July 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Despite having heard of and
used the "voltage-regulating transformer" (aka "constant voltage transformer," CVT),
I never knew exactly how they work. This installment of Mac's Service Shop in the
July 1965 issue of Electronics World magazine happens to address the device,
so now I at least have some idea what's going on to magically hold the secondary
voltage at an amazingly constant value regardless (within reason) of the primary
voltage. It is a purely passive device that uses ferroresonance for feedback. The
price to be paid for such a convenience is explained to Barney by Mac after Barney
gets befuddling readings on his voltmeters. Mac invokes the name of the inventor,
Joseph Sola, who manufactured CVT's in the Sola Electrics Company premises. According
SolaHD (merger of Sola and Hevi-Duty) About Us webpage, "Sola Electric
opened in 1930 as an emerging laboratory in two modest size rooms in a Chicago bank
building. It was a small beginning, but Joseph G. Sola had big ideas. Beginning
in the 1930's, he practically invented the field of transformer magnetics and went
on to revolutionize the fledgling electrical industry - winning 55 U.S. patents
along the way."
Mac's Service Shop: Voltage-Regulating Transformers
These special transformers, as opposed to conventional
transformers, operate on the ferroresonant principle.
By John Frye
Barney came bustling into the service department with a heavy object cradled
in his arms. "There's something I've wanted a long time," he announced, depositing
a husky transformer from which dangled a metal-encased capacitor on the service
"What do you need with a voltage-regulating transformer?" Mac, his employer,
"My single-sideband ham receiver drifts a few cycles on 15 and 20 meters when
the line voltage changes. Plate voltage of the receiver v.f.o. is regulated with
a VR tube, and since the frequency change is a slow affair I'm sure it's the result
of oscillator filament voltage change. Even a few cycles of drift is annoying on
sideband; but after I plug my receiver into this little gem - which a friend just
gave me - the line voltage can horse up and down all it wants without making me
retune. I'm told this transformer, in some mysterious way, will hold output voltage
within 1% while the line voltage is changing as much as 15%. Just for kicks, I thought
I'd check it out on our variable-voltage transformer."
Grinning expectantly, Mac watched Barney connect a 75-watt lamp to the output
of the transformer and attach leads from the v.o.m. to the lamp terminals. Confidently
the youth plugged the line cord of the voltage-regulating transformer into the output
receptacle of the service bench variable-voltage transformer, but he hastily jerked
out the plug when he noticed the v.o.m. reading. Gingerly he replaced the plug and
rotated the voltage-selecting knob of the bench transformer. Next he substituted
test leads from the v.t.v.m. for those of the v.o.m. Finally he connected leads
from both meters to the a.c. line and compared readings.
"Man, I don't get that!" he exclaimed. "This transformer is supposed to put out
118 volts, but the v.o.m, reads 133 volts, no matter if the input voltage is anywhere
from 105 to 130 volts. The v.t.v.m., on the other hand, reads a little under 100
volts for the same range of input voltage. Yet both meters read exactly 118 volts
when connected to the line. This July heat must be getting to me."
"Maybe not," Mac said soothingly. "Think a little. The a.c. meter of the v.o.m.
is a rectifying type that deflects according to the average value of a sine-wave
voltage but has a scale calibrated in r.m.s. units, The v.t.v.m. is deflected in
accordance with peak-to-peak values, but the scale indicates r.m.s, voltage of a
"A non-sinusoidal waveshape!" Barney interrupted, switching on the scope. Sure
enough, cycles of the voltage-regulating transformer output looked like slightly
domed square waves.
Fig. 1 - (A) Conventional transformer. (B) Magnetization curve.
(C) Transformer with magnetic shunt and resonating capacitor. (D) Voltage-regulating
"Your transformer is of the type called normal harmonic," Mac commented. "Output
voltage of such a transformer contains from around 14% to more than 20% harmonic
content depending on the manufacturer, loading, etc. From the looks of that wave,
I'd guess this one is close to 20%. Voltage from these transformers is set with
a dynamometer type of voltmeter, and readings from any other type of voltmeter will
not be the same. Remember, multiplying peak voltage bv 0.707 to get r.m.s. voltage
and by 0.637 to get average voltage only holds true for a sine wave. Actually, in
a square wave the peak, r.m.s., and average values are all the same. Let's see what
will happen if you feed your receiver from this transformer."
Mac connected a full-wave bridge instrument-rectifier across the 10-volt secondary
of a bell transformer and placed a 40-μf. filter capacitor across the rectified
output. A 14-volt lamp was connected across the secondary.
"Voltage across the filter capacitor will represent d.c. voltages in your receiver,"
he said. "The lamp bulb will represent filaments, and the reading of this lightmeter
beside the bulb will indicate any change in filament temperature. When the primary
is connected straight to the 118-volt line, I see we have 13 volts d.c. across the
filter capacitor, and both the v.o.m. and the v.t.v.m. read 10.5 volts a.c. across
"Now we switch the primary to the output of the voltage-regulating transformer.
The lamp gets a little brighter - I'd guess about 5% according to the lightmeter
- but our d.c. voltage has dropped to 11 volts! Secondary voltage has gone up to
11.6 volts measured with the v.o.m., and down to 8.8 volts measured with the v.t.v.m.
