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Re: Diode Strings
Tesla List wrote:
>
> Original Poster: David Trimmell <davidt-at-pond-dot-net>
>
> Hello,
>
> I am looking for opinions and suggestions. For my tube coil work I am in
> need of some robust, high voltage diodes. Recently in some tube coil
> experiments I have been sending back decent surges into my doubler diodes.
> Needless to say they have not fared to well. So, I am going to order some 6
> amp 1kV diodes from Digi-Key. These will be series strung using RC snubbers
> across each diode. Each string will have 20 diodes, giving me about 20 kV
> PIV -at- 6 amp continuos (each diode is rated for 400 amp surge for 8.3 uS).
> My plan is to use the individual strings in a doubler circuit using max of
> 3250 VAC, and four strings arranged as a fullwave rectifier for use in a
> 6500 VAC circuit -at- 700 mA max.
>
> My questions are: For the RC resistor value would a 1/4 or 1 watt be best?
> What about value, 300K to 500K? For the caps I plan on using 0.001 uFd -at- 1
> kV ceramic disk, sound ok?
> Also, I have seen many people mention placing a resistor in series with the
> diodes. A few ohms placed in series with each diode string sound good? I
> don't want to have too large of a voltage drop.
>
> I want to do this right the first time. This is much cheaper than getting
> commercial diodes, about $60 versus about $300 from RF parts.
>
> Anyone who has done this before, or has knowledge of potential problems,
> please let me know.
>
> As always, I am appreciative of any comments.
>
> Regards,
>
> David Trimmell
David,
Sounds like yuou've got the makings for some bulletproof diodes! I'd
recommend using 1 watt resistors in the snubbing networks since their
breakdown voltage will be greater, and 500k should be more than adequate
to balance the inverse voltages for a silicon diode chain. And 0.001 uF
disk ceramics should also work quite nicely. Since you're not
paralleling any diodes, the forward current in the series string will be
the same for all diodes so there's no need for any series balancing
resistors.
BTW, one other thing that's often done in commercial solidstate
rectifier chains is to add a high valued resistor in series with a neon
lamp (NE-2) across each diode to detect the presence of a failed diode.
In operation, all the neon lights should be lit - any that are dark
indicate a shorted diode which can be replaced before the added voltage
stress causes any of the others to fail.
Safe coilin' to you!
-- Bert --