Tail Biter

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MadAlexis

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Hi All

I am not too familiar with High Voltage circuitry. Work of the devil in my opinion!

Anyway I want to design a solid state tail biter circuit that can terminate a 10 KV pulse after 10 ms. The maximum energy transferred will be 8 Joules. Any ideas on what devices one can use?

Please don't say IGFETS or Thyristors without referencing an applicable part number. Most of these devices are only rated to 6.5 KV. Hence my post.

Alexis
 
well it depends if you want high or low frequency. if it is high frequency electricity you want try one of those curcuits out of a plasma ball. they are basically tiny solid state tesla coils. or if size isn't a problem an ignition coil would work.
 
"solid state" and 10KV rarely go together

What approach are you aiming for? do you want to "turn off" or "disconnect" the 10KV output like a switch? (usually applied to a voltage fed source) Or do you want "crowbar" the output and short it? (usually applied to a current fed source)
 
i didn't really mean solid state as it does have an onboard (coil). and the one i have must come pretty close. almost 1 cm sparks
 
hi alexis,

Can you tell us what you are planning to use this gizmo for??
 
Hi All

I'm thinking of designing a fence energiser. The safety specifications are as follows:

Maximum on time: 50ms
Minimum off time: 750ms
Maximum energy: 8 Joules.
Load: 500 Ohms

Now the way I see it, most energisers give out a 50ms pulse. This means that the peak voltage is not as high as a 10ms pulse. The voltage in a 10ms pulse can be a lot higher to dissipate 8 Joules than 8 Joules across 50 ms.

In order to "be safe" I want to add a tail biter, so that I am sure that I discharge the fence after 10ms. Because the fence has resistance and capacitance (2nF) residual voltage could remain. One could use a spark gap but I'm looking for something else. I have been pondering whether one could use a magnetic amp, by biasing the core with a small DC voltage one changes the saturation point.
 
hi Alexis,
You could consider using an old car ignition coil with its ballast resistor and capacitor.

A 1sec relay pulser circuit driving a relay would give you the timing and pulse energy you require by adjusting the supply voltage and dwell time.
As you probably know the HV is generated when the relay contacts open and break the heavy current in the coil.

When the relay driver is powered OFF, the relay coil contacts are closed and as the ignition coil is wound as an autotransformer the fence will discharge back thru the coil/relay contact to 0V.

Does this help?
 
yeah, as ericgibbs said you could use a ignition coil and a 555 timer and a big transistor that can handle the current. Set it to pulse every .... and your done. As you probably know ignition coils can't kill you
 
things said:
yeah, as ericgibbs said you could use a ignition coil and a 555 timer and a big transistor that can handle the current. Set it to pulse every .... and your done. As you probably know ignition coils can't kill you
Click to expand...

They can if the jolt causes you to fall off a cliff, or fall on your cocked and loaded hand gun, or spaz a screwdrive into your eye socket or... or... The courts will place the cause of death on the person responsible for the fence design and construction.......
 
You are all missing the point. To generate the HV pulse is easy. Anyone can do that. To TERMINATE the pulse and LIMIT the energy to keep it within safety regulations is what I am asking about...
 
Hi Glyph

I initially thought of a thyratron. Trouble is that they are very pricey, and secondly the grid current required is very high. Other than that a thyratron would be ideal.

Regarding the pulse generation, I calculated that a 520 mF capacitor is required on the 12V side to generate 8 Joules. A car coil will not sustain a 10ms pulse for long until it breaks down. The rise time into a 500 ohm load at 2nF is in the order of 1us. To discharge is about 2.2us. Average current to charge the fence is approximately 20 Amps. Peak current is thus approx. 40 Amps. Thereafter at 10KV the current is 80mA. A car coil will break down, it is designed to give short pulses at a high frequency. What is required is a step up transformer with mylar insulation between the windings.

Alexis
 
hi Alexis,

The energy levels you are describing are potentially lethal, why is it necessary to have an electric fence of this strength?

Is it for animal control or what.........?

>> LIMIT the energy to keep it within safety regulations is what I am asking about...

What safety regulations are you referring to??

As Lefty points out, its not always the electric jolt that kills, its the reaction,
the 'shock' produces.
 
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The specs I have quoted are within safety regulations for humans. The fences for cattle and game are rated at 32 Joules. Here are some of my calculations - 20 amps sounds high - but convert it to Joules over the rise time...

To calculate the rise time of the voltage into 500 ohms 2 nF:
t = RC
t = 500 * 2 nF
t = 1 uS.

To calculate the current required to meet this rise time:
Internal stray capacitance is about 12pF.
Load capacitance is about 2nF.

I average-rise = [C internal + C load] dV/dT
I average-rise = [12 pf + 2 nf] [10 000 volts / 1 us] = 20.12 amps

To calculate the Joules used:
V * I * s = Joules
Joules = 10 000 volts * 20.12 amps * 1uS = 0.201 Joules.

Regards

Alexis
 
It's quite interesting to calculate exactly what a Joule is - here are my calculations to determine the outrush current of the capacitor and the size of the capacitor at 12 VDC..

Charge Capacitor size:

Joules = W * s
Therefore: 800W * 10 ms = 8 Joules
Supply voltage is 12 volts:

800 / 12 = I
I = 66.66 Amps

Equation 1:
F = (A * s) / V
F = (66.66 * 10 ms) / 12
F = 55 mF

Equation 2 to verify using coulombs:
F = (C * C) / J
F = (666.66 mC * 666.66 mC) / 8
F = 55 mF.

Therefore charge capacitor must be 55 mF with a 16 Volt rating
 
Interesting note - if my calculations are correct:

8 Joules will move 8Kg 1 meter high, or 1 Kg 8 meters high. Thus it will lift 80Kg 10cm high. That's why it's called "High voltage" *evil grin*
 
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hi Alexis,

The only other idea that comes to mind is a Xenon discharge tube.

I have made 'Rescue' lights using Xenon tubes to discharge a capacitor, charged to a high voltage, using a trigger transformer.

Xenon tubes are available that are capable of 100J+.
Also the discharge times can be microsecs.

A possibilty??, Lets know you think.
 
What's the breakdown of xenon? is there a tube of sufficient length to hold 10kv without spontaneously firining?


Xenon sounds like a great idea. It will even give the fence a "warning" signal indicating the fence is energized.
 
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