4000 Joule capacitor bank


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.
Capacity 2423uF
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.

Voltage Energy Terminals
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.


Shot record

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 iPod
1x C10A miniature breaker

Crushed cans, sparks and explosions

Steel wool

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)

9 Responses to 4000 Joule capacitor bank

  1. Pingback: First shot with 4000 joule capacitor bank! | Kaizer Power Electronics

  2. Pingback: 4000 joule capacitor bank fired at maximum energy | Kaizer Power Electronics

  3. Lars Lundbæk says:

    max peak Wow

  4. Alex says:

    How much did your pearson current monitor cost Mads? I had a look on the internet but they appear to be quote only (probably means they don’t sell to average joes like me lol).

  5. Mads Barnkob says:

    Hey Alex

    It requires patience to find a cheap on ebay, I was lucky to find this model 101 that was reasonably priced. Its connector was badly damaged in the shipping so the seller refunded me 33%. Took me about a year to find my first model 110 and then another year to find the 101 🙂
    I did have a spare model 110 but sold it to another 4hv member.

    Kind regards

  6. Alex says:

    I had a “play” with my capacitor bank yesterday and noticed something that most people who are into high voltage/high power don’t really mention.

    The two 3ft wires that connect to the discharge area under go quite a bit of magnetic force when the high peak currents flow through them, in my case the slightly messy shape of my grounding stick conductor made it jerk with quite a bit of force. The adjacent HT wire 1ft apart also moved a little bit (its much neater and straighter).

    I just found that quite interesting that the peak currents can be high enough that even a straight piece of wire can make such an effective electro magnet, even with a small 130 joule bank like mine (325v 2480uF).

    Maybe it is a product of capacitance too, since I have seen similar pulling together type of behaviour with homemade HV foil capacitors.


  7. Mads Barnkob says:

    Hey Alex

    It all depends on the construction, if the wiring is made stiff enough or prepared to “bend” away from the short circuit area, you are not likely to see them move because of the magnetic forces which follows the right hand rule in electro magnetism.

    Kizmo had some very big cables fly around with his very big electrolytic capacitor bank, so there are many cases of it, just not many that write about it 🙂

    Kind regards

  8. Hamid says:

    Dear Mads,

    I have an ignition exciter unit with double 3k volts outputs.
    How i can measure it`s output energy,by simple way,in Jules Unit?

    Best regards,

  9. Mads Barnkob says:

    Hello Hamid

    This sounds like a transformer or switch mode power supply, the current it should be able to deliver must be stated on the unit. Be aware that this could only be rated for pulses since it is a igniting unit.

    One Joule is the definition of one Ampere passing through one Ohm in one second.

    Kind regards

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