Solid State Voltage and Current Regulator

From: 	Richard Wayne Wall[SMTP:rwall-at-ix-dot-netcom-dot-com]
Sent: 	Friday, January 02, 1998 4:35 AM
To: 	Tesla List
Subject: 	Solid State Voltage and Current Regulator


Happy New Year ALL!

Currently my main project now is a solid state voltage regulator and 
current controller for noncurrent limited transformers.  This type of 
control is not only applicable for tesla coiling, but even applies 
itself to control of transformers employed in HV DC rectified power 
systems.  Something that will prove useful in future iterations of 

My approach is simple.  Transformer Pin = Pout, excluding losses.  
Control HV secondary P-V-I by controlling the transformer's primary low 
voltage main supply.  I use surplus BPT modules purchased from Marlin 
P. Jones.  These modules are usually $20 - $30 NOS.  They are generally 
used to control and switch large motors and inductive loads.  I have 
three different types manufactured by Fuji and Powerex.  

My first unit is a single Darlington which handles 400 A continous up 
to 1200 V.  It has about a 3 1/2" x 4" footprint and disapates over 
3100 watts.  I mounted it on a huge multifinned Al heat sink.  No 
overheating so far in my tests.  The Darlington has reverse EC and EB 
diode protection of 1200 V and 400 A and 4000 A peak.  The transistor 
turns on and off in 30 uS at 400 V into a resistive load.  The standoff 
voltage of the module case is 2500 V.  

Using my single Darlington BPT module, I connect it with a large high 
power diode bridge that I made up.  The collector is connected to the 
positive side of the bridge and the emitter is connected to the 
negative side of the bridge.  The AC source is connected in series to 
the load (transformer primary) then in series to the two AC sides of 
the bridge.  The bridge configuration always assures current flow in 
one direction from positive to negative, ie. C ==> E.  Voltage 
regulation from full on saturation to cut off of the BPT is controlled 
by a small variable resistor from C-B. A VOM across CE measures from 
full main voltage at cut off to 1.8 VAC at saturation.  Current is 
monitored and controlled from the emitter side.  A 0.5 ohm 300 w 
resistor is in series between the emitter and the negative side of the 
bridge.  Pulsating DC voltage developed across this resistor drives the 
base of a 3055 BPT through another variable resistor.  The 3055 
collector is connected to the base of the power BPT by a small variable 
resistor (can be a fixed resistor).  The 3055 emitter is connected to 
the negative side of the bridge.  Current limiting is done with the 
variable resistor to the base of the 3055.  It's important to select 
the main power resistor so that the voltage drop doesn't exceed the 
0.6-0.7 v base drive of the 3055 prior to the peaking of the mains 
voltage.  This allows full mains voltage and current through the load 
up to the mains voltage.  Then as the load current reaches the set 
limit the 3055 kicks in and controls the base drive of the main power 

This solid state voltage regulator/current limiter is floating and is 
therefore somewhat protected from transformer kickback.  As such, it is 
riskier to operate while it is hot.  I have added RFI/EMI protection on 
both sides of the transistor module as well as to the mains power 
source to limit kickback and RF interference.  I have considered 
isolation transformers and small diode bridges on both sides for the 
controlling circuits for safety, but haven't done it yet.  

I have tested this current limited voltage regulator with many loads, 
including resistors, light bulbs, neon bulbs, ordinary transformers, a 
small HV x-ray transformer and a 12 kv 60 ma neon small TC.  I have 
tested it without a variac and voltage regulation is fairly good.  It 
would be better with a small isolation transformer, diode bridge and 
voltage divider to drive the base.  Not to mention, a little safer.  
The module stays cool to mildly warm on the big heat sink.  A cooling 
fan could easily be added at higher powers.

So what do we have here?  A workable alternating voltage and current 
regulator for small to moderate AC loads for about $40.  A Variac and 
inductive current limiting (electric welders) can be eliminated.  I do 
not claim that the prototype is perfect because there will be many 
improvements and modifications.  But, it has a great deal of potential.

My other two modules have two Darlingtons in a half wave bridge 
configurations.  I am now experimenting with two modules wired as a 
full wave bridge.  This eleiminates the above diode bridge and promises 
higher power output as each of the four Darlingtons work far less.  
Only two Darlingtons fire with each half wave, one on each module.  
Driving the bases is a little more difficult and requires an additional 
isolation transformer, probably a center tapped Tx or two antiphased 
transformers.  But, it's safer for the operator.  I'm sure power output 
can be extended to high power systems.


1.  Marlin P. Jones Catalog, # 97-7,p. 113.  1-800-652-6733.  Nice 
folks to deal with.  They faxed me all the data sheets (#7) at no 

2.  Herbech and Rademan 

3.  An easy source is old large UPSs like BEST manufactured.  Often 
when the backup batteries fail these UPSs are discarded and can be had 
for free or nominal charge.  They also have a wealth of other 
components, a very nice transformer, transistor module, heat sink and 

4.  Modules also come and go in other surplus catalogs.  Keep an eye 

5.  Hamfests