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.