Exploding resisitors at 100 meteres / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Exploding resisitors at 100 meteres

1 2 3 next

Move Thread

LAN_403

Deceitful_Frank

Thu Jan 29 2009, 10:39PM

Registered Member #1941 Joined: Tue Jan 27 2009, 10:13PM
Location:
Posts: 4

Hello again guys.

Today my new toy arrived via eBay, an 820uF 450V radial capacitor with screw terminals, very cool!

My plan is to charge it to full energy (82J) and then dump all the juice in to an 1/8W resistor on the end of a length of thin cheap speaker wire up to 100 metres long, with the hope that the resistor will EXPLODE. Not just overheat but disintegrate with great violence.

The rules of this board prevent me from saying why I want to achieve this but I am sure many of you could figure it out.

I have a few queries.

I plan to use the circuitry from a disposable camera to charge the capacitor. It kicks out 330V using a 1.5V AAA. Would the tolerances of the circuitry allow me to feed in 2.4V (2 rechargable AAs) for a 500V+ output and simply moniter the voltage across the capacitor and simply charge untill it reaches 450V?

Also, I have been shocked a few times by photoflash caps at 330V 150uF and didn't really enjoy the experience of 8J through my fingers. How would 82J compare and how dangerous is this capacitor to a healthy adult male?

I figure that 100m wire there and back is around 10 ohms so if I use a 100 ohm resistor I can say that 400V will be across it yes?

...I guess that this will force the 1/8W resister to try to dissipate 1600W. Would its destruction be suitably violent?

Also I am considering a bleed resistor to go permanently across it and gather that after 5 time constants it will be safe eough to handle. I wont ask to be spoon fed here as I can do the math but what do you guys feel is appropriate for one time constant for a bleed resisiter? 1 min, 5 mins.... ten?

Lastly would the 100m or speaker wire act enough like a capacitor to hinder the voltage rise across the resistor that I am hoping to destroy? If so are there any solutions besides using thicker wire or just seperating the twin flex over the entire length?

I know these are a lot of questions but sensible guidance would be appreciated!

Thanks and regards!

101111

Thu Jan 29 2009, 11:49PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

Hi, welcome to the forum :)
First of all: You say that you want to 100 meters of cable before the "resistive victim". This is a very good attitude when it comes to high voltage and pulse power but for safety margins you won't need this much, 1,5-2 meters should be more than enough. But then it is also the thing if you are going to use this as a detonator for explosives, then 100 meter is a really good idea.
Anyway, I have messed around with a 200 joules capacitor bank and the "explosions" of resistor, wire, small capacitors and of course wire in salt water can absolutely be loud and but as long as you use ear protection and have a little distance you should be fine.

When it comes to explosion of the resistor: I am pretty sure that it will explode, that much energy at so short time just make the whole thing vaporise -> disappear in a burst of flames.

I don't think that feeding 2.4v into a little disposable camera would make it pump out 500v but there is other alternatives. Uzzors have made made two excellent circuits for use of charging capacitors:

If you are unlucky enough to get 80 joules from one hand to another you are most likely ending up dead.
Se

for more information.

Bleeder resistor should always be used in capacitors bank, depending on use 1-3 minutes should be fine.

I wouldn't worry about the wire acting like a capacitor, but rather the wire act like an inductor, I am not sure but if you get unlucky enough you might get a reverse back charge. Freewheel protection will protect from this.

Now this story might be irrelevant but I say it anyway, it is about one of my accidents: When I was a newb I charged out my capacitor bank in a cup of salt water, the salt water had too much salt in it so when I shorted the wires in the salt water this resulted in water coming up straight in my face and continued to the roof, the cup completely shattered and I got 2000 volt from my left arm to right arm, I consider this as a near death experience.. After that I have always treated high voltage and high current with a huge amount of respect.

Hope this helps :)
Regards

rp181

Fri Jan 30 2009, 01:06AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

A resistor won't "explode" its a resistor. It will draw current and slowly burn up. A much better explosive result would be from a single strand of wire.
By 100 meters, im assuming you mean a explosive, so don't do it.

