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