Naked Science Forum
Non Life Sciences => Technology => Topic started by: ScientificSorcerer on 08/11/2013 09:29:58
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I was thinking about making a high voltage device, similar in shape and size to a table top Tesla coil or van de graaff generator except my device would have a ball on top and the neck would be one big long capacitor, the capacitor would simply be connected to a battery, a dc power source. I would call it a Leyden tower.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftikalon.com%2Fblog%2F2011%2FLeyden_jar.gif&hash=c4eb25066dbfc90790577f05d7b545de)
I got the Idea from the famous Leyden jar and I wanted to scale it up, and use modern capacitors instead of the using the old fashion jar.
I wanted to "make" the capacitor my self so that I can have a firm understanding of capacitors, I know that capacitors are very simple electrical devices, it's basically rolled up insulator and aluminum foil. But I'm having some trouble. I know that capacitors have 2 ratings farads and voltage, the more volts you have the less farads you get in vice verse, But I don't know what determines these ratings inside the capacitor, I want the big tall capacitor to have as many volts as possible I don't care about it's farad rating (charge rate). How do I go about doing this?
It's my guess that a capacitor's voltage/farad rating depends on the thickness of the dielectric I guess the more insulating the dielectric is the more volts you get? I don't know for shore, If anyone reading this knows what determines the voltage rating in a capacitor your help would be greatly appreciated.
As for the physical properties of this tower I can only wonder, Its my understanding that capacitors store static electricity so I hypothesize that If you are not grounded and you touch the ball on top would that mean that you become highly statically charged? If this were true then would my leyden tower is just a remarkably simple non mechanical van de graff generator? also I've been looking at small capacitors on the internet and what I found suprized me.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimage.made-in-china.com%2F43f34j00JZGEAeyaMioK%2FCeramic-Disc-Capacitors-472-20kv-472m-472pf-Diameter-26mm-P14-5mm.jpg&hash=1f7fce220e7f8ee7ba86861cf934627c)
tiny capacitors like this one can produce upward of 20,000 volts! I can only imagine the voltage rating of a foot long capacitor. But I've seen foot long capacitors and their voltage ratings are often far less, just with a better farad rating.
How do I make a super high voltage big capacitor, with a voltage upwards of 500,000 volts? I was thinking that I could use 2 sheets of aluminum foil and some kind of insulating material, like boric acid coated paper IDK what insulating material would be best for my application. What do you think I should do?
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You have the basic idea right - a generic capacitor (http://en.wikipedia.org/wiki/Capacitor#Overview) can be imagined as two flat sheets of conductor, separated by an insulator.
- The larger the area, the larger the capacitance (for a fixed spacing)
- The smaller the spacing, the larger the capacitance (for a fixed area)
- Inserting an insulating dielectric between the plates increases the capacitance (for a fixed spacing and area), by two mechanisms:
- Selecting a dielectric with a high permittivity stores more charge for a given voltage
- A good dielectric will have a high breakdown voltage (http://en.wikipedia.org/wiki/Capacitor#Breakdown_voltage) (usually measured in Megavolts per meter of thickness)
- Any imperfections in the dielectric (eg pinholes) will cause a breakdown at that point.
For a higher voltage, you need a thicker dielectric. The original Leydon Jars were made of glass bottles.
This leads to the relationship you have observed - high capacitance and high voltage means large size.
The challenge is to provide the largest area in the smallest space, which is often achieved by rolling the plates into a cylinder.
would my leyden tower be just a remarkably simple non mechanical van de graff generator?
It takes energy to separate electric charges and produce a voltage. That's why voltage sources need energy input in the form of mechanical energy for a Van de Graaf generator, chemical energy for a battery or light for a solar cell.
A Leyden Jar only stores electricity. It does not generate a high voltage. You would need to charge the Leyden Jar from a high-voltage DC source like a Van de Graaf generator. The Tesla Coil will not help, because it generates a high frequency AC signal; you want DC.
I am a bit doubtful that the picture shows capacitors. The scale is not visible, but a spark is likely to jump between the leads if the separation is less than 2cm (the breakdown voltage of air is sometimes as low as 1 MV/m). 20kV will quickly leak away through surface dust and moisture if the metal leads are less than several cm apart, which is why capacitors only store charge temporarily. You may have a Metal Oxide Varistor (http://en.wikipedia.org/wiki/Metal_oxide_varistor), which is designed to discharge high voltages, not store them.
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I'm pretty sure these are indeed capacitors, probably 470 picofarad with 20 kV breakdown voltage.
