Electric fences deliver a whole lot more than 70 volts. On the more standard units that I have worked on the primary voltage is about 100 volts, and the secondary of the step-up transformer has a turns ratio of about 1:50, or possibly much more. The spark from a properly functioning fence charger will easily jump a quarter inch, indicating about 2500 volts. The "high powered" fence chargers, designed to electrify a mile of fence around a horse pasture, use a capacitor charged to about 700 volts to pulse the primary of a much larger transformer. The output of that device will jump over a half of an inch with a loud snap.
As for the possibility of injuring an innocent person, that is unlikely since the system is only turned on when I would press and hold the button. Releasing the button switches the system off immediately. So unless some old lady points a gun in my direction and grabs the door handle there is no way they could get a shock. But, of course, the "Black Max" was taken off the market because somebody felt that it dould be dangerous. They refused to consider that a carjacker with a gun is dangerous. Bottom line: YOU CAN'T FIX STUPID. (note that I did not invent that phrase, but it certainly fits.)
The Black Max is/was a good idea; except - when the bad-guy tried to open the drivers door and got zapped his immediate response was to jump backwards - into traffic. A satisfying end to a potential car-jacker, though tragic to granny who approached the wrong car. I think they have been outlawed. I built a zapper to disuade squirrels from using our bird feeder using a flyback transformer from a TV, a capacitor, resistor, relay and a push button. It never killed a squirrel - quite - but it took them several minutes to recover. Output was in the 12-18KV range and produced just one pulse. Squirrels can fly! But they learn quickly. I think electric fences are 70V open circuit and limited to 60mA. Considering how unpleasant an electric fence is, I can't imagine how painful a Taser must be.
Absolutely :-) BTW I was sitting on my chair writing my previous post with my ear plugs in and as I rolled the chair back a little I could hear eletrical noise in the ear plugs and there was an electrifying sensation as sparks flew from me (my ear canal :-( )) to the metal parts of the ear plugs which got stronger as the chair moved more. That was a deterrent too, I didn't move my chair the rest of the night! We've had a very dry gusty northerly the last few days and the humidity is down below 40%.
The Black Max system as I would implement it would be controlled by a normally off momentary switch. So it would never be active unless it was intended to be active, even safer than a gun because it would not be able to go foo accidentally. It would only be activated when I saw a person with a gun approaching, or attempting to pull the door open. So the little old lady would be perfectly safe unless she pulled a gun. It is intended to be an active response to a specific threat, not a burglar alarm sort of function. Big difference there. Besides that, the system would be installed in a manner such that it could be removed and disposed of in a hurry, being replaced by an electronic siren of similar dimensions.
The only purpose of a "black Max" type device is to discourage a carjacker. And as long as the discomfort is enough to stop them withing a very few seconds that is the only requirement. 40 amps from an automotive battery for even a whole minute would not be a big deal, which is why the power is not higher.
Besides, throwing a solid arc to a bad-guy's hand is a great psychological deterrent, don't you think?
Critic, with a model number like that I am certain that the intention is to mislead the potential buyer, which is something frequently done these days. Intentionally giving a false impression while technically not telling a lie. It happens all the time.
Great explanation, etmax. I didn't know that. Thanks much for the information. It could come in handy as we do future articles about defibrillators (which we have done on several occasions in the past).
Isn't it possible the stun gun is generating very high voltages (through capacitive discharge) but the distance seperating the elctrodes is simply governed by practical limitations (size of device, handling, ...etc) as opposed to the max voltage generated/dielectric breakdown of air (20-30K per inch)? In other words, I could generate 1MV (just an example, not feasible in such a small pachage without serious insulation), but choose to seperate the electrodes 1 inch only.
Hi Will, your calculations are sound but typically a 50,000V generator in a Blackmax type device while being able to deliver 10mA will only be able to supply 5W which means that the voltage will collapse to around 500V.
Of course 500V is plenty to push that 10mA through your skin and 10mA will feel decidecly uncomfortable
The 50kV is so that it will strilke an arc that will penetrate most clothing and while the voltage drop across the clothing may be as high as an additional 500V that still means 5mA which is a deterrent.
Re why only 5W?
Well from a car battery 500W means >40A@12V to produce it and also higher power cause death and liability :-)
On the side, if you removed the skin from a person's right and left hands (skin resistance is high) and attached their hands directly to the terminals of a 12V battery. you would most likely get enough DC current through the heart to defibrillate it. :-) it's all ohms laww
Interestingly, 120VAC is actually more dangerous generally than 240VAC because for vetricular fibrillation to set in the current has to be > than 60mA but less than 200mA. I've often received electric shocks from 240V and usually get local burns because the current is so high. This is a saving grace because it clamps the heart rather than rattling its rhythm.
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