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DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Return to index
Sheet #: D7f
Title: RESISTORS
Updated by: Joel Goldberg
Drawings by: Joel Goldberg
References: Any electrical or electronics text
"Model Railroad Electronics",
Peter Thorne, Kalmbach Pub. Co.
GENERAL
October 1999
First Issued: May 1955 (D7c.41)
Updated:
All resistors consist of an insulated core, a resistive
material that is placed around this core and two wires or
Originally
Compiled by: Ed Ravenscroft, MMR
terminals are connected to the ends of the resistive
Page: 1 of 5
material. There are many different types of resistive
materials. Among these you will find resistance wire,
resistance coatings and plated materials that exhibit resistive qualities.
Resistors are identified in four different ways. One of these is the amount of resistance, measured
in units of the OHM. The symbol for the Greek letter Omega is used to represent the ohmic term.
This symbol is 'Ω'. A second way is the amount of heat, or power, that the resistor is able to
withstand before it fails. The letter 'W' is used to indicate the power rating in units of the Watt.
These are the two most common ratings for resistors.
This power rating is measured in units of the WATT. A third way, one that is not often shown, is a
voltage rating. The resistor is rated in the maximum amount of VOLTAGE that can be placed
across its terminals, or wires, before it fails. The final way is its TOLERANCE rating. Resistors are
given values using a standard industry numbering set. They also are rated with a tolerance value
ranging from ± 0.5% to ±20%. What this means is that the measured value of the resistor may vary
from its marked value by the percent of the tolerance. Most modelers will be concerned about the
Ohmic rating and the Wattage rating.
The Ohmic value and Wattage value ratings are independent of each other. You may find a 10Ω
resistor that has a power rating of 1 Watt and another 10Ω resistor with a power rating of 50 Watts.
Typically, the physical size of the resistor is an indicator of its power rating. Physical size increases
with its power rating.
RESISTOR COLOR CODE CHART
A standard Electronic Industries Association
(EIA) color code is used for all electronic
components. This color coding is shown in
the chart at right. Low power resistors utilize
this color coding to identify their Ohmic
values. It is important to recognize that the
third color code band is NOT a specific
number, but it is a multiplying factor. For
example, a resistor having a color coding
marking of red, yellow and orange would
have an Ohmic value of 24,000 Ω. This is
developed when using the chart: the first
band represents the number 2, the second
band represents the number 4 and the third
band is the multiplier. In this example the
multiplier is 1,000, so the value is
determined by 2 4 x 1000, or 24,000.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7f
Title: RESISTORS
ELECTRICAL
Page: 2
FUNDAMENTALS
of 5
Band 1 gives the first number
Band 2 gives the second number
Band 3 gives multiplier
(number of zeros)
Band 4 gives tolerances, if used
(no band indicates +- 20%)
EIA Resistor Color Code Markings
RESISTOR SIZES AND RATINGS
Resistors are commonly identified as being
either low power or 'power' resistors. Low
power resistors are identified by their color
coding markings and their physical size. Four
physical sizes for low power resistors are
shown to the right. The smallest one is rated
as a 1/4 watt device. The standard ratings
increase with their size to ½, 1, and 2 watts.
TYPICAL RESISTORS
The symbols for resistors of any value are shown below. There are some variances with this
symbol. They, too, are shown. A difference in symbols also may be found. For example, the
resistor symbol used in electrical work is simply a box, as shown. A different graphic symbol is
used on an electronic diagram, also shown below. For the purposes of this Data Sheet, electrical
symbols will be used. You will find a component identified as a VARIABLE RESISTOR. This, too, is
shown below. The variable resistor consists of a resistance element and a contact strip that can
move along the surface of the element. Total resistance is determined by the value of the
resistance element. The amount of resistance between either end of the element and the contact
strip is changed by moving the contact. This, in effect, divides the total resistance into two
segments, these being one end of the element and the contact and the other end of the element
and the contact.
