How to Make Power Transformer Substitutions
April 1959 Popular Electronics
transformers have not changed too much since this article was written
in 1959. Efficiencies might be up a bit, and maybe sizes down, but
other than that, the equations and implementation methods are about
the same. True, you will not find a lot of new transformers with
secondaries that sport vacuum tube filament and plate voltages,
but their equivalents for ±15, ±12, ±5, etc. are there.
April 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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How to Make Power Transformer Substitutions
By Eugene Richardson
Transformers - those bulky heavy, and expensive items - are found
in all types of electronic equipment. Whatever your interest in
electronics, you're sure to have noticed one or more of these steel-clad
components quietly humming to itself off in a corner of the chassis.
When a replacement is needed, or you're searching for a particular
item for a construction project, it's necessary to know the inside
story of the power transformers; how interchangeable are the units
and what do the specs mean?
The purpose of a power transformer
is simply to convert the a.c. line voltage into the higher and lower
voltages required by electronic circuits. The two types you'll come
across are known as high-voltage plate and filament transformers.
In receivers and amplifiers you'll find the functions of both combined
in a single multiwinding general-purpose unit. A typical power supply
is in Fig. 1.
Size and Frequency. Physical
size of a power transformer depends upon both its power handling
capacity and operating frequency. The higher the unit's power rating,
the larger the diameter of the wire needed in its windings and the
greater the amount of iron laminations in its core. A transformer
designed to power a one-tube audio preamplifier may measure a little
over an inch on each side and may weigh but a few ounces. However,
the power transformer of a moderately large television receiver
will measure several inches on each side and be quite heavy.
The lower the frequency, the more iron is needed in the transformer
core to maintain operating efficiency. Hence, transformers designed
for 25-cycle line operation are larger and heavier than the more
familiar 50-60 cps units. Since higher frequency transformers require
less iron, military equipment transformer frequencies may range
from 400 to 1000 cps.
Unless you live in an area supplied
with 25-cycle power (a common frequency of hydroelectric power plants),
the chances are you use 60-cycle transformers in all your projects.
However,400-cycle (or other high frequency) transformers are encountered
on the surplus market. As a general rule, a transformer will overheat,
and may burn out, if used at frequencies appreciably lower than
those for which it was designed. Thus, a 60-cycle transformer may
overheat if connected to a 25-cycle line, as will a 400-cycle transformer
connected to a 60-cycle source.
Fig. 1. Schematic wiring diagram of a typical power supply.
Specifications. Aside from operating frequency,
a power transformer's electrical specifications are given in terms
of primary voltage, secondary voltages and rated currents. In some
cases, the unit's power-handling capacity may be indicated in watts
or volt-amperes (va.=primary voltage multiplied by current in amperes).
typical filament transformer may have the following specifications:
Primary, 105 - 120 volts, 60 cycles; Secondary, 6.3 volts, CT, 2
amps. Such a transformer is designed for operation on a standard
60-cycle power line. Although line voltage is nominally 115 volts,
it may vary from 105 to 120 volts, depending on local conditions.
This unit's center-tapped (CT) secondary winding has a nominal
rating of 6.3 volts, and is capable of delivering a current of 2
amperes without overload. The exact secondary voltage will vary
with the applied primary winding voltage and the secondary load.
If overloaded, the transformer will supply more than 2 amperes,
but the voltage will be low and the unit may overheat. Conversely,
if less than 2 amperes are drawn, the secondary voltage may be somewhat
higher than 6.3 volts.
Frequently, the presence of a center
tap is indicated in the secondary voltage specification rather than
the abbreviation of "CT". For example, a transformer might carry
the following specs: Primary, 105-120 volts, 60 cycles; Secondary,
350-0-350 volts, 50 ma. This transformer has a standard primary
winding and a secondary winding delivering 350 volts on each side
of its center tap; rated secondary current is 50 milliamperes. The
secondary winding could also be described as 700 volts-CT, 50 ma.
Fig. 2. Standard color coding for power transformer leads.
a multi-winding power transformer is used, such as in Fig. 1, the
voltage and current rating of each secondary winding are listed
separately. A typical set of specs reads as follows: Primary, 105-120
volts, 60 cycles; Secondary No.1, 300-0-300 volts, 50 ma.; Secondary
No.2, 5.0 volts, 2 amps; Secondary No.3, 6.3 volts, CT, 3 amps.