If you have a high voltage AC source you can use an array of capacitors and diodes called a voltage multiplier to generate a much higher voltage than any of the individual components can withstand. This technique was used in CRT displays and elcgtron microscopes and I have used it to build precision x-ray generators up to 300 kV. Any decent electronics manual will show you the circuit, but the trick of getting reliable outputs is in the engineering of the multiplier stack: try paraffin wax as an easy-to-use insulator.
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Careful now ...
This Leyden jar is made from a 5 gallon bucket and stores enough electricity to kill you. (https://www.youtube.com/watch?v=f6_ADvpW0CA)
I strongly recommend building a smaller one unless you know what your doing.
The leyden jar is a very old design dating back to the late 1700's Its simply put a high voltage high capacity capacitor.
Built properly and treated like a loaded gun you too can make your own miniature lightning bolts.
Trust me when i tell you that a discharge from this device is loud enough to damage your hearing so wear hearing protection.
I have experimented with high voltage for many years this is one of the more dangerous projects I have completed so please be careful. I accept no responsibility for your injury or death.
http://www.instructables.com/id/Leyden-jar-of-DOOM/
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I may have worded some of my first post wrong, what I meant to say is that this Leyden tower would be basically one large dc capacitor, this capacitor would have an extremely low farad rating (low amp) and extremely high voltage, I would be happy with a max voltage of around 500kv. It would be less dangerous with less amps in my opinion. if in the event that I did get zapped it would be like getting a jolt from a large van de graaff. I DO NOT want amps in this thing. I want to get as many volts as possible.
I also wonder if I can pass the current through a transformer, I know it sounds like a transformer won't work, but induction works in dc as long as it's a quick jolt, kind of like a faraday ring. It will transform the amps into volts, but I would much rather not use the transformer idea.
How do you make a large capacitor with pieco farad capacity and ultra high voltage.
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... what I meant to say is that this Leyden tower would be basically one large dc capacitor, this capacitor would have an extremely low farad rating (low amp) and extremely high voltage, I would be happy with a max voltage of around 500kv. It would be less dangerous with less amps in my opinion. if in the event that I did get zapped it would be like getting a jolt from a large van de graaff. I DO NOT want amps in this thing. I want to get as many volts as possible.
Energy Stored on a Capacitor ... http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html
The gallon-bucket Leyden-jar (https://www.youtube.com/watch?v=f6_ADvpW0CA) could hold the same amount of energy (joules) as a defibrillator, which could stop your heart.
... a bank of capacitors to approximately 1000 volts with an energy content of 100-200 joules ...
https://en.wikipedia.org/wiki/Defibrillator#Move_to_direct_current
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I may have worded some of my first post wrong, what I meant to say is that this Leyden tower would be basically one large dc capacitor, this capacitor would have an extremely low farad rating (low amp) and extremely high voltage, I would be happy with a max voltage of around 500kv. It would be less dangerous with less amps in my opinion. if in the event that I did get zapped it would be like getting a jolt from a large van de graaff. I DO NOT want amps in this thing. I want to get as many volts as possible.
Let's go back to basics here - Ohm's law. The human body model that we use for safety in medical electronics is 1000 ohm in parallel with 0.1 microfarad. Plenty of people have been killed by electrocution so the numbers are well established.
Now you have a source of 500,000 volts, so the initial discharge current will be 500 amp, regardless of the capacitance of the Leyden jar. The question is whether the current can be sustained long enough to kill you - say 1 second at 100 mA hand-to-hand. 0.1 A x 1 s = 0.1 coulomb.
So we need to store the lethal charge at 500 kV. C=Q/V = 0.1/500,000 = 0.2 microfarad.
I'll leave the design up to you. C= eA/t where e ~3 x 10-11, A is in square meters and t is in meters.
I like the idea of aluminium foil on a plastic bucket - virgin polyethylene is an excellent dielectric so a brewer's tub would be good but builders' buckets, or indeed anything made with black plastic, are likely to be conductive.
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... The human body model that we use for safety in medical electronics is 1000 ohm
When you're dealing with kV the arc can pass through mm/cm of air which is usually an insulator (1016 ohm),
with such high voltages the 103 ohm resistance of human skin is irrelevant and is not going to provide any protection.
Apparently (http://youtu.be/qEJoAJFdz4o?t=5m19s) some Leyden jars using a 2.5 litre container had a capacitance of 6 nanofarad ,
so a five litre (gallon) bucket container could easily be 10 nanofarad.
