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EBWs, capacitor discharge, wire blowing


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Topic Summary: High Voltage Discharge

 

Posted by: Deceitful_Frank Posted on: August 28th, 2005, 7:06pm

 

I want to build a small, handheld and cheap circuit to charge a 300 volt 100uF capacitor to full power using

a few "AA" cells and give me 9 joules of energy to send down 200 feet of blasting wire and activate a homemade

electric cap. I've researched this but have found all the information to be a little overwelming! Not wishing to be

spoonfed but could somebody give me some ideas or point me in the right direction?

Thanks in advance

 

EDIT: ... could I dismantle a photoflash lamp or a camera?

 

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Posted by: Blackhawk Posted on: August 30th, 2005, 8:47am

 

You could use the photoflash of a camera, it will supply pretty much exactly what you said. What kind of ignitor

are you detonating? If you are trying to make an exploding bridgewire detonator you will need a lot more juice, if

you are using it to ignite a nicrome/lightbulb igniter why would you need a HV burst when you could use a big

battery?

 

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Posted by: Deceitful_Frank Posted on: August 30th, 2005, 11:32am

 

It is a nichrome or other high resistance wire that I want to heat to initiative a primary, obviously the wire would be

coated with a thin layer of laquer to prevent it from reacting with my HMTD in the cap!

I know that I can acheive this at the end of a length of THICK (to prevent voltage loss) cable, when powered by

maybe 1 or two lead acid batteries but as I said in my post, I want the unit to be handheld plus I'd like to use thin

blasting wire and need a high (100's) voltage to deliver enough juice to a cap hundreds of feet away.

So a photoflash unit will fully charge a 300 volt 100uF capacitor eh... so would just the one capacitor do it or do

you reckon I'd need to rig up a few in parallel?

 

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Posted by: Blackhawk Posted on: August 31st, 2005, 8:38am

 

My concern is that the HV pulse will be gone through the nichrome so fast that it will not heat up to a great enough

temperature to both break down the coating and ignite the primary, by all means test it out though because it would

be interesting to see if it worked.

 

Anyway, a camera photoflash unit will come with it's own capacitor; generally they are between 300 and 350V

and 200-300uF. That said there is no reason you can't link more capacitors onto the charger unit to increase the

total capacity, the only difference adding more caps will make is an increase in charging time.

 

When you disassemble the camera for the flash unit, be careful not to shock yourself (it's hardly dangerous, but it

really hurts when you don't expect it), take the battery out and short the capacitor leads with a screwdriver to make

sure everything is good and discharged before you play around with it.

 

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Posted by: rogue_chemist Posted on: August 31st, 2005, 2:57pm

 

Oh come on now... It does not hurt that much. I like that sort of electrocution. Also, you may end up making an

EBW machine here if your caps are large enough.

 

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Posted by: waxman Posted on: January 16th, 2006, 12:30pm

 

If it is 9 joules you want, 200 MFD capacitance is needed at 300 volts.

E = 1/2 C (V)squared

Parallel capacitors to the photoflash unit to total 200 MFD.

Ensign-Bickford made (make?) cap discharge "initiators" using a tiny low-value resistor that vaporizes nicely.

 

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Posted by: Dragonman586 Posted on: January 18th, 2006, 4:22am

 

So how many joules would I need to make an EBW? Is it within the reasonable range to pile photo flash capactiors

on top of each other in a parallel circuit?

 

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Posted by: Boomer Posted on: January 18th, 2006, 5:37pm

 

Depends a lot on the bridge wire construction. When using TR5 fuses (1-2.5 Amps) I needed 1500 Amps,

1500 Volts reached within 600 ns. That means photoflash's are out. With a shorter/thinner wire (they use 1.5 mil

gold in commercial caps) you can use much less power. They start to function at 200A, so you can use much less

energy than the 16 Joules I had in the cap too. Electrolytics might work with those.

 

Remember it is not the energy - a gram of thermite has KILOjoules - it is the power, i.e. how fast the energy is

delivered. Above ratings from my box equate to a power rise speed of 3 TERAwatts per second,

or 4 *billion* horsepower per second. In other words the peak power of 2-3 megawatts is reached (and over)

within a microsecond.

Posted by: evilgecko Posted on: February 1st, 2006, 6:40am

Doesn't switching become an issue as well? I guess you could use the xenon bulb in the flash unit as your switch.

 

They work by having your 330V potential seperated by a 5mm or so gap filled with xenon (I think). When a very high

voltage is applied around the center of this gap (in the 1000's of volts, there is a very little transformer on the PCB

with a wire sticking out the top of it and connected to the shiney metal plated around the tube which produces this

voltage), it ionises the air and allows the 330V to jump across. However, this loses energy in the form of light and

heat.

