DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
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GENERAL
Sheet #: D7c
Title: TRANSFORMERS
Updated by: James A. Whatley
Updated: October 1999
First Issued: April 1956 (D7c.13)
D. A. Hoffman
Originally
Compiled by:
References: "Electrical Handbook for Model
A transformer is an electric device which converts
Railroads", Vol. 1, 3rd Ed.
alternating current of one voltage to an alternating
by Paul Mallery, MMR
Various Electronic supplier catalogues
current of another voltage by means of electro magnetic
Page: 1 of 4
induction. The most commonly used transformer in
model railroad work is the low voltage, stepdown type
incorporated in most power packs. It is used to reduce
the usual 110 volt house current to a value suitable for operating a model railroad.
BASIC COMPONENTS
In its most basic form, a transformer consists of a primary winding and a secondary winding wound
on a core of laminated electrical steel. The primary winding is connected to the input source or
supply line and the secondary winding is connected to the output or load. A typical transformer is
shown in Fig. 1a and its schematic drawing symbol is shown in Fig 1b. Transformers can also
have multiple windings wound around the same core, for example one primary and two
secondaries. Another common configuration is for the secondary winding to have a center-tap
connection as shown in Fig. 1c.
SEC
PRI
Figure 1a:
Pictorial representation
of a basic transformer
SEC
Figure 1b:
Schematic drawing symbol
of a basic transformer
PRI
C.T.
Figure 1c:
Schematic drawing symbol
of a transformer with a
center tapped secondary
FUNCTION
Introduction of an alternating current in the primary winding causes a current of the same
frequency to be induced in the secondary winding. The values of the voltage and current in the
secondary winding have a relation to the number of turns as shown by the following equation:
Primary Turns = Primary Volts
=
Secondary Turns
Secondary Volts
Secondary Amperes
Primary Amperes
DATA SHEET
Sheet #: D7c
Title: TRANSFORMERS
Page: 2 of 4
© NATIONAL MODEL RAILROAD ASSOCIATION
RATINGS
Transformers are usually rated in "Volt-Ampere" capacity:
Volt-Ampere rating = Primary Volts x Primary Amperes
or
= Secondary Volts x Secondary Amperes
Thus a transformer having a 60 VA rating operating on 115 volts will draw about ½ ampere from the
line. If the output is at 12 volts, the safe continuous load on the secondary would be 5 amperes.
Sometimes the term "watts" is loosely used to rate transformers in place of the correct term "VoltAmperes."
To assure trouble-free operation the load should not exceed the rating of the transformer.
Exceeding the rated input voltage or the rated output amperes will cause overheating and if
continued too long will ultimately burn out the windings. Ensure that a transformer is adequately
rated for the intended load. The frequency rating also is important -- in North America, the
standard house current is 60-cycles per second or Hertz, whereas 50-cycle is used in some other
parts of the world.
Some transformers, such as are used for doorbells, have their windings and core so designed that
when one attempts to overload the secondary, the output voltage drops sharply thereby limiting the
output current to a safe value. Even a short circuit in the secondary of such a transformer may not
result in serious overheating. On the other hand, since such transformers do not deliver overload
currents even for short periods, they generally cannot be used as a source of power for high
amperage devices such as switch machines, even though the rated output voltage may be
adequate.
VOLTAGE REGULATION
Well designed transformers are quite efficient and the losses due to heat within rated limits are
quite nominal. Likewise the voltage regulation, or stability of output voltage under varying loads is
very good. The drop in output voltage from no load to full load seldom exceeds 5% in well designed
transformers.
AUTOTRANSFORMERS
Autotransformers have have only one coil with fixed or adjustable taps or a slider arranged for
changing the output voltage. The output voltage may be higher or lower than the input voltage,
depending on tap position. When one of the output leads has a contact that slides over the coil,
the output voltage can typically be varied from 0 to about 120% of the input voltage. They are
available under the trade names: "Powerstat," "Autostat," "Variac," Etc.
