series connection of high-voltaage diodes

[kelee]

hello all, a techtronic oscilloscope has been sitting on my shelf for about three years now because of a problem: a 9kv diode in the EHT acting as a rectifier is bad and there is no way to get a replacement.

Is it feasible to use three 3kv diodes in series (of the same current rating with the one being replaced), maybe with some additional component(s), without causing trouble for me; especially when one takes into account the high frequency and high voltage nature of the pulses rectified
If yes, how will the connection look. Your assistance will be highly appreciated.
kelee in nigeria.

 

[Sebi]

You can canibalize a good high voltage diode from old TV or B/W monitor flyback transformer.

 

[Dean Huster]

Series diodes

Kelee,

It might be helpful if you post the model number of the Tektronix scope you're dealing with. A lot of folks that browse this forum (including me) are former technicians who worked for Tektronix or for independent calibration labs.

I'm assuming that you're working with a rectifier that's individually accessible vs. a potted PDA HV multiplier which is actually a voltage multiplier vs. a standard rectifier.

You can nearly always substitute lower-voltage, series-connected rectifiers for a higher-voltage rectifier. Just make sure that all substitute rectifiers are identical. The voltage rating of the series set will be three times the voltage rating of a single diode and the current rating (you won't be concerned about that in this application) will be the same as that of an individual diode. It's also prudent to put a 10M ohm resistor in parallel with each diode so that the reverse voltage is distributed evenly across each diode in the event that one of the diodes has a LOT higher reverse resistance that the others, which could stress it. Some applications have you also place a 0.01µF disc ceramic capacitor in parallel with each rectifier also, but you should have to do that in this case. Those are for bypassing any HV spikes, but in the Tek scope, there shouldn't be any of those. That was more for amateur radio transmitters and linear amplifiers. Besides, the caps have to withstand the reverse voltage of an individual diode and they'd take up too much space.

If we know the Tek model number, there might be some kind soul who would have a spare rectifier they'd be willing to mail to you. I've sent plenty of items to Canada, Australia, Italy, the U.K., etc. like that. Freebies!



Dean