 

Spark gaps might be an option, but then again they are just like the flash tubes except in the open air. But 330V

is a little to low for a spark gap.

 

SCR's would be the ultimate big bertha. They are silicone controlled recitifiers. When you apply a small voltage to

the gate of an SCR, the anode and cathode become closed circuit and conducts electricity. They have very little

internal resistance and as its solid state there are no sparks involved, saving you power for the bridge wire. However

SCR's are quite expensive at high current ratings. They are used in rail guns, coil guns, and EMP generators.

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The above is what I saved from the old board. Here is a follow-up, already pm'ed to someone here. When I find the time there will be more theory.

 

What current flows if I discharge a capacitor? Generally speaking, it is voltage / resistance, no matter what capacitance. Only for a small capacitor, the current will end sooner, for a bigger it will last longer.

 

What you need to calculate it is the CHARGE stored in the cap, i.e. voltage * capacitance. One Farad stores one amp-second (one Columb) per volt. E.g. a 5µF cap charged to 2500 Volts (small EBW system) stores 5*2500 = 12500 micro-Amp-seconds, or 12500µC (12.5mC, 0.0125C). It could therefore deliver 12.5µA for 1000 seconds, 12.5mA for a second, 12.5A for a ms, or 12.5kA for a µs (the latter being unlikely, see below).

 

One point to remember is that the current is not constant until the cap is empty, but starts at U/R and goes down exponentially, time constant being R*C. I.e. said 5µ cap will discharge through 200 Ohms in 1ms towards 1/e or 37%. In the next ms it will go down to 37% of that, and so on. We are only concerned with the first third of the discharge curve or less, since if we need the last ‘drop’ of charge our cap is undersized! So the estimate of U/R is reasonable, with a drop of 20% towards the end.

 

The last thing to keep in mind is stray inductance. An inductance as a component is also called a choke, and for a reason. It chokes the current rise speed! Here we have the parasitic inductance of the wires and the intestines of our capacitor. The unit is the Henry (H), and as with capacitors, you will see smaller units like micro- or nano-Henries, i.e. a millionth or billionth of a Henry. Calculating is done the opposite way of capacitors, the rules are mirrored! It is this simple: One Henry slows the current rise down to one Amp per second for every volt. With our 2500V cap, if the circuit has 1µH, and zero resistance, the current will rise with 2500V/1µH = 2500000000 Amps per second, or 2kA/µs. Remember that the cap discharges, so the voltage goes down while the current rises, which rises more and more slowly for that reason. At 5590A the cap is empty, but the current through the inductance has ‘momentum’ and carries on, decreasing again while the cap charges in the other direction. This value comes from preservation of energy: U^2C must be I^2L, with no losses (R=0). At –2500V and zero Amps the cycle repeats with opposite polarity. Then again with positive cap voltage and so on. You guessed it, we get a sine wave, since we just invented the parallel resonant circuit.

 

With real circuits we have losses, since there is resistance. Small losses will make the sine wave go down in amplitude over time, while large losses dissipate all energy in the first half cycle. The time constant for this is …. (I stop here since this was enough theory!).

 

Example: Our 5µF/2500V cap discharges into a bridge wire, total resistance (parasitic resistance inside cap + wire + bridge wire) is 1 Ohm, parasitic inductance is 0.5µH.

 

First approximation: 2500V/1Ohm = 2500A, falling exponentially within 5µs (R*C).

 

Approximating inductance effect: 2500V/0.5µH = 5000A/µs.

 

Combining: This is more than what the resistance allowed, so joining both curves you get a current that ramps up from zero to 2500A in 0.5µs, then falls to 63% of that in 5µs. Roughly a triangle with the right side sinking in. You round the edges at the start and the tip and you get close enough to reality, or what you would see on an oscilloscope screen.

 

Next lesson will tell us whether (or why) this is enough to explode the bridge wire, by learning what ‘burst action’ means! (Just let me tell you this example would work, and quite well.)

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Try another way...

 

Use RG58 coaxial cable for the run to the explosive.

Put the primary explosive in a caliber 22 empty hull.

cut the cable with a clean face having the center conductor

in the center, and the shield around it with NO little wires

shorting it out.

 

Press the cable end into the 3/4 filled hull and crimp lightly

about 1/16 inch from the open end of the hull. OR, if squeamish,

use crazy glue to seal the cable to the hull. Put the cable in the

hull end, a little bit, put a drop of glue to wet the part outside,

and push it in, and let it set.

 

Use an ordinary stun gun on the far end of that cable.

hook the center conductor to one stungun terminal, the shield to the

other. Strip the cable to allow this and leave a couple inches of the

center cable insulation on the center wire.

