The feeder has a hatch which opens and closes under mcu control. If the livestock attempt to jam open the closing hatch with their heads the feeder will deliver a high V low I charge to the lip of the closing hatch to make them pull, ther heads out.

The mcu controls the operation of the hatch and when to shock. The shock will not be anything greater than that delivered by a standard electric fence energizer. Im in the processes of getting the ISO standard for fence energizers and intend that the shock delivered conforms to them.

My usual forum is a mcu based one. They found the HV all a bit too hard it seems, a forum friend recommended I come here for advice.

I have been experimenting with this circuit

Before I go any further can anyone advise me if this sort of fly back circuit is a suitable basis to build on or is it inherently flawed for my application. If it is flawed can anyone advise me on a alternative?

I propose to use the circuit to charge C3 (or alternative rated cap) to a suitable v. How would I ensure that C3 doesn't get over charged given that I won't be using the triggering part of the circuit, C2, Xenon , T2 etc. Once charged C3 would dump its charge into the hatch lip.

How might I control the dumping of the charge at a predetermined v. My first thought was a zener diode that would break down at a given v. I can't find a zener rated at a couple of V though.

I presume that I would use a R to limit the current to a safe level?

I'd control the on off/operation of the circuit with the mcu and a Q.

Any input is much appreciated.

Brendan.

Dr. Slack

Wed Aug 05 2009, 12:01PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

That's as good a circuit as any to use to charge a capacitor with, however this low voltage (350v) backed by a C is the wrong way round to do it for shocking livestock. The low voltage means that reliable contact is difficult through dry skin, across fur etc, then when there is good contact, the C can deliver damaging currents unless R limited.

You would be much better using a larger flyback to store the energy and deliver the output. When the primary current is interrupted, it will deliver a very high voltage, as high as it needs to be to get across small airgaps through fur etc, and delivers a very low current. I suspect you'll find that electric fences are done this way once you start looking into their specs.

A very good flyback to use for this purpose is an auto ignition coil. They are relatively cheap even if bought new, even cheaper if scavenged. Energising the primary from a 12v battery and a relay with mains rated contacts is the absolutely simplest way to do it, and provides reasonable isolation of the 12v and the HV from the MCU. Use an auto-type snubber across the relay contacts to prevent them burning out quickly. You could put a mains switch in parallel with the relay contacts for a very simple way of manually delivering a shock. The energy stored can be reduced by a small resistor in series with the primary. Use a safety gap of a few mm from the HV output to ground to prevent the coil over-volting if it fires with no load on the output.

A potentially more reliable way would be to use a FET instead of a relay, no contacts to burn, but you need a bit more knowledge to build it. A relay can laugh off abuse and misuse, but will steadily degrade over 10^3 to 10^6 operations. A FET OTOH can go on for ever, but will also die in an instant if abused. and you can't see anything moving to tell you whether it's working. You still need the snubber, and a 500v FET; don't make the mistake of thinking the FET only needs to handle 12v and a bit, there's several hundred volts on the switch when it opens. With a FET, you can vary the stored energy by varying the "on" time of the coil before the break, a relay doesn't really have the speed or repeatability to do this fancy control.

Steve Conner

Wed Aug 05 2009, 01:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The only potential flaw I can see is the assumption that a cow, sheep, whatever, will know you want it to pull its head carefully out of the hatch when shocked. As opposed to, say, panicking, thrashing around wildly, hurting itself and destroying the whole setup. (What would you do if you got your head wedged in an electric fence?)

I'd be more worried about that than any electronics issues. I hear Temple Grandin's consulting rates are quite reasonable

Bjørn

Wed Aug 05 2009, 01:21PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If the livestock attempt to jam open the closing hatch with their heads the feeder will deliver a high V low I charge to the lip of the closing hatch to make them pull, ther heads out.

I have seen many cows and some dogs getting shocked and often they do it once and they stay away from the situation that caused the pain for years or the rest of their lives. So if your shock is powerful enough to make sure they remove their heads you have a significant risk that they learn to stay away from the feeder. It is not the pain that keeps cows off the electric fence, it is a psychological effect caused by the first contact so you need to consider this carefully and see if there is an option.

When it comes to the design you need to have significant isolation to stand off the high voltage over time, everything in close contact with cows have a tendency to break down for several reasons. A few years ago my wireless internet connection suddenly got really bad and at the same time my metal detector started beeping for no apparent reason, by triangulating with the metal detector I found a 4 cm plastic isolator that was arcing over and using the electric fence as a huge antenna.

Arcstarter

Wed Aug 05 2009, 10:07PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I would agree with everything that has been said here, about scaring the livestock away from the feeder completely.

But, i do have an idea of how you could make something high voltage that would be able to jump reasonably far across an airgap such as fur, hopefully. You could use something like that circuit you provided. That could charge a capacitor to whatever voltage would be suitable, then have a sidac or a diac with triac/SCR (am i confusing you now?) dump the capacitor into the primary of an auto ignition coil. The capacitor could be charged to around 300 or so volts, and a camera flash circuit could do it, even. The capacitor would have to be a small enough value to insure the ignition coil does not produce more voltage that it can internally or even externally withstand, and to keep the current output down. This would give an extra level of high voltage protection. The camera or other flyback driven transformer would provide galvanic isolation of at *least* 300 volts, which would insure no electronics or the user was not damaged.

That is similar to a taser circuit, but go for much lower current. You can sort of test the current by bringing the ground lead to the HV lead and if it is loud, it'd hurt. If it is quiet, probably not as painful.

Make the capacitor that discharges into the ignition coil primary small enough that it does not really hurt, but provides a rather uncomfortable shock. Though, you would not be able to test (do *not* touch the thing, i am fairly certain you should not put your life on the line) it yourself.

This is just a suggestion for a circuit. If you try it and it makes your livestock break its legs trying to get away, don't blame me or the forum :P.

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