Introduction
The idea behind a capacitor bank is to use many capacitors in parallel and/or series, to be able to charge up as much energy as possible. This energy can then be used to short circuit 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 materials in the conducting path into vapour.
To get an idea about how much (or little) energy 2600 Joule is, here is a couple of examples.
- The human heart consumes 1 Joule of energy per heartbeat.
- An AK-47 bullet leaves the riffle with a muzzle energy of about 2000 Joule. Bullets are kinetic energy. Energy stored in a capacitor is potential energy, but when released into something, it becomes kinetic energy.
- 2600 Joule can light up a regular 7 Watt LED light bulb for 6 minutes.
- Instant release of 2600 Joule, even with a high rise time of 1 ms (1/1000 of a second) would generate a peak power of 2.6 MW!
Safety
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.
Considerations
When discharging a capacitor into a circuit with a inductance, which also includes the capacitors own equivalent inductance, the voltage will ring between the capacitor and the inductive part of the circuit. This forms a LC circuit that will oscillate at its resulting frequency, producing 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. The cheap, easy and not very safe solution, is to blow the circuit open. Leaving a part of the work coil, load or other thing placed at the discharge terminals, be so thin that it will immediately be blown apart, open the circuit and prevent voltage reversal to the capacitor bank.
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.
Specifications
| High voltage supply | Microwave oven transformer 2050VAC, full-wave rectified for 2900 VDC |
| Capacity | 830 uF |
| Full charge voltage | 2500 VDC |
| Stored energy | 2600 Joule at 2.5 kV 1660 Joule at 2.0 kV 934 Joule at 1.5 kV 415 Joule at 1.0 kV |
| Stored charge | 2.075 Coulombs at 2.5 kV |
| Trigger mechanism | Solenoid operated spark gap switch |
Schematic and datasheet
Calculations
The Vishay PhMKP 400.3.12,50 PFC capacitors are manufactured for power factor correcting applications. They contain 3x 82.9 uF capacitor elements wired in Delta, for 3 phase AC connection directly to them. To use this capacitor with a DC voltage, we need to short out one of the elements, and can use two of the capacitor elements.
The capacitor is rated for a maximum voltage of 1000 V, being a self-healing MKP type, they can withstand some abuse. The 1 minute terminal-to-terminal test voltage is given as 2.2 times nominal voltage 415 VAC. Doubling that AC figure to DC is 900 VDC, as its the same voltage swing. This results in a datasheet-based maximum voltage rating of 1800 VDC for two of these in series. I want to abuse them a little more.
Update 21. May 2024: Measurements of the individual capacitor elements, showed a capacitance of 106 to 117 uF. This is due to the element being measure, have the two other elements connected in parallel as a series string. This fooled me when building the project and made the video. I wrongly calculated the total capacitance to 1100 uF, where it was the first anticipated 830 uF all along.
Wiring the two 83 uF elements, in a single can, in parallel, doubles the capacitance to 166 uF. Wiring two of these cans in series, results in 83 uF at the double voltage rating. Having ten of these arrangements, results in a capacitor bank of 830 uF rated for an estimated 3000 VDC. Reality turned out to be somewhat lower, as internal arcing could be heard around 2700-2800 VDC. 2500 VDC seemed like a stable charge level.
Estimation of the possible short circuit energy. There is no ESR given in the datasheet, so we have to piece together some information. Inrush current is given as 300 times nominal current. Nominal current for the PhMKP 400.3.12,50 is 18 A (at 415VAC 50 Hz). By Ohms law we can calculate the internal resistance from the inrush current figure.
Charging a pair of capacitors (2 elements in parallel, 2 of those in series) to 2500 VDC will result in the following short circuit current .
Having 10 of those pairs in parallel, should in theory make the possible short circuit current 10 times higher. In reality, resistance of the rest of the circuit will limit the current and only measurements with a pulse current monitor can reveal what the true values are.
Construction
I love to reuse as many of all the old parts I already have, from teardowns or findings at the scrap yard. This entire PFC MKP Capacitor Bank, wiring and components comes from the scrap yards. Only the analog meter is a NOS from a estate sale. The SKKD 81/16 half-bridge rectifiers comes from large UPS systems. The power resistors from a x-ray machine. The high voltage transformer from a microwave oven. The capacitors from a power factor correcting electrical cabinet.
The high voltage power supply is a 230VAC to 2050 VAC microwave oven transformer, with a full-bridge rectifier from SKKD 81/16 half-bridge modules. 1 kVDC is produced at 78 VAC input, 2 kVDC is produced at 159 VAC and 3 kVDC is produced at 239 VAC.
The charging circuit is manually placed and disconnected BEFORE firing the capacitor bank. Leaving the charger connected across the capacitor bank, can result in unwanted current paths. Current the wrong way will most likely blow up your charging diodes.
The analog meter is rated for 1 mA DC for full scale swing. To use this meter for voltage measurement, we have to add a series resistor. We need to develop a voltage across the meter, corresponding with 1 mA. I used a string of 14x 270K resistors, for a total of 3.8M Ohm. By Ohms law it now takes 3800 V for full scale.
The solenoid operated, spring return, spark gap switch is operated by 230 VAC. The arm is a piece of plastic with a 20 mm distance between transformer core and short circuit contactor surfaces.
Shot record of PFC MKP Capacitor Bank
In order to keep track of capacitor lifespan, here is a list of the different energy levels and work loads, that the PFC MKP Capacitor Bank has been shorted into.
| 1660 Joule | 1x aluminium foil |
| 2600 Joule | 3x aluminium foil 1x can crushing (5 turn 1.5 mm2) |
Crushed cans, sparks and explosions
12th May 2024
The 4 first short circuit tests, aluminium foil and a crushed can.
Conclusion
The energy release speed of this PFC MKP Capacitor Bank is VERY different an electrolytic capacitor. The much lower ESR leaves you with an ear deafening loud blast when it fires. Near 4 kJ energy discharges in a confined space, really let you feel the pressure wave from the blast, I would say that beyond this point, a bigger place or outside is needed.
Short circuit current is estimated to be around 30 kA, possibly higher, as MKP is superior over other capacitor types used for pulse applications. The 30 kA figure is very similar to what I have measured on my other capacitor banks, but this is for now, only a estimated guess.
Demonstration
12th May 2024
Video of construction, aluminium foil explosions and a crushing a can.