 

Pressing the stun gun switch will cause a sassy electrical arc at the

other end, which will detonate any primary explosive including low explosive mixtures.

 

To get the feel of all this take 50 or a hundred feet of RG58 cable and apply the stun gun to one end and watch the spark at the other end. It's pretty sassy because the cable capacitance is about 30 micro micro farads per foot, and it charges up to about 6 kilovolts before it arcs over. The longer the cable the longer the spark lasts. It is approximately the length of the cable divided by .6 the speed of light as to time duration.

 

No big cables, no big blasting box, and the RG58 cable is used by all CB radio buffs, and probably won't cost as much as the large wires you would use trying to fire a shorting wire.

 

IF stun gun is outside your budget, use a small 120 volt to 12 volt (or such) transformer and hook the 120 volt side to the RG cable, and put the 12 volt leads in series with the lamp in a camera flash unit, so the flash current goes through the 12 volt winding, and you should get a spart even sassier than from

a stun gun at the other end.

 

Try it with just the cable until you get a sufficiently pleasing spark.

 

Remember the size of the spark is only important if you are firing something like black powder. For primaries, as soon as a single molecule goes, (and a bunch of them at the spark path will do that), the rest can't help going also.

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That is about 'spark gap' detonators, and has nothing to do with capacitor discharge (if you don't count the cap in the stun gun).

 

Having a flash cap discharge 'reversed' through a transformer did work for me too, it was actually my first remote ignition. Problem is you blow the insulation of about 90% of all transformers, they are meant for 120/230 volts, not kilovolts.

 

"The longer the cable the longer the spark lasts. It is approximately the length of the cable divided by .6 the speed of light as to time duration"

 

Yeah sure. So a two-foot cable gives you a 3ns arc, one that's over before the air even *starts* conducting? Sorry bullshit. Your stun gun has a ~300nF cap discharge through a spark gap into the primary of a HV transformer with a ~1:100 turns ratio. Your RG58 cable capacitance adds to the output capacitance of the gun (which new ones are required to have), forming a parallel resonant circuit. The cable capacitance is transformed back at 1:100^2 towards the transformer primary. Get the input inductance and calculate the frequency of the sine wave which gets truncated by the spark breakdown. Now it gets complicated, you got two coupled resonant circuits, the primary cap with the transformer primary, the secondary cap including the cable with the transformer secondary, both coupled loosely by the stray inductance of said transformer (which is quite big as it is open core). Sorry no light speed in the signals here.

 

Where you are right with 0.6 times light speed is when the reflection of the bridge wire current of an EBW travels back to the source, adds up to the cap voltage, and travels again towards the EBW as 2 times cap voltage/cable impedance. This effect gets important only for cables 'long enough' (at 200000000m/s !) compared to the burst time of the wire. Otherwise you see the waveform described in my first post.

 

P.S: Discuss detonators in the HE section please.

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  • 6 months later...

Quick Question,

 

If I were to arrange for a fully charged 330V 230uF photoflash capacitor to be discharged across a 100 ohm 1/4 watt carbon film resister on the end of 100 metres of speaker wire with a total round trip resistance of say 20 ohms, Do you people KNOW if the current would overload the resister enough to cause it to "explode", or atleast heat up in an instant enough to initiate a primary explosive?

 

Ok I think I am reasonable proficient with ohm's law and such but please bear with me...

 

Total energy contained within the capacitor E=1/2QXV^2 or half the charge times the voltage squared. I make this about 12.5 Joules

 

OK so with that wiring setup and assuming the internal resistance of the capacitor is negligable, we would get 275V across the resister at the start of the discharge causing a current of 2.75 amps to flow and a power of 756 watts to need to be dissipated!

 

Now this is 3024 times the power rating of this resister so would it "blow"?

 

Do my numbers add up?

 

Any other thoughts, advice or constructive critisism welcome as always :D

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I wonder if the thin 0.5 or 0.7mm pencil leads can be used as bridgewires in ebws. I have made them explode only just by shorting them on a drill accu.
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I've been trying to make a small railgun(if it launched a plasma spark out the end I'd be pleased) for a while. I've tried to do research and enlighten myself to the mysteries of electricity but I found that I'm completely retarded in that field. So I haven't even been able to charge my capacaters. I was wondering how this could be done to get a spark out of my project that I have almost given up on. I was planning to just charge the capasitors and then touch the wires together(not with my hand of cource) to complete the citcuit and vaporize some aluminium foil between the rails. I have 6 capasitors that I think have a 350volt 2,700 uF capasitance. Does anyone have suggestions to help me? Thanks alot.
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