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: Wed, 01 Nov 2006 16:53:22 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: The Geek Group High Voltage Capacitors, making a HV  switch,
      (fwd)

At 01:34 PM 11/1/2006, High Voltage list wrote:

>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.


A NST and rectifier makes a fine charging supply. I charge my many kJ 
bank with a lowly 15kV 30mA NST.

Heed some warnings:
1) Contemplate the consequences of an unexpected flashover.  That's a 
lot of stored energy, and if it gets dissipated in, say, your diodes, 
they WILL explode.

2) I put a 300 ohm 300W resistor in series between the bridge and the 
cap.   Actually, it looks more like:
NST: bridge: relay in series: relay to short to ground: resistor: 
relays to select caps: array of caps.

the 300 ohms means that if something shorts in the NST/bridge area, 
the current is limited to 20 kV/300 ohms, or a few hundred 
amps.  Big, but not "exciting".
In fact, it's how I discharge the caps when I don't have a load.. I 
just short the output of the bridge rectifier to ground, and the 
energy is dissipated in that resistor, which does get warm.

3) If you want to limit the heating in the resistor during charging, 
the NST is ideal.  If you charge from a "stiff" source, the energy 
dissipated in the resistor will be equal to the energy stored in the 
capacitor.  However, the NST is more like a constant current source, 
with the voltage gradually rising, so the energy dissipated in the 
resistor is much less.

4) You can measure charging current by putting an AC Ammeter in the 
PRIMARY circuit of the NST and calibrating it in terms of secondary current.

5) assume that some part, some day, is going to fail.  Figure out 
what will happen when it does.  Wires fall off, relays stick closed 
or open, and the last thing you want is a fully charged system 
sitting there waiting for something *bad* to occur.