44KV today

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guest

44KV today

Post by guest »

This weekend I set up a jury rig HV supply using the three Westinghouse precipitator transformers set up on a thick wood base and the secondaries wired in series and fed to a diode bridge made from strings of 60 x 1N4007 diodes.

I'd done a bit of research on series diode strings using the ARRL handbooks 1973/78/2001, the last edition advised that using parallel resistors across each diode was no longer necessary for modern diodes, I wasn't sure this applied to the ancient 1N4007 but I figured at around $2.50 US to try the theory out it couldn't hurt so I built four diode strings each using 60 diodes to give a theoretical 60KV rating. These were made by getting two strings of diode still in the bandoleers and arranging them anode to cathode with one string above the other, this allowed me to solder alternate pairs and trim in about five minutes!!. They were then cleaned of flux and potted in clear polyester resin with a 1/8 brass screws as as terminals. If you use this method I can recommend casting a bottom layer first rather than using thin plastic sheet spacers as I did as they came loose from the bottom and had to be back filled later on.

Today I wired the three tranny's primaries together and then to an isolation transformer and then to my 1KVA variac. The secondaries were wired in series with 1KV rated wire with a 1" rubber radiator hose sleeve externally and then to the four diode strings wired as a bridge. A HV probe was hardwired to the dc output and the whole lot placed on a sheet of nylon and then on a plastic milkcrate where I could watch from the variac. The assembly worked nice til about 10KV where I could hear arcing and a quick inspection showed an output wire was arcing through the rubber radiator hose to the earthed chassis (so much for 3/8" thick rubber!!, it has carbon black in it doesn't it?) I powered down and pushed the wire away with a piece of plastic, after that I was able to bring the voltage up to about 28KV for ten minutes, after that I blew the 5 amp variac fuse (1.2 KVA) trying for more voltage (at about 96% of max Vin).

This experimental rig had no filtering capacitance and according to my ARRL 1978 book my voltage probe circuit was measuring average voltage so I came up with the following calculation: 28 000/0.636 = 44.025 KV peak output.

I have no idea as to why the fuse blew at this stage with no load, I will try each transformer separately to see if I can isolate a unit. Possibly the polyester resin has some inherent leakage at these voltages, does it do something nasty like give off water during curing?. As these transformers are out of precipitator units (W'house precipitron) which are prone to arcing during normal use I'm wondering if they are constant current like neon tranny's, the circuit they were used in had a half wave rectifier, a filter cap (leaking oil and smelling PCB-ish so I didn't salvage) and a string of about 20 x 20W series resistors and then to the plate stack. Also no precautions were taken against corona discharge.

Regards
Mark Harriss
guest

47KV now

Post by guest »

I ran the rig a second time tonightwith the lights out and spotted leakage wherever the black rubber sheath over the wires came in contact with wood, the rack metal frame or whatever, I tied them using string to suspend them and was able to reach 30 KV average (47KV peak) but had arcing on the loose brass nuts to the diode terminals, at this point I decided to take a break for the day.
JohnCuthbert
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Re: 47KV now

Post by JohnCuthbert »

There are two sorts of polyester resin. The usual one stinks of styrene and doesn't give any water on curing.
The other needs to be baked to cure it.
Are the rubber hoses black? if so they may be filled with carbon black which would explain the conductivity.
guest

Re: 47KV now

Post by guest »

The resin does have that strong smell when curing, it's made by fibreglass international and is their clear casting resin. The black rubber 1" water hose appear conductive enough to use as high value resistors, I not only get corona discharge but a nice hot spark through a length of it so I'd say it's loaded with carbon, thats the reason I didn't paint the finished diodes black although I was tempted to.
guest

Re: 47KV now

Post by guest »

I swapped the black rubber for clear poly tubing and cured the corona/arcing problems completely and was able to go up to 52KV peak, no load. The variac fuse blowing was caused somehow by the isolation transformer, it appears to be faulty as it will blow the fuse with no load so I'll have to check it out with a megger.

I'm very interested in how conductive this black rubber automotive heater hose is, it may be possible to use it for a high wattage resistor if it's at all consistent and doesn't age too much.
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