Lead Identification. Connections to transformers
are made to fixed terminal lugs or to wire leads. If you purchase
a new transformer, you'll find that lead connection instructions
are printed on the box, on a label on the transformer, or on a separate
sheet. But if you have a transformer salvaged from another project
or taken from a piece of used equipment, these connections probably
will have to be determined anew.
If the unit is of recent
manufacture, and equipped with color-coded leads, you can identify
the leads by referring to the standard power transformer color code
given in Fig. 2. Filament center taps are not included in all transformers.
Fig. 3. Basic test which you can make to identify power transformer
windings using an a.c. voltmeter.
Fig. 4. Two tricks to use in making emergency substitutions: (A)
two filament windings can be connected in series to supply higher
voltages; (B) a filament winding can be connected in series with
the primary to lower all secondary voltages.
transformer is not equipped with color-coded leads, a simple technique
will indicate which is which. With the transformer disconnected,
use an ohmmeter to determine pairs of leads and center-tap connections.
Check the resistance of each winding. The winding having the highest
resistance is usually the high-voltage secondary and may read from
25 to several hundred ohms. The winding having a medium resistance
- generally from 5 to 25 ohms - is the primary. Finally, the lowest
resistance windings, usually less than I ohm, are the filament windings.
Having made a tentative identification, connect a standard
100-watt lamp in series with the leads chosen as the primary leads
and a source of line voltage as in Fig. 3. The lamp should light,
but not at normal brilliance. If it lights to normal brightness,
either the transformer is shorted (and should be discarded) or you've
made an error in choosing the primary leads.
an a.c. voltmeter, check the voltage across each winding, including
the primary. The ratios of these voltages will help you to identify
Fig. 5. How to obtain an "artificial" center tap on a filament winding.
As a final step, remove the series lamp, applying full
line voltage to the transformer primary. Use your a.c. voltmeter
to check the unit's output voltages and to identify each winding
positively. Remember that the voltage will read slightly higher
than normal because of the absence of a load.
Substitutions. A substitute power transformer should be
used only if the specified component is unobtainable.
choosing a substitute, make sure that its secondary voltage specifications
are the same and that its current ratings equal or exceed those
of the original.
For example, a transformer with a
250-0-250 volt, 60-ma. secondary may be used as a satisfactory replacement
for a unit rated at 250-0-250 volts, 40 ma. Similarly, a filament
transformer rated at 6.3 volts, 3 amperes, is a satisfactory substitute,
electrically, for a unit rated at 6.3 volts, 1.5 amperes.
A 5% difference in secondary voltage ratings will usually not affect
circuit operation. Thus, if a project calls for a transformer rated
at, say, 360-0-360 volts, substitutes with ratings of either 375-0-375
or 350-0-350 generally will be satisfactory.
a special transformer having several secondary windings is required,
and an exact duplicate is unobtainable, separate transformers can
be used in place of the single multi-winding unit, provided that
adequate mounting space is available. The transformer's 117-volt
primary windings are connected in parallel.
special filament or bias voltages are required, two (or more) windings
can be connected in series to supply the necessary voltages, as
shown in Fig. 4(A). Connect adjacent winding leads together temporarily
and check the output voltage obtained between the "free" leads,
using your a.c. Voltmeter. If the output voltage is less than expected,
the windings may be "bucking." In this case, interchange the connections
to one winding.
It may be necessary to reduce the
circuit's B+ voltage after installing a substitute transformer.
There are several ways of doing this. A small resistor (5 to 25
ohms, 10 watts) can be connected in series with one of the primary
leads, or the effective turns ratio of the transformers can be reduced
by connecting one of the filament windings in series with the primary,
as shown in Fig. 4(B). The preferred methods would be to substitute
a rectifier tube with a larger internal voltage drop or lower the
value of the input filter capacitor (C1 in Fig. 1). If the hum level
in the d.c. output goes up; raise C2's value.
a center-tapped filament winding may be needed, but may not be available
on the substitute transformer. In such a case, an electrical "center-tap"
can be obtained by connecting a 50-to-100-ohm adjustable wire-wound
resistor across the filament winding, as shown in Fig. 5. The adjustable
tap is centered on the resistor.
It is not necessary to
use all the windings available on a multi-winding power transformer.
For example, suppose you need a general-purpose power transformer,
and find one with secondary specifications which match those of
the needed unit but with an "extra" 6.3-volt filament winding. Simply
ignore the extra winding, taping its leads to one side (taking care
that they do not short together).
are important only when the substitute unit is used as a servicing
replacement or in the construction of equipment where the component's
physical size and shape are important. These specifications include
overall dimensions, weight, and type of construction or mounting.