The idea behind a capacitor bank is to charge up as much energy as possible to short circuit that energy through small coils, aluminium paper, steel wool, wire and a lot of other things that can conduct a electric current. The short circuit current is enormous for a very short time and that big amount of energy can turn the conducting paths material into vapour.
To get an idea about how much energy 4000 Joule is, here is a couple of examples.
A human heart consumes 1 Joule of energy per heartbeat.
4000 Joule could lit up a 60W light bulb for 66 seconds, using a low energy 11W bulb it could be lit for 6 minutes.
4000 Joule is just enough for cooking 50 gram of water, to bring it from 20 degree Celsius to 100 degree Celsius.
WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.
Read this document about safety! http://www.pupman.com/safety.htm
DANGER!: High energy discharges can be lethal, the amount of energy released overwhelms what a human limp or life can withstand.
A long time have passed since I built my first capacitor bank, the 333 Joule microwave oven capacitor bank. Since then I wanted to build a larger, but these large capacitors does not turn up often at a reasonable price. Patience was all I needed before picking up a Maxwell energy discharge capacitor from ebay at 40 Euro. It is not a pulse discharge capacitor, but will be well up for the little wear it will see in this use.
When discharging a capacitor into a circuit with a inductance, which could also just be its own equivalent inductance, the voltage will ring between the capacitor and the inductive part of the circuit, resulting in voltage reversal which can be very harmful for the capacitor. There are many ways to counter this and some of them are complex and expensive, for now I will ensure that a part of the circuit will be a thinner wire, that will always explode, and thus cutting the circuit.
Wear and tear on spark gaps is highly dependent on the energy transfer taking place. It is not as much affected by very high peak currents or energy levels, but the charge in Coulombs. Using the capacitor energy calculator on this site shows you both values and you will quickly discover that a high capacity low voltage bank will have a high stored charge versus a low capacity high voltage bank, despite they have the same energy stored, the factor in stored charge is 10 times higher for the high capacity bank.
|High voltage supply||776 to 1855 VDC from two transformers in step up setup and a voltage doubler.|
|Full charge voltage||1800 VDC.|
|Stored energy||4050 Joule.|
|Stored charge||4.5 Coulombs.|
|Trigger mechanism||Spring loaded spark gap switch.|
Having a capacitor bank with a large capacitance, I found it attractive to be able to charge it to different energy levels without being forced to use a variac. Knowing the exact energy stored will also make analysis of the discharges more precise.
By using the multiply input voltage taps on the step down transformer with an input voltage of 230VAC I get a varying output that I put into a step up transformer where I use the 656VAC output tap. Doing this I get a output voltage range from the voltage doubler from 776 volt to 1855 Volt.
Voltage² (Volt) • capacity (Farad) • 0,5 = energy (Joule)
1855² • 0,002423 • 0,5 = 4168 joule
In the following table I have all the charge possibilities listed.
|776 Volt||729 Joule||1 and 7|
|813 Volt||800 Joule||2 and 7|
|854 Volt||883 Joule||3 and 7|
|889 Volt||957 Joule||1 and 6|
|928 Volt||1043 Joule||2 and 6|
|970 Volt||1139 Joule||3 and 6|
|1028 Volt||1280 Joule||1 and 5|
|1067 Volt||1379 Joule||2 and 5|
|1123 Volt||1527 Joule||3 and 5|
|1855 Volt||4168 Joule||2 and 4|
I wanted to as many of the parts I have already at hand and focus on using parts that are odd and will have a hard time to find a place in other projects. The first focus of this was on the power supply, looking through my stack of transformers I found two that could be used for a step up transformer arrangement with the possibility of lowering the charge voltage as described above under calculations.
Capacitors and diodes for the voltage doubler is all salvaged from old electronic equipment and is right about on the edge of their ratings. Each string of diodes can withstand 2400 volt and the capacitors can withstand 1800 volt. I hope my decision about the capacitors is good enough, I did it to use a minimum of components and still maintain a capacity large enough to smooth the DC properly.
The spring loaded spark gap switch and charger switch is a copy of the switch I built for the 333 joule capacitor bank. I was a little worried that the rather small construction was not good enough for roughly 12 times the energy. I reinforced the switch with some heavy copper pieces and larger gauge wires and mesh. The biggest advantage of this switch is that I avoid making a protective circuit for the charger as its completely disconnected when the capacitor discharges.
In order to keep track of capacitor lifespan, here is a list of the different shots that have been made with it.
|100 Joule||3x steel wool + 1 Ohm resistor|
|300 Joule||2x steel wool + 1 Ohm resistor|
|700 Joule||2x 10 strands of AWG40|
|1000 Joule||8x steel wool
1x 200cm 0.25mm copper wire
|4000 Joule||4x steel wool
1x C10A miniature breaker
Crushed cans, sparks and explosions
5th August 2012
The was the first test shot, 1 kJ into a small twist of steel wool.
11th August 2012
The current measurements was done with a Pearson current monitor model 101 connected to a 10x probe and a Rigol DS1052e oscilloscope. Attenuation on the oscilloscope was set to 1x. So numbers should be multiplied by 10.
Current measurements of steel wool, 60 mm length, 10 mm diameter, with 1 kJ energy discharged. Measured peak current 13 kA.
Current measurements of steel wool, 60 mm length, 10 mm diameter, with 4 kJ energy discharged. Measured peak current 29 kA.
A close up of the trigger spark gap doing a shot and the wear caused to it from about 10 high energy shots.
Having only fired the capacitor bank once, at 1 kJ, it is a little early to draw any other conclusions than it works as planned and its an ear deafening loud blast when it fires.
Having now conducted a 4 kJ shot, I can only say that it is a far as one should go with energy discharges in a small room. Feeling the pressure wave from the blast is the point where this continues outside.
Having measured 29 kA through a piece of steel wool makes me very satisfied with this capacitor, it is around 3 times more than I expected from this. Further measurements of short circuits through heavy conductors show level of around 25kA to 30kA.
5th August 2012 – 1 kJ shot into steel wool
19th October 2014 – 4kJ shot into steel wool
19th October 2014 – 1kJ shot into 200 cm 0.25 mm copper wire
19th October 2014 – 4kJ shot into a iPod
19th October 2014 – 4kJ shot through a Mini circuit breaker (SEKO DZ47-63 C10 1P+N)