200kV seems possible with "domestic" Van de Graff generators.
a 10 nanofarad capacitor charged to 200kV stores 200 joules ...
[ Invalid Attachment ]
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html
200 joules is well into heart-stopping defibrillator territory (https://en.wikipedia.org/wiki/Defibrillator#Move_to_direct_current).
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http://www.ncbi.nlm.nih.gov/pubmed/7377095 (http://www.ncbi.nlm.nih.gov/pubmed/7377095) suggests a considerable difference between a therapeutic and a lethal dose. Not that I'm in favour of delivering either in a domestic kitchen, nor would I use a high voltage to do so in any circumstances - 500V is a lot more manageable.
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The above discussion has focussed on capacitance and resistance.
Another important factor is inductance. Your basic Leyden jar has very little inductance, and so can deliver high currents quickly.
If you try to make your capacitor as small as possible by rolling it into a cylinder, this could add considerable series inductance, and discharge relatively slowly. Manufacturers try to connect the foil of a rolled capacitor all along its length, to minimise inductive effects.
I also wonder if I can pass the current through a transformer
You can certainly pass the current from a capacitor into a transformer. This is how Tesla Coils work - A charged capacitor discharges into the inductance of the primary, causing a resonant oscillation. The secondary inductance and capacitance is tuned to resonate at the same frequency.
Be careful of "polarised" capacitors of the electrolytic type (http://en.wikipedia.org/wiki/Electrolytic_capacitor). These have very high capacitance rating in a small volume, but if the voltage goes negative such as in the Tesla Coil, they tend to break down and the electrolyte explodes into steam, spilling corrosive fluids into your circuit.
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http://www.ncbi.nlm.nih.gov/pubmed/7377095 (http://www.ncbi.nlm.nih.gov/pubmed/7377095) suggests a considerable difference between a therapeutic and a lethal dose.
It wouldn't be necessary to cause “morphologic damage” (cook) someone’s heart with electricity to kill them, just apply enough to disrupt its natural electrical activity to cause dysfunction : fibrillation (https://en.wikipedia.org/wiki/Fibrillation) / asystole (https://en.wikipedia.org/wiki/Asystole) , [cf. ECT (https://en.wikipedia.org/wiki/Electroconvulsive_therapy#ECT_devices)]
AC (https://en.wikipedia.org/wiki/Alternating_current) electricity is far more likely to cause fibrillation than the momentary (sub-millisecond ?) pulse which would be delivered by a capacitor , but given the considerable amount energy which can be stored in nickademuss's bucket (http://www.thenakedscientists.com/forum/index.php?topic=49521.msg423601#msg423601) their “may cause death” warning seems appropriate.
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I think the OP has fundamentally misunderstood what capacitors do.
"the capacitor would simply be connected to a battery, a dc power source."
Suggests that he's planning something well under a thousand volts as a source.
"tiny capacitors like this one can produce upward of 20,000 volts! "
No they don't.
"If this were true then would my leyden tower is just a remarkably simple non mechanical van de graff generator? "
No.
Capacitors don't make voltage, they just store it.
The voltage rating of a capacitor is an upper limit, it's not some "set in stone" voltage they "produce.
If you were to build some mighty capacitor and charge it up with a 12 volt battery, the capacitor would be charged to 12 volts.
Unless you have a high voltage power supply, the builder's bucket capacitor is going to have no more than a tiny charge on it and practically no stored energy.
There is a way round this, but it's a lot more complicated.
One of these
http://en.wikipedia.org/wiki/Electrophorus (http://en.wikipedia.org/wiki/Electrophorus)
will allow you to "upgrade" a charge to a much higher voltage, but it's going to take a lot of time and effort to produce an dangerous charge (and there's every chance that leakage currents will mean you never get a very high voltage on your bucket).
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Let's cut to the chase
http://www.instructables.com/id/High-Voltage-Multiplier/ (http://www.instructables.com/id/High-Voltage-Multiplier/)
shows you how to generate lots of volts from an AC source. You can get a 5000 or 7000 volt transformer from a neon sign supplier, and multiply it up to about 60,000 V before the engineering problems become significant.
If on the other hand your forte is mechanical rather than electrical engineering, I commend the Wimshurst machine as a fun way to charge your Leyden jar. http://steampunkworkshop.com/how-build-wimshurst-influence-machine-part-1 (http://steampunkworkshop.com/how-build-wimshurst-influence-machine-part-1)