ELECTRICAL SYMBOLS
Fixed value resistor
Variable resistor
ELECTRONIC SYMBOLS
Fixed value resistor
Variable resistor
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7f
Title: RESISTORS
ELECTRICAL
Page: 3
FUNDAMENTALS
of 5
GENERAL - continued
Power resistors are also commonly identified by their
physical size. Power resistors do not utilize the color
banding coding system. Typically, their actual Ohmic value
is printed on the body of the resistor. Several different sizes
and shapes for power resistors are illustrated to the right.
FIXED AND VARIABLE RESISTORS
The power resistors illustrated above are considered to be
'fixed' values. Their ohmic value is measured across the
terminals of the resistor. You will find a group of resistors
that are identified as 'variable'. One of these is illustrated.
Variable resistors have three terminals. Several types are
shown to the right. One of the terminals is connected to
each end of the resistance element. The third, middle
terminal, is connected to a rotating contact arm. These
resistors are available in several different physical sizes
and ohmic values. Their use in a circuit will vary, depending
upon the application.
Two of the most common applications for variable resistors
are: rheostat and potentiometer. Only two of the terminals
are utilized when it is used as a rheostat. The total
resistance in the circuit is changed, depending on the
placement of the rotating arm. This will vary the circuit
resistance and control circuit current. The potentiometer
circuit uses all three terminals, as shown on the following
page. This type of connection will vary the circuit voltage
instead of circuit current.
TYPICAL POWER RESISTORS
TYPICAL VARIABLE RESISTORS
The same variable control may be used in a circuit in one
of two ways: either as a RHEOSTAT or as a
POTENTIOMETER. The difference is in the manner in
which is it connected into the circuit. The method of wiring
the rheostat and potentiometer is also shown on the next
page. In the rheostat circuit, the amount of resistance will
change the total resistance in the circuit, thus controlling
the current flow.
In the potentiometer circuit, the control is placed in parallel
with the voltage source and is used to control the amount
of voltage at its output. Few, if any, model railroad circuits
in present use utilize the rheostat circuit for control of train
speed. Most of the circuits are now using transistors to
control operational conditions.
POTENTIOMETER
DATA SHEET
Sheet #: D7b
D7f
Title: RESISTORS
ELECTRICAL
Page: 4
FUNDAMENTALS
of 5
© NATIONAL MODEL RAILROAD ASSOCIATION
+
+
From
Power
Source
From
Power
Source
To track
-
To track
RHEOSTAT CONFIGURATION
POTENTIOMETER CONFIGURATION
ELECTRICAL CIRCUITS
The basic electrical circuits are: Series, Parallel and a combination of Series/Parallel. Each of
these are illustrated, along with the mathematical formulas required to determine total resistance
in each of the circuits.
R1
R2
R3
IN SERIES: R=Overall Resistance
R1
R2
IN PARALLEL: R=Combined Resistance
R1
R =
Rn
R=R1+R2+R3+ … +Rn
R3
Rn
1 _
1 1_ 1_
1_
_
= + + + ... +
R R1 R2 R3
Rn
R1 x R2
R1 + R2
R1
also
R2
R1 =
R x R2
R2 - R
SPECIAL CASES: R=Combined Resistance
R2
R3
R1 ( R2 + R3)
R =
R1 + R2 + R3
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7b
D7f
Title: RESISTORS
ELECTRICAL
Page: 5
FUNDAMENTALS
of 5
RESISTOR FAILURES
The most common failure for a resistor is due to overheating. Heat is created when current flows
through the resistor. This is caused by the opposition to current flow, or the resistance of the
element. When the amount of heat exceeds the power, or wattage rating of the resistor, a change
in the amount of resistance will occur. The process that will eventually lead to the total destruction
of the resistor occurs in the following manner. As the current flow through the resistor increases,
its manufactured value will tend to decrease. This will create an additional flow through the
resistor and the rest of the circuit, generating additional amounts of heat. Eventually, the value of
the resistor will increase and you may even see that it has broken in two parts. Evidence of this
activity prior to breakage is the charred and darkened body of the resistor when you inspect the
circuit. Often, the cause of the increase in resistance is due to the failure of another component
that is connected in the same circuit. Therefore, just replacing the burned resistor may not repair
the circuit.
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