Re: HV DC power supply design

To: tesla-at-pupman-dot-com
Subject: Re: HV DC power supply design
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Mon, 10 Jan 2000 07:13:12 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Some general advice..

#1) Use single HV rectifier blocks, rather than strings of smaller diodes. 
It works better, and the blocks aren't that expensive.  K2AW Silicon Alley
sells 14 KV, 1Amp blocks for, as I recall, $15.

#2) Filter capacitors will be the expensive part.  You need to define how
much ripple is acceptable (0.001%, 1%, 10%, etc.) at a given load, then,
you can calculate the size of the cap needed. Example, 5 kV out, 500 mA
load, 1% ripple.  1% of 5 kV is 50 volts. With a full wave or bridge
configuration, the ripple frequency is 120 Hz, or, in time terms, 8.6 mSec.
At 0.5Amps, to keep the ripple less than 50 Volts, you'll need 86 uF of
filter capacitance. This will store about a kiloJoule, which will be mighty
exciting if it flashes over.

#3) As a practical matter, HV power supplies that use LC filters, rather
than a straight Capacitor input filter, work well and don't have as
exciting a flashover characteristics.

#4) Current metering is usually done in the low voltage (i.e. closer to
ground) leg, so that the meter isn't floating at many kV off ground.

#5) Voltage metering is trickier than it seems, because you need a HV
resistor for the meter.  Use lots of smaller resistors in series. Figure a
max of 1-2 kV per 2Watt resistor (you won't be dissipating that much power,
but the 2W resistors are physically large). I use strings of 10 Meg
resistors.  Make SURE you put a resistor from the bottom of the string to
ground so that if the meter goes open, or a wire falls off, the meter
doesn't float up to HV.

#6) You will need something to limit the surge current when you turn the
supply on and the filter caps are discharged.  The something could be as
simple as the series impedance of the transformers (most, if not all, MOT's
actually limit the current).
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: HV DC power supply design
> Date: Saturday, January 08, 2000 7:34 PM
> 
> Original Poster: "Tero Ranta" <tero.ranta-at-bigfoot-dot-com> 
> 
> Dear List,
> 
> as you might have noticed, I'm trying to design a stiff DC power supply
> using two MOTs and a voltage doubler pair. It's not only for TC use. I'm
> looking for general HV DC power supply design guidelines, how to do the
> diode/capacitor strings, how to calculate needed values for filter caps,
how
> to connect the voltage/ampere meters etc.
> 
> I have a 1996 ARRL (radio amateur) handbook, which has a tiny section on
HV
> power supplies, it tells me what I should do in general, like use many
> diodes in series, but that's not enough, because I'd like to know what
> cap/resistor values should I connect in parallel with them etc.
> 
> I have a tendency to screw up things, that's why I want to even once do
> something that ends up working well for years. :-)
> 
> If you know a good source of information regarding the subject, please
let
> me know.
> 
> Thanks in advance,
> Tero.
> 
>