Re: The Geek Group High Voltage Capacitors, making a HV switch, (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Thu, 02 Nov 2006 07:08:12 -0600
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: The Geek Group High Voltage Capacitors, making a HV switch,  (fwd)

High Voltage list wrote:
> Original poster: Steven Roys <sroys@xxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ---------- Date: Wed, 1 Nov 2006
> 08:27:45 -0800 (PST) From: C. Sibley <a37chevy@xxxxxxxxx> To: High
> Voltage list <hvlist@xxxxxxxxxx> Subject: Re: The Geek Group High
> Voltage Capacitors, making a HV switch,  (fwd)
> 
> With respect to charging, how large of a power supply does one need?
> I'm considering two of the geek capacitors in parallel, for 14.8uF.
> If I were to charge with a 60mA 15KV NST to a peak voltage of
> (24500V), I calculate total energy of 4500 Joules.  Assuming a .8
> power factor, a 60mA 15K NST puts out about 720W.  Since "Joules" is
> watt*seconds, is it just as simple as dividing Joules/watts to get
> seconds of charging time?  With that I get just over 6 seconds, but
> that seems like a low number.  I assume that I'm doing something
> wrong, but even with a factor of 10 it's only a minute.  Is an NST a
> good choice for a charger, or should I look for something bigger?
> 
> Curt.
> 

Hi Curt,

Remember that the NST will only deliver charging current to the 
capacitor when it's output voltage is higher than that of the capacitor 
bank. As the bank charges, this becomes a decreasing portion of the 
output voltage waveform. In addition, an NST delivers considerably less 
current at higher output voltage due to its declining I versus E curve. 
Both of these effects can result in considerably longer charging times 
when using NST's. Charging times of a minute or more are typical. 
However, you can speed up the charging process, even when driving from 
an NST supply, by using a voltage doubler or tripler circuit instead of 
a simple diode bridge.

Don't skimp on the PIV rating of your rectifier chains - use _at least_ 
2X the peak voltage from your NST's on each leg. You may also want to 
consider using pulse-rated ceramic tube or rod resistors from Kanthal 
Globar or HVR Advanced Power for your bleeder and/or charging resistors. 
These can safely absorb tens of kilojoules of energy. They handle 
transient energy peaks MUCH better than wirewound or film resistors 
which can fail open (sometimes spectacularly).
http://www.globar.com/ec/resistor.php.html
http://www.hvrapc.com/rodandtube.asp

BTW, HVR Advanced Power has a nice tool on their site for calculating 
pulsed power rating of their various resistors. This is very nice for 
various pulse and energy dump or bleeder applications:
http://www.hvrapc.com/WebModules/Calc/Series.aspx

Heed Jim and Peter's words well - design the front end and controls for 
your system very robustly, and be sure to add a series 
charging/discharging resistor between your HV supply and the bank. Have 
an alternative fail-safe bleed off method (and a supplementary crowbar 
switch/resistor if you really want added insurance). A well insulated 
shorting stick (connected to bank ground through a high power pulse 
rated resistor) can be used to probe a (supposedly) dead system before 
touching/reparing anything.

Design your entire system so that, if any system component fails, 
explosive destruction of the system (or experimenter) will not occur. 
This includes the caps in the capacitor bank. High energy discharge caps 
can typically absorb the energy from only one or two other identical 
units without a case rupture. However, larger paralleled banks can have 
"exciting" failure modes. Plan adequately for bank location, protection 
of adjacent caps, and retention of oil or cap innards in case of a 
catastrophic failure.

Make sure that you can always accurately measure the voltage on the 
capacitor bank at all times and add bleeders across any cap(s) used in 
multipliers. Use a dedicated voltage measurement system, NOT a hand held 
voltage probe! Do not "haywire" a high energy system. You may also want 
to measure current through the bleeder resistor to provide a bit more 
redundancy.

The short circuit energy in these capacitors cannot be adequately 
described in an email. They will not give you a second chance... never, 
ever, forget this.

Bert
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