My Tesla coil DRSSTC1 exploded 12 days before a show, panic!

Can you guess the song that was played? Silent death during a MIDI playback, once power is turned back on, the bridge short circuits and …

AC vs DC fuses, high current, explosive destructive testing and theory

Littlefuse 300A 32VDC fuse destructive test Protistor 200A 690VAC fuse (naked) destructive test: Protistor 200A 690VAC fuse (housed) destructive test: High Voltage Fuse Teardown video: …

5kJ capacitor bank fired at 12kVDC / 3.5kJ charge

Third experiment with the bank of 35 electrolytic capacitors connected in series. Results are better than theoretical estimate, which was 6000 A limited by ESR, …

5kJ capacitor bank, 1.5kJ BANG test at end!

For now all details during development and testing can be found on the forum thread: https://highvoltageforum.net/index.php?topic=25.0 35 capacitors in series, each 450VDC/1000uF, for a 48 …

4000 Joule capacitor bank

Introduction

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.

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

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.

 

Specifications

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.

Schematic

 

Calculations

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

Construction

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.

 

Conclusion

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.

 

Demonstration

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)

333 Joule MOC capacitor bank

Introduction

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.

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!: Microwave oven transformers are a high voltage supply without current limiting.

Considerations

Microwave oven capacitors capacity is relatively low for their voltage rating, but they are designed to be applied around 300% of their rated DC voltage for 60 seconds. Most are rated 2100VAC at about 0,8 to 1 uF capacity.

A typical data sheet for a common microwave oven capacitor is as follow.

Technical Specifications Values
Capacitance 0.8 ~ 1.2 uF. +/-3%
Rated Voltage 2,100 VAC
Frequency 50/60 Hz.
Dissipation Factor 0.0035 maximum
Operating Temperature =”-10 ~ +85 C.”
Insulation Resistance : T- C 1,000 MOhms
Test Voltage: T – T T – T: 9,030 VDC for 60 seconds
Test Voltage: T – C T – C: 9000 VAC 10seconds

Microwave oven capacitors got a built in bleeder resistor which discharges the capacitor fast, which is why its important to discharge the bank as fast as possible when the wanted voltage is stored over the bank. On the other hand its a good safety feature that the bank will discharge itself within 30 seconds and that might save your life.

The amount of microwave ovens needed to build this bank is over the edge, but as they can be obtained for free from containers its still worth the time for the sake of the experiments.

All the critical parts in this project comes from microwave ovens and therefore it can be build with very little spend on materials.

Microwave oven capacitors are not build to sustain these hard short circuits, so they will take damage for each short circuit in the form of lowered capacity as the dielectric material in the capacitor is damaged, this might end fatal with a shorted capacitor that in the worst case will happen with a violent explosion. Take care to shield off the capacitors as they are housed in a metal can, fragments from these is not something you want flying around you if there is a failing capacitor.

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 6500 VDC from a single microwave oven transformer with a full wave voltage doubler.
Capacity 18.5 uF combined from 21 microwave oven capacitors in parallel.
Full charge voltage 6000 VDC.
Stored energy 333 Joule.
Stored charge 0.111 Coulombs.
Trigger mechanism Spring loaded spark gap trigger.

Schematic

Calculations

With the knowledge of the capacitors being designed to work with voltages 300% higher than their ratings for shorter periods of time, I have chosen to charge them to 6000 VDC as its convenient to build a charger for this voltage. This voltage can be achieved with a microwave transformer with a full-wave voltage doubler on the secondary side. The transformer delivers 2300VAC RMS.

2300Vac • √2 • 2 = 6505 VDC

The bleeder resistors in the capacitors loads the transformers and brings the voltage down to 6000VDC when its charging on the 21 capacitors my bank consists of.

The 21 capacitors are not the same make or capacity, but varies from 0,85 to 1 uF. I have measured the the capacity of the bank to be 18,5 uF with a LCR meter, the stored energy in the bank is then.

Voltage² (Volt) • capacity (Farad) • 0,5 = energy (Joule)

6000² • 0,0000185 • 0,5 = 333 joule

Construction

The capacitors are split into 3 strings with 7 capacitors in each. I soldered 4mm² copper wire between the terminals of the 7 capacitors and the 3 strings are connected with 6mm² wire bringing all 21 capacitors into a parallel coupling. Heavy gauge wire is used to ensure that it can withstand the huge current doing the short circuit and it gradually gets heavier the further into the connection towards the short circuit point we get. All connections are intended to be as round as possible to avoid corona losses when working with voltage into the kV range.

I build a wooden case that is split into 3 sections to separate the capacitors, charging circuit and dis- / charge mechanism.

As mentioned the capacitors got a built in bleed resistor which makes it crucial that the discharges happens very fast after charging is over. A solution could be to keep charging while discharging, but this poses new problems where we have to protect the charger circuit against the ringing current when the capacitors are short circuited.

I chose to make a spring loaded trigger that is pulled to the charger and when I let it go it springs to a brass plate where it short circuits the capacitors into the coil, look at the picture underneath and the schematics to get a better understanding.

 

Crushed cans, sparks and explosions

Now that everything is put together and calculations have been done to some extend, its time to harvest the sweet fruits in the shape of wonderful bangs, sparks and explosions. To get fully rewarded it is necessary to have some means of filming / take pictures of the sparks and explosions. A DSLR camera is by far the best for taking pictures with long exposure but there are great alternatives for Canon camera’s, its called CHDK. CHDK is a third party software that unlocks the power of the powerful processors in most of Canons digital compact cameras. Read more about CHDK.

Crushed cans

Winding a small coil with 3 windings of 2.5mm² hard copper wire, wound to fit a beer can tight, will make us able to crush a can with the very powerful and intense magnetic field that is generated when the capacitor bank is short circuited through the coil.

Aluminium paper

Short circuiting the capacitor bank through a small piece of aluminium paper will make it vaporize in a loud bang and very bright flash, it is hard to capture this properly as the light from the explosion is very bright and the aluminium paper burns up almost instantly.

Steel wool

The procedure is the same for steel wool as for aluminium paper, but steel burns slower than aluminium. It is possible to see the sparks with the naked eye, but the pictures of this is absolutely remarkable!

Water and fruit

You can see discharges primarily in water which results in loud explosions from the instantly vaporized water, the amount of steam developed expands very fast and that makes it so loud.

 

Conclusion

I am satisfied with the results I have achieved with a capacitor bank that was constructed almost for free as all materials come from things that were thrown out.

The steel wool sparks makes it worth all the work put into this project, and the dis- / charge mechanism turned out to be simple and effective.

Future improvements could count a higher charging voltage, if its raised to 8000VDC the bank would gain about 200j of energy.

Its important to use a work coil with large enough distance or isolation to avoid flash overs, this picture clearly shows what happens if this is not taken into consideration.

 

Demonstration