Charging a capacitor

[BrendanP]

Regulars will remember I posted re. a HV generating circuit a few weeks back. Ive been experimenting with a few options.

Ive settled on adapting a kit/circuit that is used to flash a xenon tube. Nothing new here, a capacitor is charged via a small transformer and a oscillator circuit. The charge in the cap is dump into the xenon tube when the tube is triggered.

pdf here.
http://kitsrus.com/pdf/k163.pdf

(N.B. that in the above C3 is listed as 33uf 350V. The part supplied is actually a 225 /630V device).

I propose to use the charge in the capacitor to produce the shock effect I need. The cap is 225 /630V device. It gives a meaty jolt, I've tested it several times.

As the circuit is now the tube flashes every half second or so.

What will happen if I let the oscillator/transformer part of the circuit simply keep on charging C3/ 630V cap with no flasher part of the circuit connected? Will this damage the cap? Or does the cap simply reach its rated 630V and then develop a internal r that means it can't take any more charge?

 

[Andrew Cowan]

The cap will charge to whatever voltage you are putting in, then develop a sort of internal resistance to charging. The voltage in a cap rises and then levels off (is that inverse exponential)? The capacitor in your circuit will charge to 300v - the transformer voltage.

Safety: If I was doing this, I'd add a resistor to stop it being potentially deadly - which it currently is. You are relying on the resistance of your skin. 70ma can easily kill you - 50ma is still very dangerous. To limit it to 50ma, add a 6k resistor in series with the shock pad. 10k is better, but still very dangerous. You definitly need a resistor.

I've played with camera flash circuits and found they take quite a current even with no load - as much as 100mA.

Make sure you keep one hand in your pocket during testing and trials - you want to minimise chance of current going through your heart. You need to think what would happen if a child touched the HV pad and the (gounded)? body of the feeder, when it was raining, and design it so it won't kill the child.

Be careful!

Andrew

 

[BrendanP]

Thanks for the tip Andrew, I've had the circuit connected to my psu and it like your camera flash draws over 100 milli amps.

I gave myself a few zaps on the hand from it and it left small marks where the wires made contact, I suppose thats indicative of "...danger,danger Will Robinson.....".

 

[BeanieBots]

The circuit uses a flyback method. In theory, such a design has no voltage limit. Certainly many kilovolts could be reached. If you exceed the capacitor rated voltage, it will short internally and be permanently damaged.
Such circuits with some method of clamping. This could be as simple as a neon but that might limit a bit early (~90v) for your purpose.
You could replace the zenon trigger with a load resistor.

 

[boriz]

A shocker has thousands of volts but very few joules. No significant current flows and there is little or no permanent tissue damage. What you’re building is not a shocker, it’s a killer. Just a couple of milliamps can kill. Exactly what are you trying to achieve?

 

[manuka]

" It's the Volts that jolt, but the mils that kill". Over here in electric fence country NZ, fleeting 5-10kV zaps from rural e-fences are an extremely unpleasant experience, but I don't recall any local electrocutions from them. Maybe us Kiwis have thick hides? Even 25kV car ignition systems are only at low current.

However I'd squirm at the prospect of someone with a dickey ticker (= heart problems) being felled by just a few 100V at significant current. Witness the manslaughter charge being levelled => http://www.stabroeknews.com/2009/news/local/05/14/jp-charged-over-electrocution-death/ by someone who'd organised a back yard man trap. Sure -it was meant to deter rascals, but ended up killing one of them...

 

[boriz]

Did you never wonder why 220v mains kills, but a 50000v tazer does not?

 

[Dippy]

Brendan, please take note of the above posts.

That xenon flash circuit you linked will trigger at a certain voltage, keeping the cap voltage within its rating.
But simply putting the unregulated HV into a capacitor could potentially take it over its voltage rating.

Its all very well making a high voltage, but electrically feeble, generator.
But when you start adding energy storage (capacitors) you are making something potentially lethal.

I dread to think how many schoolboys have read this and it now gives them ideas on how to zap their Geography teachers.

Please be careful. This is no defibrillator

 

[BeanieBots]

Please be careful. This is no defibrillator

Well, actually, it's not all that far off the same output
A defibrillator on a low setting will give out about 50J @ 200v.
(Citation http://hypertextbook.com/facts/1999/CindyAnnRomanowich.shtml).

Let's have a look at the stored joules.

E = 1/2 * U^2 * C
Where
E = Energy in capacitor in joules
U = Voltage in capacitor in volts
C = Capacitor capacitance in farads

Let's now see what Brendan's circuit can do.

...C3 is listed as 33uf 350V...The cap is 225 /630V device

If the cap is marked 225, that means 22uF, so let's use the slightly worse design value of 33uF.

