Teardown of a 1970’s Microwave Oven from Husqvarna. The model “Cupol” was a iconic design from the Swedish designer Carl-Arne Breger. Additional information and pictures …
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.
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.
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.
|Capacitance||0.8 ~ 1.2 uF. +/-3%|
|Rated Voltage||2,100 VAC|
|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.
|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.|
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
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.
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.
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.
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.
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.