.::MOT Power Supplies::.

In the past, theonly way to get a more powerful power supply for your Tesla coil is to either parallel NSTs, at the cost of one of them failing, or to buy a large PT (potential transformer) or pole pig (pole-mount distribution transformer). Today, there is a new alternative: the Microwave Oven Transformer (MOT). MOTs are typically low voltage and high current, making it almost impossible to make a spark gap that will hold up at high currents and extremely small gap settings. My experience has been that in order to get a single MOT to arc the electrodes have to be touched together. However, coiler Tim Johnson has done some extraordinary "research" and experimentation into putting MOTs in series. This allows for a higher voltage to be developed. Greg Hunter has done this before using a "Twin Pack" MOT power supply using a voltage double to develop 9.6KV DC. The only problem with this is protecting the doubler diodes from high voltage transients. What Tim has done is put 4, 6, or 8 MOTs in series for 9.6KVAC, 14.4KVAC, and 19.2KVAC repectfully assuming 2400VAC per MOT. Most MOTs develop between 400mA to 750mA, but typically develop 625mA, and that is what will be used here. Assuming 625mA, the 4 pack, 6 pack, and 8 pack will consume 6KVA, 9KVA, and 12KVA. One KVA is equivalent to roughly 1000 watts.

Here is a brief overview of what is entailed in each power supply

4 Pack MOT Power Supply:

9.6KVAC
625mA
6KVA
Power requirements:
240V 30A
120V 60A
Dryer Plug can be used, 4-wire is preferable (explained below)

6 Pack MOT Power Supply:

14.4KVAC
625mA
9KVA
Power requirements:
240V 40A
120V 80A

8 Pack MOT Power Supply:

19.2KVAC
625mA
12KVA
Power requirements:
240V 50A
120V 100A

Here are some schematics that Tim has provided detailing hookup of the High voltage wiring, and the primary wiring on the 4 pack:

Instructions and specifications are on each of the diagrams. Click on one to enlarge it.
*NOTE: On the wiring diagram for the 4 pack power supply, it shows the primarys being wired to ground. This is a code violation. They should be wired between one "Hot" side and NEUTRAL. A 4 wire plug should be used for this having two hot terminals, one neutral, and one ground. Using the ground will put anything that is grounded at potentialy lethal voltage, and should not be done (they wouldn't have the code if it didn't mean anything :-)

**This is what the 4-pack diagram should look like**

Here are pictures and links to movies of the power supplies in action.

This is a movie of some simple arcing from the power supply: Click Here A JL operating from the power supply: Click Here

[For Use in a Tesla Coil]

An async-rotory should be used for maximum spark length with a RPM of 3600 or more. A 10,000 RPM angle-grinder with phenolic disk and 4 brass bolt electrodes comes to mind here. I intend to use a Gary Lau Vortex gap with brass electrodes when I get around to using my 4 pack power supply. It provides enormous amounts of air-flow at high pressure so quenching shouldn't be a problem. For a capacitor, Tim uses a variation of the party cup capacitor. He uses thin walled 10 Litre buckets from the dollar store! Use aluminum foil tape to coat the outside of each bucket, the stack together, voila! With 10 plates, over 100nF can be achived, and it is good to 15KVAC in air. No messy oil! This is perfect for a 4 pack power supply. For the 6 pack the unit should be put in oil or each layer doubled up to provide twice the dielectric thickness. Here are two pictures that Tim gave me to help illustrate this concept. The pictured capacitor has a capacitance of 50nF, and has 10 plates, but the buckets are about 5 litres judging by the MOT next to them.

[Testimonials to MOT power supplies]

Tim Johnson: Tim J thinks around 200nF is best when using the 4 pack, and around 180nF is best for the 6 pack. Tim uses a 165nF for the 4 and 6 pack himself, and runs the bucket cap under oil, increasing the rating to 30KV. He uses an 8" secondary with 1986 turns of 27 AWG wire, and admits that it isn't the best secondary arrangement, but it still works. His current spark gap consists of a 11,200 RPM angle grinder with a 6" diameter PE cutting board. Four electrodes were originally used, but when he increased it to 8 electrodes the arc length really improved due to the shorter charging time, meaning (to me at least) more power is getting to the streamers, and the increased bang rate quickens streamer formation. His topload consists of one 10" x 36" to dryer duct toroid on top of a 3" x 12" toroid. Typically, he gets 11ft streamers, and on one occasion got a 12ft streamer using the 4 pack arrangement. With the 6 pack, he gets 13ft arcs, but believes 14ft is attainable considering that it draws 9200W

Dave Hartwick:
40" arcs to ceiling
Fo = ~130 kHz
Power = ~3 kVA
C = 0.025 ufd
8" secondary
air blast gap

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