Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 07:15 AM 9/21/2006, you wrote:
Original poster: "Jim Mora" <jmora@xxxxxxxxxxx> Hello Group, I am getting antsy to power up my 3 phase DC coil PS. The PS is from Raytheon, Nikie era. It puts out 24KV DC. I think I posted a picture of the beast once. Bert Hickman called it "Knurly", (California speak) and it is! In its former Cold War Radar/tracking configuration, it used (6) 8020 vacuum tubes for 6 pulse rectification. It has a 150 H inductor on the output which does not play well by way of Richie's Burnett's equations. If my math was not wrong, I would have a max 180 BPS (too small) and .4uf cap (too big). A more ideal inductance would be say 75H so two of these in parallel or a smaller one would work. I don't know if there is any value in DC reasonance charing or just brute force. The design is cool. The tube insulators step up to longer insulators namely 4" to 10" with the anode caps. Anyway, most tube suppliers suggest going to solid state diodes which I plan to do. Where's the beef,... getting to that? I have (36) HDb7.5 puck, stud diodes which are 7500v 1.4 amp rated. I would use 6 per leg (delta) wye input.
Are the 8020s dead? If not, I'd use them. They're a burly tube that has long been used in HV power supplies.. I think the specs are like 40kV, 300mA.. but more to the point, tubes have a bit more cushion against accidents. A bit of over voltage, and they survive. A transient overcurrent, and they survive.
OTOH, if they die, replacing them at $200 each (or probably more.. that was 10 years ago that I priced them from Richardson) is a good reason to switch to semiconductors.
So here are the questions: 1. Is there any point in resonance charging or for that matter can it be avoided in some form?
With a 6 pulse rectifier, you've got pretty steady DC. You'd just wind up drawing a power arc, because there's nothing to quench it.
2. What should be across the individual rectifiers in the strings to add failure prevention and equalization?
These days, with very consistent devices, the usual strategy is just to be conservative with how many diodes you put in the string. Say you've got 6kV PIV diodes and you've got a 30 kV application. Use 8-10 diodes in series rather than squeaking by with 5. The concern is with reverse recovery time (turn off). The diode that turns off first sees the entire voltage across the string. In a sinewave application, though, the reverse voltage comes up slowly.
Throw away all those old ARRL handbook pages that talk about capacitor/resistor networks for equalizing diodes in series strings, unless you're using diodes you bought loose at 10 different hamfests over the past 20 years.
3. Shall I bother with a de-queing diode string? (6 more).
With resonant charging, you need it.