[time-nuts] 4 KV Power Supply Recommendations

Ed Palmer ed_palmer at sasktel.net
Mon Jan 18 17:02:21 UTC 2010

Why not use diodes from microwave ovens?  A very quick check shows ~$5US 
each for 12Kv @ 300-500ma. if you buy them new.


Lux, Jim (337C) wrote:
> On 1/18/10 2:15 AM, "Bruce Griffiths" <bruce.griffiths at xtra.co.nz> wrote:
>> Mark Sims wrote:
>>> A lot of the Tesla coil, etc people insulate their wires with silicone rubber
>>> tubing (available at hobby shops as fuel lines).   Make sure it is silicone
>>> rubber and not PVC, etc.
>>> I have seen diode strings built up out of lots of cheap diodes (like 1000V
>>> 1N4007s).  The diodes are strung together and then crammed into the silicone
>>> tubing.  The ends are sealed and the tubing is then filled with mineral oil
>>> (some people use vegetable oil).
>>> ----------------
>>> power supply diode and resistor strings covered in heat shrink but don't
>>> touch as it is not good at HV.  
>> Such series strings of non avalanche rated diodes like 1N4007's without
>> suitably proportioned parallel  RC networks almost inevitably fail
> Actually no. In fact, the typical recommendation these days is to not use
> the RC networks because they actually increase the failure probability.
> Today, the parts are sufficiently well matched (coming off the same spool
> typically) that reverse recovery time is pretty much the same.
> The typical problem in these strings is that one diode turns off a bit
> faster than the others, and so, takes more of the voltage until the other
> diodes turn off  In a "rectify from sine waves" application, though, the
> reverse voltage doesn't rise instantaneously, so there's plenty of time for
> them all to turn off.  This doesn't necessarily apply for "flyback" type
> power supplies, which can have fairly high dv/dt.
>  The other thing you want to do is use a LOT of parts in series.  If you're
> using 1000 PIV parts and you want a 5 kV diode, use 10 diodes.
> Where people get into trouble is when they use 5 1000 volt parts, randomly
> selected from the surplus bin with 70xx date codes, to build a 4800V power
> supply.
> The other thing to watch out for on Cockroft Walton type circuits is current
> limiting the output if there's a fault.  Short the output and you discharge
> all those capacitors through the diodes, cooking them.  You can use a series
> inductor if your load is constant and you don't want to foul up the (already
> crummy) regulation.
> Also, as a note, there are actual HV rectifiers from philips available
> widely in the surplus world.  They're often 6kV PIV rating at a few hundred
> mA, but MUST be potted or oil immersed, because the package length is too
> short to run them in air at full voltage.  They're designed to be seriesed
> or used in voltage multiplier applications, so they have soft recovery
> that's very consistent, and decent reverse breakdown properties, too.

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