DATA SHEET
Sheet #: D7c
Title: TRANSFORMERS
Page: 3 of 4
© NATIONAL MODEL RAILROAD ASSOCIATION
AUTOTRANSFORMERS - continued
LINE
INPUT
OUTPUT
NEUTRAL
Figure 2a:
Variable Autotransformer
pictorial diagram
Figure 2b:
Variable Autotransformer
schematic diagram
MODEL RAILROAD APPLICATIONS
Besides their use in power packs, the chief use for transformers of the step-down type is to furnish
power for auxiliaries such as lights, flashers, switch machines, etc. Filament transformers as found
in electronics stores are a good source of low voltage alternating current.
The alternating current from transformers is not recommended for continuous operation of relays or
relay type switch machines because the relays, even though designed for AC operation, will hum,
creating unnecessary noise. DC will operate these relays at lower voltage and with less
overheating.
When buying transformers, it is always advisable to look for the UL (Underwriter's Laboratory) or
CSA (Canadian Standards Association) labels to be sure that the transformer can safely be used at
the stated rating without overheating or creating a fire hazard. Transformers with a tapped
secondary or primary are desirable as they allow some latitude in adjusting operating voltages to
meet varying conditions.
When connecting a transformer it is essential to distinguish between the low voltage and high
voltage leads. The low voltage leads which will carry the greater amperage are often the heavy
leads. However, the data sheet accompanying a particular manufacturer's transformer should be
consulted.
For safety, one point of every transformer secondary winding should be connected to earth ground,
for example a cold water pipe. If the transformer feeds a DC rectifier, the ground connection is
typically made to one of the DC output leads. This ground connection ensures that the secondary
winding circuit can't float up in voltage relative to ground due to the possibility of stray capacitive
coupling between the primary and secondary windings.
DATA SHEET
Sheet #: D7c
Title: TRANSFORMERS
Page: 4 of 4
© NATIONAL MODEL RAILROAD ASSOCIATION
MODEL RAILROAD APPLICATIONS - continued
Variable autotransformers are an effective way of controlling train speed, but for safety they should
always have one side grounded and be isolated from the train layout. In household wiring, the 110
V neutral (white) conductor is grounded at the service panel and the line or "hot" (black) conductor
is at a voltage of 110 V with respect to ground. It is essential to connect an autotransformer so that
the input neutral connection is at the same voltage as layout ground. However, safety is further
enhanced by isolating the autotransformer output. This is conveniently done by using such a
transformer to feed a regular step-down transformer or power pack. The latter need not contain a
rheostat. Instead of controlling the voltage applied to the track by means of a rheostat in the DC
output of the power pack, the voltage is controlled by using the variable auto transformer to control
the input voltage to the power pack. A very smooth control of train speed results but the cost is
somewhat higher than the conventional power pack and rheostat combination.
TRANSFORMER EXAMPLES
Some typical examples of transformers appropriate for model railroading are shown below.
In Fig. 3a, an open style construction type of transformer is shown, in which the primary and
secondary windings are wound around the central core of a series of overlapping "E" shaped steel
core laminations. Many modern designs use separate bobbins for the primary and secondary
windings to increase isolation safety. This type of transformer is typically mounted inside an
enclosed, ventilated metal chassis.
Fig. 3b shows an enclosed construction type of transformer. Electrically, it is similar to the open
type, but the sturdy steel enclosure provides mechanical protection suitable for mounting on top of
a chassis or cabinet.
A typical variable autotransformer is illustrated in Fig. 3c. This configuration is designed for panel
mounting. In model railroad application, extreme care should be exercised to physically enclose
and separate the 110 V windings and related wiring from other layout controls and circuits.
Figure 3a:
Figure 3b:
Open Style
Enclosed Style
Construction Transformer
Construction Transformer
(Fig. 3a & 3b Courtesy of Hammond Manufacturing Co. Inc.)
Figure 3c:
Variable Autotransformer