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#126455 - 01/15/06 10:38 AM Fido-Shock With a Variac
Spark Master Flash Offline

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
I've got a Fido-Shock, which electrifies a fence with 700 volts. It's got a transformer that steps the voltage up from 120.

If I power the Fido-Shock with a variac and reduce the input voltage to, let's say, 60 volts, or 350 volts at the secondary...when Fido gets shocked, how many amps will hit him if he's grounded?

How many amps will hit him if the input voltage is 120 and the secondary is at 700?

Is he safer with 60 volts or with 120 volts of input? Would there be a higher amp draw with 60 volts?

Basically, what I'm asking is this: what will hurt him more - 350 volts or 700 on the secondary - and why isn't it the other way? How do amps change from one voltage (350) to the other (700) in this situation?
"When in doubt, short it out"

#126456 - 01/15/06 11:36 AM Re: Fido-Shock With a Variac
Rewired Offline

Registered: 01/01/06
Posts: 567
Loc: Hamilton, Ontario, Canada
Well first of all its going to be MILLIAMPS. If you cranked a few AMPS through your dog at 350 or 700 V you are going to cook it... Volts X Amps = Watts
Watts = HEAT.

Now what Milliamperage that flows when your pet gets a " tingle" will depend on his resistance, there are all sorts of variables...

You will push more current (milliamps) through your dog if he gets hit with 700V because volts is "pressure" and you dog is a big resistor.. (ohms law)...
Chances are the device has electronic components and will not operate properly if you feed it from a 60V source... I would just leave it as is and operate it from the supply voltage its intended to be operated from..

#126457 - 01/15/06 11:48 AM Re: Fido-Shock With a Variac
Alan Belson Offline

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
Fido won't go near the bloody thing twice, so you can turn it off!

Wood work but can't!

#126458 - 01/15/06 12:27 PM Re: Fido-Shock With a Variac
renosteinke Offline
Cat Servant

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
A little knowledge can be a dangerous thing....

Not long ago, a prominent athlete had a relative electrocuted, along with the dog, when they contacted a home-made "fido shock fence."

You might notice that there are different units sold for securing dogs, livestock, and poultry. In other words. the units sold for keeping fido in are specifically tailored for the use; buy a different unit, and it won't work nearly as well.

The power units of these fences often do three things:
1) As you noted, they increase the voltage;
2) They limit the current to a safe level, and often 'cut out' when there is a problem, such as a branch falling across the line;
3) They will also often change the cycles of the power. I've worked with all kinds, and there is a very distinct difference in the effect they each have on the rate at which your muscles contract!

"Bigger" is not always better. This is one area where it's best to stick with the factory model.

#126459 - 01/15/06 02:26 PM Re: Fido-Shock With a Variac
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
The Fido-Shock is quite a bit more than just a transformer.

It takes the input voltage, steps it up to a high voltage, but then delivers a current limited and time limited pulse to 'Fido'. Without knowing the actual guts of the circuit, it is impossible to know what will happen when you reduce the input voltage. If the internal circuitry is well regulated, then the output will stay exactly the same. If it is simple resistor limited, unregulated capacitor charging, then the shock energy might decrease as the _square_ of the applied voltage. If some stranger circuit is used, you might even see an _increase_ in shock energy as the input voltage decreases.

Remember that it is the _current_ that kills, but the _voltage_ that pushes the current through the victim. The idea of these sort of circuit is to have a very high available voltage, to push a shocking current through whatever the random load resistance is, but at the same time you want to limit the current so that if the victim has a very low resistance, the actual current flowing through the victim's body remains extremely low. In the case of capacitive charge systems, the actual current is not limited, but instead the total charge (amp-seconds) is limited.


#126460 - 01/16/06 04:41 AM Re: Fido-Shock With a Variac
Bill39 Offline

Registered: 11/28/01
Posts: 77
Loc: Indianapolis, IN, USA
"Fido won't go near the bloody thing twice, so you can turn it off! "

My dog learned quicker than our neighbor lady. She knew about it but had to keep testing it out.

#126461 - 01/16/06 05:00 AM Re: Fido-Shock With a Variac
electure Offline

Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
I've had some particularly smart Fidos.
A pair of Queensland Heelers that figured out that the fence wouldn't shock them if it was raining. They would run amok for a little while, chase the horses around the pen,and then dive through the doggy-door into the house.
Their next step was to come jump into the bed with me, soaking wet and covered with mud (and manure) usually in the middle of the night.
Quite a way to wake up.

#126462 - 01/16/06 04:42 PM Re: Fido-Shock With a Variac
kdal Offline

Registered: 09/21/05
Posts: 35
Loc: Florida,U.S.A.
Which would you rather get it with, the 350 or the 700?

#126463 - 01/16/06 07:31 PM Re: Fido-Shock With a Variac
Ann Brush Offline

Registered: 12/27/05
Posts: 155
Loc: Ohia
I have sheep on the farm here. We use an electric stock fence to keep them in a 'moving' pasture. The charger plugs into a std 120V outlet and emits pluses of charge at about 10 kV (when there are no weeds on the fence)every second and a half or so. If the electric fence wire gets deeply encroached by weeds the charge the line will hold drops to around 6kV. It HAS to be well grounded (three 8' rods at least) otherwise it eventually shorts internally and has to be replaced. The current model we are using has lasted about 10 years but we have been anal on the grounding (4 rods). Unit cost ┬▒$100 and gives a nasty bite (sheep only need one zap and they keep away - and they are pretty stupid). At one point we patched a section with a bit of 12AWG romex after a kid crashed through the fence with one of those off-road go-cart thingys, you still got shocked through the insulation. To link into the dog thread here, we have beagles and they would jump up at the entrance to a new sliding glass door we installed, I put down a insulated section of wire connected to the stock charger and they only needed one zap to NEVER jump up on the glass again. I have some pictures of the charger and the "dog door-doomer", which I will ask to be posted.

#126464 - 01/16/06 07:46 PM Re: Fido-Shock With a Variac
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma

Which would you rather get it with, the 350 or the 700?

I would rather not get shocked at all, thank-you.

There is actually not nearly enough information to answer the question.

If the zapper is current or charge regulated (which is possible and which would make sense, but which would be more expensive to build), then the open circuit voltage makes little difference! If the voltage is high enough to overcome whatever insulation/resistance is in the path, then the current limiting will cause a fixed amount of current to flow, no matter what the open circuit voltage.

If, on the other hand, the zapper works by charging up a small capacitor to a high voltage, using the _limited_ energy stored in the capacitor as the method of limiting the shock to a 'safe' value, then I'd rather get hit at the lower voltage...the energy in the capacitor scales as the _square_ of the charge voltage.

Finally, the original question suggested that lowering the input voltage would result in a corresponding reduction in output voltage, however this is not at all certain to be the case. So it might be the case that if you set the input voltage to 60V, the output would still be the fixed 700V.


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