HV-DC Supply


I have been dinkin' around to produce a high current, relatively high 
voltage supply for use with the H-at- thyratrons and small Tesla coils.

Some tips follow:

1.  Try to use only a current limited power transformer.  I plan on using 
a 60ma 15kv neon transformer.  This auto-limit feature keeps big trouble 
from happening in case of shorts or lock up in the H2 Thyratron.

2.  I plan on using a variac for controlling the primary voltage to the 
transformer so that I can make the supply output variable.

3.  By ground referencing the center tap on the secondary and using two 
diodes, I can achieve full wave rectification without a bridge.  The 
downside is the output will be limited to the half  the peak voltage of 
7.5KVAC (about10KV).  This has a second advantage of making the minus 
(negative) lead of the supply, ground.  This can actually be a big 
disadvantage if you ever want to float the supply.  If floating is 
demanded then you would have to bridge rectifiy the system and use a none 
grounded, isolated secondary transformer.  Such transformers are rarely 
found in current limited models.  I have one from an old heart 
de-fibrulator unit by Hewlett-Packard (hamfest buy).

The diodes used should be fast recovery 1 amp units.  I am going to use 
series connected microwave oven diodes.  I might use some special 
unitrode 2 amp 7.5Kv diodes which I got at a hamfest.  Some 673 mercury 
vapor vacuum rectifiers would be the best of all worlds, but the filament 
transformer and the isolation required on same is beyond most casual 
coilers abilties.  The resulting 50-200 watt coil system would be a real 
weeny compared to the 200 watt filament requirement on this big  1 
foot tall 15kv 5 amp glass diodes!

4.  Metering is a must for both current and voltage.  The current is best 
metered in the negative or grounded leg of the supply or else the meter 
becomes hot.  The voltmeter is best placed in the circuit with its minus 
lead to ground and the series resistors (use about three for standoff) to 
hot.  I am speaking of dedicated d'Arsonval meter movements here of 
course.  If the floating supply design is opted for then the meters 
should be mounted in plastic and a secondary plastic face shield added 
over them to protect the operator from elevated potentials within the 

5.  The next item is the filter capacitor.  I am planning on using a 
large 1uf 20KV unit which I have on hand.  This can be a big ole nasty 
mylar unit (Red glass caps), as they make great filter caps, or even some 
old WWII paper units. If this cap is made too small, you will have no 
reserve energy storage and the output impedance of the finished supply 
will be too high for any serious work.  I would say that the filter cap 
should be on the order of 100 times the size of the Tesla resonator cap. 
(my cap limit will be .01 ufd or even a good bit less).  Note**** You 
must supply a bleeder resistor across the capacitor terminals.  I plan on 
using a 20 megohm bleeder.  (remember however, that at shutdown there is 
a 60 second delay (3 RC) before you are relatively safe. I plan on having 
a dropout relay with a 0.1megohm resistor in circuit so that when the 
variac power is killed, the resistor is thrown across the output.  Such a 
relay will have to be custom designed or a vacuum antenna switch 
used.(hamfest buy)

6.  I plan on placing formal current limiters in the system to allow the 
supply to be tailored to a number of jobs.  I will series a 20 megohm 
resistor in series with the hot lead for a 500ua max output.  I will also 
be using a 200k ohm resitor (switchable) for a 50ma max output system.  
The .5ma system can be used for relatively safe electrostatic 
investigations, while the 50ma system can be used for small Tesla coil 
systems and H2 thyratron experiments.

7.  It is pivotal to secure a .5 henry or larger HV oil immersed choke if 
you plan such a supply!!  This "must have" component is only found at 
hamfests.  This choke must be placed in series with the output at the 
last moment prior to the application.  My choke will be a 1 henry 12KV 
unit which I picked up at a recent hamfest. This choke will save your 

8.  Finally, the supply should be built on plastic or some other good 
insulator and all components mounted on standoffs of some kind to avoid 
internal supply arcing.  I plan on using some 10KV silicon probe wire for 
all interconects and silicon HV putty at every joint.

Such a supply would supply DC to a Tesla coil which would make an 
incredible system with much more output for the buck than the simple AC 
systems we are used to.  Higher rep rates would be easily obtainable.  
The usual warnings about shock hazards are still in place with the extra 
precaution that you now have a 50 joule system capacitor (1uf -at- 10KV) 
ready to kill at an instant in the high current mode. BE CAREFUL!!

I hope this helps those who have contimplated such a construction project 
for that illusive DC coil system.  If you need a schematic for this 
simple supply, you might bone up on power supply theory prior to 
building. You'll get it on your own.  Those who are old time hams 
(before the 2 meter free license weenies) or other electronic types 
intuitively know what I have planned are ready to build now.

Richard Hull, TCBOR