Now let's take off the trigger and let it run open loop.
A voltage of 500V is certainly achievable, probably more.

E=0.5*500^2*33e-6 = 4J

That is close to being within the same order of magnitude as a commercial defibrillator
Remember, a defibrillator is cabable of stopping a healthy heart just as much as it can re-start a stopped one.

Enjoy your testing!

 

[Andrew Cowan]

A defibrillator actually stops hearts - not starts them. If you have a heart attach, and you heart starts fibrillating (getting all out of sequence), the defibrillator stops the heart, and the body (hopefully) starts it again. Kinda like holding the power button down on your PC to reset it. That's roughly what happens - ask Dr Ac if you want a better description .

But I think we're going astray here. In answer to the question of why 220V ac will kill you and a 500V taser won't part of the answer is that the 220V is AC, so starts your heart fibrillating. Of course, the duration of the voltage also has an effect.

Static electric shocks are very high voltage, high currents, but only for picoseconds.

Brendan - it may be worth trying a higher voltage and much smaller capacitor. 0.1uF at 1500v = 0.11J which is a lot safer (but still not safe).

A

 

[BeanieBots]

Andrew, your calculator batteries going flat again or been zapped by static?
1500V @ 100nF = 0.11J

 

[BrendanP]

Thanks for everyones input.

I actually was going to ask for an explanation as to the term joules as I've seen it in the data supplied for electric fence energizers so thanks BB.

I understand the basic concept thats its amperage that kills not v per se. I've had some pretty severe jolts of so called bull stopper fencing units.

I need a circuit design for use with a proto stock feeder I'm working on. The shock will be used to deter animals from damaging part of the operating mechanism. I've got a lot of work to do with the project so rather than reinvent the wheel I'd rather adapt a existing design if possible.

It isn't laziness, anyone who has tried to develop a concept knows its massive task, the more unknowns I can eliminate the faster I can get a proto into the field.

Some time ago I pulled apart a off the shelf Waratah energizer that can do kms of wire, it had a cracked psb. I bridge the crack with white wire jumpers, at the time I observed the pcb had three LARGE caps mounted on it. It would appear the cap energy storage then "dumpage" is employed in the more powerful fencer units. (Gallagher is a kiwi manufacturer of fencer units)

It occurred to me today that perhaps a zener diode across the output would be a way to go. If the cap exceeds the zeners rating the zener would break down and short the charge to ground.

I need to get this thing sorted out so I appreciate everyones input.

 

[BeanieBots]

Don't know about in AUS, but in the UK you require certification for such designs and errection of such devices. BUY a proper unit.

Zener with a KV rating, hmmm.....
Think spark-gap.

 

[Andrew Cowan]

Note that those electric fence units use the resistance of the ground as a safety measure, as I said in post 2. Makes it a little safer. The net says a bull fence should use 4000v and 1.5J. The high voltage means even if the ground is dry, and the bull is standing on stony ground (say resistance 200k), the currrent can still be 20mA - a fair jolt.

I presume you'll have a ground earth spike, but as the horse will be close to the unit, around 2000v is about right.

A

 

[BrendanP]

Certification to erect such a unit? Not here in Au BB, its a God given right to head down to the local farm supply and buy the most powerful unit available (or credit card will allow) set it up and let all and sundry, human and animal, "ride the lightening".

 

[BeanieBots]

The authorities get funny about things like that over here.
The general public are not allowed to carry knives, guns or tazers.
Only convicted criminals appear to get away with it, but even they get a "good telling off".

 

[Dippy]

"If the cap exceeds the zeners rating the zener would break down and short the charge to ground. "
- what sort of zener voltage did you want?

Did you mean:- "If the voltage across the capacitor exceeds the zener volatge then the zener would discharge the capacitor down to ground" ??
Zeners don't work like that.

How about using a gas discharge tube so if the volt across cap exceeds the strike then the tube would take the charge to ground?

 

[BrendanP]

Thanks Dippy. Re. the zener thats what I meant. I thought that I could use a zener to limit the v build up in the cap.

I was thinking of a max charge in the cap of around 2K v. I had a very quick look at a zener data sheets and couldn't see anything anywhere near that level.

I'll look into gas discharge tubes as you suggest.

I was laying in bed contemplating fence energizers as I remembered the intermittent high pitch whining sound that fence energizers make as they operate which is very similar to that camera flashes make as they charge up their cap.

 

[BeanieBots]

The two systems are quite different both in design and function.
A camera flash charges and stores ready for use.
A fence energiser provides intermittent HV on demand.
Both use switchers which is why both give off similar sounds.