Coherent

Fri Jan 30 2009, 01:14AM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
Location:
Posts: 73

The resistor won't blow up. It will just rapidly overheat and catch fire while draining the cap. One idea that you can try is using a small low voltage electrolytic cap instead of a resister. Have the big cap discharge into the smaller cap and watch as the little cap explodes from overvoltage.

Fraggle

Fri Jan 30 2009, 01:35AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

A loop of thin wire encased in a mixture of match-heads and epoxy makes an excellent igniter.

aonomus

Fri Jan 30 2009, 02:08AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I read a document regarding using 1/4 watt resistors as model rocket engine ignitors, however I don't think you would get the needed shockwave to detonate secondary explosives. Regardless its illegal most likely.

Dr. Slack

Fri Jan 30 2009, 09:40AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I did some experiments with 1/8th watt resistors a long while ago to serve a similar duty.

I discovered that dumping a 100R resistor in contact with some ignitable material directly onto 240v mains ( a similar sort of voltage and impedance to what you are considering), there were two very common failure modes. The first is that usually the material didn't light, like lit less than 50% of the time, modern resistors getting too damned safe! The second is that the resistor arced and blew even a 13A fuse, which was tedious, however it appeared that the arc was so short in duration and low in voltage that it often didn't light the material.

The solution was to put an inductor in series with the resistor, I used one from a large flourescent lamp. This a) limited the current, so the fuse didn't blow and b) stablised the arc, reduced the arc current, increased the arc voltage, increased the arc time, so increased the total energy dissipated in the resistor, so I acheived near to 100% success rate in lighting.

The difference was that without current limiting, the resistor would often appear to have nothing more than a crack or two in its protective paint film. With current limiting, the entire resistor would be consumed by fire. Your 1600W figure is only true as long as the resistor is 100 ohms. As soon as it arcs, the effective resistance will drop. Try to make it a controlled burn rather than an uncontrolled bang; unless you want a shock wave, in which case you won't do it with a resistor.

It may be that the resistance of the wire is sufficient to current limit and stabilise, otherwise you could experiment with an inductor

If you had an electrodynamic (slapper) shock wave generator at the far end, then you'd need matched trnasmission line, and an electrolytic capacitor would not be fast enough, you'd need a pulse cap. With a thermal device at the other end, it's all so slow you can forget about line reactance.

Deceitful_Frank

Fri Jan 30 2009, 06:29PM

Registered Member #1941 Joined: Tue Jan 27 2009, 10:13PM
Location:
Posts: 4

Thanks guys for your input. I have posted this thread on three forums and the general view seems to be that in order to get a real shock wave I need KILOvolts across the resistor which is difficult to achieve let alone hundreds of feet away without putting myself in mortal danger. And lets not forget the financial and time cost involved to realise this!

In my time I have conducted many amateur scientific experiments and have been a lapse of concentration away from instant death, even dismemberment a few times but I always try to balance the risk involved against the gains.

This 820uF 450V cap is obviously not up to the task for which it was originally intended and would just in reality be great for firing an e-match... I can do this with a photoflash cap holding a tenth the energy and I know that the penalty for failure is burnt fingers and extreme discomfort at worst. If I fuck up with this big cap I am dead. It only cost me Â£6 so no great financial loss.

I know to many of you this may sound like a wimpy cop-out but the cemetry is full of heroes.

I will be scrapping the exploding resister idea in favour of heating a length of nichrome wire to dull red heat in the hope of "cooking it off to DDT" Whether or not those five words mean anything to you will depend on which forum you are reading this post in but thats all academic really. I have toyed with this Idea in the past with a view to igniting a simple mx of fuel and oxidiser but this is different.

All I need to do is select the right componants to build a device that will glow the 50mm length of coiled nichrome at 100 metres. The rest would be in the lap of the gods!

Backyard Skunkworks

Fri Jan 30 2009, 06:46PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

If all your looking for is a good electric igniter, you should try the good old method of wrapping a sliver of aluminum foil around a match head and shorting it across a good sized battery, using fairly large wires to carry the current.

ScotchTapeLord

Fri Jan 30 2009, 08:59PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Have you considered steel wool? That is fun to run charges through, and it should be effective if mixed with something flammable.

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob