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6v from 12v battery

kend

New Member
Hi, this may be a softball from an electronic illterate but here goes.

I would like to rig an electric fence charger that is supposed to run from a 6v lattern battery (read as expensive and does not last long) to work from a 12v deep cylce battery that I already have. I do not have the specs on the device, but can see that it generates the zap from swinging a wheel attached to a clock spring past a coil/magnet setup; so I don't think it pulls many amps.

Are there any parts that I could get from Radio shack that I could solder in to covert it to run on 12v?
 

joecool85

New Member
I'd imagine a simple voltage regulator chip would do it for you real well. RadioShack carries the LM317T which could be set up to output 6v at 1.5 amps, and they're cheap too.
 

duffy

Well-Known Member
For a horse corral?

You are going to need a switching regulator or the efficiency will be so bad you may as well just use a resistor. Fortunately, they sell a cheap little switching regulator module that takes +12V and turns it into +6V:

Digi-Key - PT78ST106V-ND (Texas Instruments - PT78ST106V)

This thing and a capacitor would do it. I'm guessing something around 470µF, 16V for the cap (Radio Shack will have that one). Cap goes on the +6V output to ground, in parallel with the fence charger, and is polarized so mind the markings.
 

kend

New Member
trying to keep cows in.

I picture soldering the part (or parts) into a set of lead wires between the 12v battery and where the current + and - connections for the 6v are. I know what a resistor looks like; does this other part you mention have just two wires like a resistor? I'm afraid I need pretty specific instructions..
 

crutschow

Well-Known Member
Most Helpful Member
For the low duty cycle of the fencer (perhaps a few percent for a once per second pulse) the quiescent power required by a switcher (they require some power even when there is no load) may give a worse efficiency than the 50% of a simple linear drop of the voltage. I would suggest just using some silicon diodes in series (say 9 or 10) to drop the voltage to 6V. You might add a large capacitor to provide the high current required during the pulse.
 

duffy

Well-Known Member
Can't pull more than 1.5A continuous, the lantern battery would run down in less than one day!

Kend - you got the right idea, solder the part on the lead wires like that. The regulator has 3 terminals, Vin, Vout and Com. Vin is (+) from the 12V battery. Vout is (+) to the electric fence. Com is (-) for both battery and fence.

The capacitor goes across (+) and (-) on the 6V for the fence. It's necessary because the fence delivers pulses and we have to make the regulator look like that battery. The capacitor has a low impedance that will do the job.

As to which regulator is the most efficient, generally speaking a switching regulator is the most efficient because it works like a transformer. Everything else just "burns off" the remaining voltage - HALF of it in this case.

That said, crutchow's point about the quiescent current is an important one, but I don't think it will be significant. You would have to have a pretty big idle draw from the regulator and a very low average draw from the fence to make it worse than an LM317T or a bunch of silicon diodes.
 

Video Warrior

New Member
I don't think it would be 1.5A continuous. Just when the fencer is on its "sizzle cycle" time. Still might not be that much. I am wondering about the wisdom of some sort of flyback or despiking diode as part of this circuit though. Seems like there may be some considerable reverse emf flying showing up on that primary. The regulator may already have something for that, but doesn't hurt to over-protect.
 
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duffy

Well-Known Member
The capacitor is there to handle the pulse current. Backspike problems will have been handled on the interruptor switch by the designer of the electric fence circuit so that it doesn't burn the contacts.
 

Andy1845c

Active Member
Have you already purchased the battery? They do make 6 volt deep cycle batterys. That may be the easiest solution. Many automotive battery chargers have a 6 volt setting so recharging it would be easy.
 

kend

New Member
ok, thanks guys. I'm heading to the store at lunch to get a PT78ST106V and a capacitor around µ470.

Andy, I do already have the battery and the 6v deep cycle are unfortunately over $100; I guess because they are less common than the 12v

I think it will pull much less than 1amp because it would run for a couple of months on a lattern battery (they stopped making the battery that it uses). I think the current comes from the pendulum/coil type mechanism in the charger not from the battery. The battery just rocks the pendulum back and forth kind of like a clock. The charger is about 35years old but works much better than the newer ones I've tried. The new ones won't arc to your finger like the old one, by the time the bull gets shocked he is already into the fence and may run through it instead of away which tears up the fence and loses the bull. The new one I have is rated at .5 Joules but I have found some that are 4-6 Joules for several hundred dollars more.
 

crutschow

Well-Known Member
Most Helpful Member
That said, crutchow's point about the quiescent current is an important one, but I don't think it will be significant. You would have to have a pretty big idle draw from the regulator and a very low average draw from the fence to make it worse than an LM317T or a bunch of silicon diodes.
According to the efficiency of the PT78ST106V at low output current, it's internal circuit operating current calculates to be about 18mA. This will draw 13AH from the battery in a month, which seems significant. If we can determine what the lantern battery capacity is, we would know how the switcher current compares to the draw of the fencer by itself.
 

duffy

Well-Known Member
Kend, they aren't going to have that thing at Radio Shack, I don't think. That's something to get online.

Carl, if you're trying to project that line out on the left side of that diagram, it's no more than a guess.
 

duffy

Well-Known Member
Kend, was this battery about a foot across, 8" tall and 3" deep with screw terminals on each end?
 

kend

New Member
Duffy,
That sounds like the size of the 6v battery that it used.
You were right, the guy at radio shack looked at me like I was crazy
 

duffy

Well-Known Member
I remember those batteries, buddy of mine used them on his pony pen. I remember coming across a HUGE mountain of those batteries rusting away in the woods where he used to throw the dead ones. Sorry to hear they quit selling them.

Another battery they quit selling was that 60V battery used in sponge painters.
 

crutschow

Well-Known Member
Most Helpful Member
Carl, if you're trying to project that line out on the left side of that diagram, it's no more than a guess.
I disagree. The efficiency is, of course approaching zero as the output drops to zero. But the quiescent operating current of the switcher likely doesn't change much as the output current drops to zero since, at that point the losses are mostly due to the power used by the active circuitry. Thus you can select a low output current point, such at .12A where the efficiency curve stops, to calculate the circuit current from the efficiency at that point. I doubt that the circuit operating current changes much from that point to zero current output, since most of the power losses due to the output current such as from switch and inductor resistance are now very low.

It may not be completely accurate but it's much better than a guess.

I believe the battery is similar to the RayOVac 641 RAYOVAC© INDUSTRIAL CATALOG. It has about 43.8AH to a 3.6V end voltage. This would indicate the charger takes about 20AH a month if the battery lasts for a couple months.

On this basis the linear drop is slightly more efficient since it will draw a total of 20AH from the battery a month. The draw from the 12V battery for the switcher will be 13AH plus the 10AH for the fencer (at 100% efficiency), giving a total of 23AH.

If you want to use a switcher, you need one with very low quiescent current. Since the average fencer current is only about 30mA you could use a very small switcher with a large output capacitor to handle the peak current.

Incidentally, it looks like you can still buy the battery on-line.
 
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Boncuk

New Member
You were right, the guy at radio shack looked at me like I was crazy
How about asking for two free samples at TI? One for later use and one to fry in experiments. :)
 

kend

New Member
Duffy, I got the pt78st106v and it has 3 prongs to solder to, but they are not marked. Do you know how I can tell which is which?
 

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