A discharge tube should be used as a 'last resort' safety measure. It should NOT be your primary voltage regulation. For a fence, your design should generate (almost instantly, as little charging is involved) at the desired voltage.

 

[papaof2]

Off Topic comment related to firearms

The authorities get funny about things like that over here.
The general public are not allowed to carry knives, guns or tazers.
Only convicted criminals appear to get away with it, but even they get a "good telling off".

Guess I'll stay in the US where my state allows legal carry of almost anything, although firearms require a permit for concealed carry (permit requires fingerprinting and background check). There are some restrictions on places you can carry a firearm http://www.georgiacarry.org/cms/georgias-carry-laws-explained/frequently-asked-questions/. Most of the states that allow concealed carry have reciprocal agreements with other states that allow concealed carry http://www.usacarry.com/concealed_carry_permit_reciprocity_maps.html.

If visiting any of the states that allow carry (link above), don't be surprised if some grey-haired guy who walks past you holding the hands of his grandchildren has a pistol on his belt (or under his coat). After all, we were one of the rebellious colonies and some of the descendants are still fiercely independent ;-)

Note that people who have permits for concealed carry are almost never involved in gun-related felonies. They have great respect for the potential power in a firearm and are more likely to walk away from a fight than to participate in one - just don't be the person who puts one of them on a life-threatening situation.

John

 

[LizzieB]

The circuit uses a flyback method. In theory, such a design has no voltage limit. Certainly many kilovolts could be reached.

How did you come to that conclusion?

 

[BeanieBots]

It 'charges' the inductor/transformer and then 'lets go'.
V is proportional to rate at which flux can decay.
Depends on many factors (mostly inductive) but can be very high.
The main point is it's not as simple as transformer action.

Simple length of wire shorted across battery produces sparks.
What's the highest voltage you can get with a 1.5v battery and a length of wire?
Same principle.

 

[LizzieB]

OK, I see, you were referring to the (blocking) oscillator action, it just didn't look like a flyback transformer to me.

 

[212]

I have a cheap fence shocker that has sent some cats to the great scratching post in the sky. The cheap one keeps shocking them when they bite the wire that stung them and they can't let go. The better shockers pulse the shock, and let whatever got popped let go.

 

[boriz]

"God given right"

No right's for atheists?

 

[BrendanP]

A picaxe forum regular suggested this forum would be a good place to get advice on HV topics.
http://4hv.org



"...The maximum energy that can be produced by the security system energiser is limited by design to a maximum of five joules into a load of 500 ohms. The energiser is designed and required to comply with this output even under abnormal operating conditions such as component failure. The overall system is governed by AS3129 & AS3016 with due consideration given to the duty of care associated with such systems. This includes clearances, numbers of wires, the use of warning signs etc. The energiser is designed to be fail safe. The system is constructed with due consideration for duty of care. Warning signs are applied at appropriate intervals...."

How does this compare with earlier observations re. the danger of a 4 joules output from the circuit I linked to?


from:
http://www.qldsecurityfencing.com.au/questions.htm

 

[manuka]

5 Joules may sound puny- as Work= m x g x h this 5J is akin to dropping a 1kg weight under gravity on to your toe from ½ metre. Ouch- but not lethal! Electrically the danger relates to the "shockee" susceptibility, as significant currents may be lethal to some individuals-kids especially. Muscular paralysis and heart fibrillation are the killers. You can NOT predict the shocking circumstances-folks survive 11kV shocks, while others have died on a mere 50V (ac).

Mmm- I'd say you CAN however predict that someone,who's just had a nasty shock, will come & punch your nose in.

 

[Dippy]

Does that quote give a spec for voltage?
What does it assume for the source/storage of that energy?

I reckon we're lacking some schoolboy physics here.

4 Joules (or 5 joules or a 100 joules) is meaningless here without some context.

We all (should) know that a capacitor could store 4 joules at low voltages. (BB showed this in his equation on Page 1 of this thread). It would give a nice spark when shorted with a screwdriver yet totally safe to put your finger across it.

We all (should) know that the damage done to a target depends on the time-period of the transfer of the energy and whether the target can dissipate the energy safely and quickly enough.

You could put 1000J through Stan's ears safely ; but only if the time period was long and not some ridiculous voltage.
And this is related to resistance/impedance and the source type.
You all know about time-constants so you should be able to envisage the situation with a capacitor as the energy source...
However, if you put the same 1000J through Stan's head in a microsecond the he would be pushing up breadboards.
(Or have such terrible brain damage that he turns into a Geography Teacher).

After all, how do you think some pulsed lasers can have kW or MW specs?

 

[BrendanP]

Thanks Dippy and Stan. I am going to buy a copy of the AS3129 standard so I know what I am aiming for. I'll post again once I have that in hand.