How to wire two Raytheon CK5517 cold cathode tubes in series for higher PIV? - Page 1
Forum section: electronics / projects

author: KJ-90
author_rank: Contributor
timestamp: June 05, 2017, 06:19:17 pm
content: I am building a vintage 1940s speed light unit reproduction that operates on 1kV and has a 25uF 2kV high voltage capacitor. I know this combination of voltage and capacity leads to a very, very dangerous charge being stored in the capacitor, so I know all the high voltage safety precautions. The transformer has an output of 2.2kV at about 5mA. The original half-wave rectifier circuit uses a 2X2 tube. However, I was wondering how I can wire two Raytheon CK5517 cold cathode rectifier tubes in series for a higher PIV. Those tubes don't have a high enough PIV if only one was used. The problem that I have is how to determine the voltage equalizing resistor that is paralleled across each tube. Two such resistors are used, presumably like with solid state rectifiers. The data sheet for the Raytheon CK5517 is available at:https://frank.pocnet.net/sheets/138/5/5517.pdfThe only issue with the data sheet is that it doesn't give the tube voltage drop, which is what I presumably need to calculate the resistor value. How do I calculate that value?I'm sorry that I forgot to mention a few things that I should have posted first ( other members pointed them out ). The power transformer operates from the 120 Vac power mains, and therefore outputs a 60 Hz sinusoidal waveform, at its high voltage. Therefore, no reverse recovery time issue exists. The transformer secondary has a DC resistance of almost 11,200 ohms. This was presumably done by the manufacturer to limit high voltage fault current in case of a rectifier, capacitor, or other secondary circuit component failure. Also, the CK5517 tubes are intended to have a surge resistor in series with each tube; the data sheet recommends no less than 2K ohms. This is recommended because if a circuit failure happens, all of the photoflash and filter capacitor energy would be discharged in the rectifier tubes- quite a bang at that power level. The transformer's DC resistance and also the surge resistors should prevent any overvoltage issues with "instant on" rectification, unlike with the 2X2 tube which doesn't have that problem.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2017, 07:57:49 pm
content: I guess I'm not confident the PIV can be equally split. If it isn't, you will have a big boom with such a large filter cap.With series-connected HV diodes (of any type) I have a high value HV resistor (1-10MEG) across each to ensure equal reverse voltage. Since each diode does not have exactly the same leakage current, you want to ensure PIV is equally distributed.I think the hard part would be the start-electrodes on the two series CK5517's.You want the same amount of ionization for each tube and having a middle-node resistor to GND allows tube 1 to start, tube 2 starts afterwards. But during reverse blocking, the middle-node resistor puts DC across tube 2 and the xfmer AC on tube 1, so I don't see things equally split up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2017, 11:57:56 pm
content: If nothing else, put resistors and capacitors in parallel with each device. Values are determined by variations in device leakage current, electrode capacitance, and deionization time.It still might not work very well. Idunno. There's probably a good reason why they made so many types of rectifiers (of different V, I and $)...Why not use silicon diodes instead, just a few more stacked up (preferably also with parallel R or C)?Note that gas tubes and semiconductors are subject to recovery time, and this won't work at high frequency (over maybe low ~kHz). Only high speed diodes, or hard vacuum rectifiers, will be suitable.Also also, hard vacuum diodes have tons of voltage drop (like 100V+). Will that be a problem for the circuit? Keep this in mind if you replace with gas or semiconductor diodes.Tim

author: calexanian
author_rank: Super Contributor
timestamp: June 06, 2017, 03:32:39 am
content: These were used in the same fashion as solid state diodes in a typical diode multiplier circuit with caps and balancing resistors as mentioned before. Easier to just use silicon rectifiers. These tubes are pretty much useless above 400 hz aircraft operation as the internal conduction speed can't keep up with high frequency very well.

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2017, 06:18:32 am
content: Also, the old 2X2 has a warmup time that might be essential if your load is also tubes. An instant-on rectifier might have B+ skyrocket until the load warms up to load things down, very hard on filter caps.I would use some scrap microwave oven rectifier diodes (usually 4-12kV 300mA) with maybe time-delay relay on the big plate transformer.