flyback transformer and microwave diodes

[AllenWallace]

I have salvaged an industrial flyback transformer which was used to power an xenon industrial lamp. It's much bigger than traditional flyback transformers. I want to see if I might power my fusor with it. I wired a transistor driver, but when I observe the rectified voltage on my 'scope, it appears that the microwave oven diodes are not working. Is it possible that the MOT diodes are only good for 60 hz and not good for a few khz AC?

[Richard Hester]

That's most probably correct. You will need some fast recovery rectifiers to get a HF high voltage supply working right. Microwave diodes are rough and tough, but they don't switch worth a damn. I've gotten some nice deals on HV fast recovery diodes off of Ebay. You have to wait, then pounce when the oportunity presents itself. BTW, even fast recovery HV diodes don't really recover all that well - you have to treat them with kid gloves. The combination of very high breakdown voltage and reasonable recovery characteristics seems mutually exclusive with silicon diodes. Tubes might be a decent alternative if you can round up enough HV, high power rectifiers. Since you're already doing a switching front end, it's not too difficult to do a push-pull oscillator to drive a transformer with several low voltage, high isolation secondaries using high voltage wire. These will power any number of floating filaments/cathodes for a Cockroft-Walton multiplier using tube rectifiers. TV flybacks used to use a similar trick with a single turn winding to drive the filament of a 1B3 or 1K3 rectifier.

[Richard Hull]

Yep, silly-con with its solid state carrier limitations don't like to switch all that well at high current/high voltage. Sorta' like trying to make a tight turning radius battleship.

There are a few 1 amp 2kv diodes out there which are common that bear the moniker of ultrafast (UF suffix or prefix) A 1KV 1amp example is the IN4007UF. I use 'em in geiger supplies where I am working HF drivers and need a multiplier circuit.

Nothing beats the good old hollow state devices.........NOTHING. (provided you don't need to put the 20kv supply in the size of a cigarette pack.

The ancient, but venerable 3B24 diode would be ideal (hamfests only) as it can handle a gang o' mils in a small package. The IB3GT would be a stretch for a fusor as it is rated at only a mil or so of current. But, hey, you can force the thing's plate into red heat and it may groan, but it will work.

350% over power is often OK in the tube world, especially if you don't operate 24-7.

Richard Hull

[ChrisSmolinski]

I've got a stash of BYD33M HV diodes (1kV, 1A, 300nsec). Not ideal for building that 50 kV multiplier, but some nice uses on the low end of "HV".

If anyone needs a few, let me know, and I'll be glad to send some your way.

I used to have some 10 kV diodes, but don't believe I do any longer. Back from the days at the old job designing power supplies for 250 kV x-ray sources. I think I still have the videotape of the CEO testing one for handling arcs by shorting the outputs to ground with a 6 ft lucite rod. (the multiplier stack was in the oil filled source tank, electrostatic fields made the oil swirl around like oil in a deep fryer, quite the sight!)

[DaveC]

It is common practice in HV multipliers to put current limiting resistors in series with the ouput. A typical value is 1 ohm/volt of output, which keeps fault currents to 1 A or less. So a 20 KV multiplier would use a 20 k ohm resistance. Usually this will fall below the surge current rating of t ypical HV diodes.

Transformer type supplies can also use the current limiters to good advantage. Just be sure to size them to limit currents below the surge ratings of your diodes.

Since full HV output appears across the resistors in the event of a fault or arc, they should be HV resistors.. preferably several in a string to prevent HV flashover.

Carbon composition resistors ( you don't want carbon or metal film or inductive wire wounds) are quite good. DigiKey has one type with a few kV DC operating voltage and up to 14 kV surge voltage ratings. Prices are less than $1.ea. Getting real high wattage may be a problem. My applications typically get by with a 1 Watt (in oil or potting compound) rating or even less.

Dave Cooper

[guest]

You could try some TV20, 20 kilovolt diode sticks. These are
designed to rectify the output of old B&W flyback circuits. They'd
be fast enough, but from memory are only about 50 mA or so.