Besides demonstrating the futility of trying to measure non-sinusoidal voltages
with conventional meters, this experiment leads us to expect d.c. voltages will
be down by about 15% in your receiver while the filaments burn a little brighter.
"Normal-harmonic-type voltage-regulating transformers are not recommended for
use where a high harmonic content may lead to difficulty or where the output is
to be rectified. Such is not the case when the output is used for heating or to
operate relays and solenoids. In your case, why not use a separate filament transformer
working off this gift horse of yours for heating critical filaments in your receiver?
You could arrange it so these filaments burn all the time and keep heat-sensitive
circuitry warmed up. That will make your receiver even more stable than it is right
"You know, you're not so dumb for an older man," Barney said. "I'll do it. Now
can you tell me how voltage-regulating transformers work?"
"I'll try, but it's not easy," Mac warned. "Not much has been published on the
subject in popular magazines. Most of what I know I've learned from literature furnished
by the Sola Electric Company, a pioneer in the field and a major manufacturer of
"Let's start by reviewing conventional transformer action," Mac said while he
drew some sketches on the blackboard at the end of the bench. "Fig. 1A here
is such a transformer; Fig. 1B is the magnetization curve of the core material.
A voltage Ep1 across the primary causes a current Ip1 to flow
through the primary, producing magnetization flux Φ1. This flux links
the secondary and, in accordance with Faraday's Law, produces a voltage Es
across the secondary proportional to the primary/secondary turns ratio. Secondary
current flowing through a load produces a secondary flux that tends to cancel primary
flux so that primary current must increase to maintain the original flux level."
"The conventional transformer is a linear device operating on the linear portion
of the magnetization curve. Increasing primary voltage to Ep2 produces
current Ip2, causing a flux increase to Φ2resulting in
an increase in secondary voltage directly proportional to the increase in primary
voltage. But if the transformer is operated to the right of the knee of the magnetization
curve, it ceases to be a linear device. Now a similar change in primary voltage
causing an Ip3 to Ip4 increase in primary current results
in a much smaller increase in flux and, consequently, in secondary voltage. Such
a saturated transformer provides a degree of voltage regulation, but it is not practical
for any considerable amount of power because primary current approaches short-circuit
value as the core saturates. What we need is a transformer in which secondary flux
could be saturated without materially affecting primary flux."
"This is accomplished in Fig. 1C by what is called a magnetic shunt or bypass
built into the core. Here, as you can see, only part of the primary flux links the
secondary, and vice versa. Next, to increase secondary flux saturation, we connect
a large value of capacitance across the secondary and establish a condition called
ferroresonance. A ferroresonant circuit cannot be exactly the same thing as an ordinary
resonant circuit because the inductance in the ferroresonant circuit is non-linear;
yet the two circuits have many common properties.
"For example, the resonant tank circuit of your amateur transmitter develops
very high voltages across it because of heavy circulating tank currents. So long
as the excitation voltage is maintained above a certain minimum value, changes in
the level of that exciting voltage have little effect on voltage across the tank
"High voltage across the ferroresonant circuit is also chiefly due to heavy circulating
currents through the capacitor. This voltage is much higher than would be expected
from the primary / secondary turns ratio. You will find around 600 volts across
that capacitor. And voltage across the ferroresonant secondary is little affected
by changes in the primary voltage. Isolation of primary and secondary magnetic fluxes
allows the heavy saturating secondary current to be produced without heavy primary
"But there is always some increase in secondary voltage with an increase in primary
voltage, and this leads to the circuitry of Fig. 1D. Here we see a portion
of the voltage developed across the resonant circuit fed to the load in series with
a bucking winding wound over the primary. An increase in primary voltage leading
to a slight increase in voltage across the tapped portion of the resonant winding
also produces a slight increase in voltage across the bucking coil. Since the bucking-coil
voltage opposes the voltage of the resonant winding, the two voltage increases cancel
"Those are the highlights of the regulating transformer story, but there's much,
much more. For example, if one of these transformers is overloaded, the magnetic
field collapses and output voltage falls to zero without primary current increasing
enough to do any damage. When the overload is removed, normal operation is automatically
restored. And there is another type of regulating transformer, costing slightly
more, called the sinusoidal that uses additional 'neutralizer' windings to reduce
the harmonic content of the output to less than 3%. It can be used for those applications
where the normal-harmonic type is not recommended since it has the same excellent
voltage-regulating characteristics. Other voltage-regulating transformers are de-signed
to furnish various filament voltages, or combined plate and filament voltages, and
for 400-cycle use."
"Is Sola the only manufacturer of these transformers?"
"No. Other companies that manufacture or have manufactured voltage-regulating
transformers include Stancor, Triad, Acme Transformer, General Electric, and Raytheon."
"Well, thanks for all the information - I think," Barney said. "I feel a little
like the fellow who asked for a slice of bread and got a bakery. But you just let
me mull over what you've told me for an hour or so, and I'll bet I can come up with
"No bet!" Mac answered quickly.
Posted October 21, 2022
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of none other than John T.
Frye, creator of the Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.