Resistor Selection Application Notes Facts and Factors

A resistor is a device connected into an electrical circuit to resistor to withstand, without deterioration, the temperature introduce a specified resistance. The resistance is measured in attained, limits the which can be permit- . As stated by ’s Law, the current through the resistor ted. are rated to dissipate a given wattage without will be directly proportional to the across it and inverse- exceeding a specified standard “hot spot” temperature and the ly proportional to the resistance. physical size is made large enough to accomplish this. The passage of current through the resistance produces Deviations from the standard conditions (“Free Air . The heat produces a rise in temperature of the resis- Rating”) affect the temperature rise and therefore affect the tor above the ambient temperature. The physical ability of the wattage at which the resistor may be used in a specific applica- tion. Selection Requires 3 Steps

Simple short-cut graphs and charts in this catalog permit rapid 1.(a) Determine the Resistance. determination of electrical parameters. Calculation of each (b) Determine the to be dissipated by the Resistor. parameter is also explained. To select a resistor for a specific application, the following steps are recommended: 2.Determine the proper “Watt Size” (physical size) as controlled by watts, , permissible temperatures, mounting conditions and circuit conditions.

3.Choose the most suitable kind of unit, including type, terminals and mounting. Step 1 determine resistance and watts Ohm’s Law Why Watts Must Be Accurately Known 400 Stated non-technically, any change in V V (a) = I or I = R or V = IR current or voltage produces a much larger change in the wattage (heat to be Ohm’s Law, shown in formula form dissipated by the resistor). Therefore, 300 above, enables determination of the the effect of apparently small increases resistance when the required voltage and in current or voltage must be investi- atts current are known. When the current and gated because the increase in wattage voltage are unknown, or the best values may be large enough to be significant. 200 not decided on, at least two of the three Mathematically, the wattage varies as terms in Ohm’s Law must be measured the square of the current, or voltage, as in a trial circuit. stated in the formulas (b). For example,

2 an increase of 20% in current or voltage Percent Rated W 2 100 (b) P = I R or P = VI or P = V will increase the wattage 44%. Figure 1 R below graphically illustrates the square in watts, can be determined from law relation. Hence, the actual current the formulas above, which stem from must be used in figuring the wattage and 0 Ohm’s Law. R is measured in ohms, V in the increase in wattage due to appar- 0 100 200 volts,I in and P in watts. ently small changes, then determined in Percent Current order to select the proper size resistor. or Voltage Allowance should be made for maximum possible line voltage. Fig. 1: Rapid increase of wattage with current or voltage.

Step 2 or Physical Size of Resistor

A resistor operated at a constant wattage will attain a steady tem- the “National Electrical Manufacturers Association” (NEMA) and the perature which is determined largely by the ratio between the size “Underwriters’ Laboratories, Inc.” (UL) can be described as follows: (surface ) and the wattage dissipated, The temperature stabilizes The relation of the “Free Air Watt Rating” of tubular type, vitre- when the sum of the heat loss rates (by radiation, convection and ous enameled resistors to the physical size, is to be set at such a conduction) equals the heat input rate (proportional to wattage). The figure that when operated at their rated watts, the temperature rise greater the resistor area per watt to be dissipated, the greater the of the hottest spot shall not exceed 300°C (540°F) as measured by heat loss rate and therefore the lower the temperature rise. The rela- a when the temperature of the surrounding air does tion between the losses varies for different resistors. not exceed 40°C (104°F). The temperature is to be measured at Free Air Watt Rating the hottest point of a two-terminal resistor suspended in free still air The wattage rating of resistors, as established under specified stan- space with at least one foot of clearance to the nearest object, and dard conditions, is defined as the “Free Air Rating” (“Full Rating” or with unrestricted circulation of air. “Maximum Power Rating”). Several standard methods of rating are A slightly different definition of temperature limit used as a basis in use based on different service conditions. The method of both for wattage rating, and which results in a slightly higher attained temperature, was originally established in military specification MIL-

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R-26 for wirewound resistors. load-life test. Characteristic V resistors are required to dissipate rated wattage in It is also assumed that the temperature rise at a given wattage an ambient of 25°C without exceeding a maximum operating temper- is independent of the ambient temperature in which this wattage ature of 350°C at the hottest spot. This corresponds to a temperature is being dissipated. Therefore, for high ambient temperatures, the rise of 325°C in a 25°C ambient. Although MIL-R-26 permits a 25°C operating wattage should be limited in accordance with the curves greater temperature rise than NEMA or UL, the reference ambient for of Fig. 3. Although the assumption that temperature rise is indepen- the latter two is 15°C higher. Consequently, the difference in attained dent of ambient is not exactly true, the approximation is sufficiently temperature between the two systems is only 10°C. The curves in close for all practical purposes and, therefore, has been adopted for Fig. 2 show the relation between temperature rise and wattage for 100 various specifications. Note the differences in the permissible rise for °F °C Ind. & Comm. Std. Mil-R-26: Char. V. 400 Bare Resistor— 80 EIA: Char. E, H & V 700 NEMA; 25°40° A 375°C 675°F Corrib & Powr-rib 350 atts U.L.-NEMA Std. Ind. & Comm. Std. for Resistors 600 ° ° Mil-R-26: Char. V 60 B 325 C 585 F EIA: E, H & V ° ° mperature 300 C 300 C 540 F U.L.-NEMA Std. Mil-R-26: Char. U

Te 500 for Resistors EIA: Char. G 40 250 D 250°C 450°F Mil-R-26: Char. U EIA: Char. G

400 Percent Rated W Ambient 200 20 300 275° 340° 150 0 200 100 0 50 100 150 200 250 300 350 Ambient Temperature, °C

mperature Rise above 100 50 Fig. 3: Derating for ambient temperature. Te

0 0 derating purposes. 0210 0430 0650 0870 090 100 Despite the above variables, figures may be cited in terms of Resistor Load — Percent Rated Watts “watts dissipated per square inch of winding surface” for a given Fig. 2: Approximate hot spot temperature rise of a resistor in free air for various speci- temperature rise. For power type resistors operating at 300°C rise fications. above ambient, this figure varies between approximately 6.3 watts per square inch for large resistors (175 watt) to about 9 watts each specification. per square inch for smaller resistors (12 watt). It should also be The absolute temperature rise for a specific resistor is roughly observed from Fig. 2 that temperature rise is not directly proportional related to the area of its radiating surface. It is also dependent upon to wattage dissipated. Note, for example, that at 50% rated wattage, a number of other factors, however, such as of the temperature rise still remains about 70% of that at full rating. the core and coating materials, factor of the outer surfaces, The wattage ratings used in this catalog, unless otherwise stated for ratio of length to diameter, heat-sink effect of mountings, and other certain types, are on the basis of a nominal operating temperature of minor factors. 350°C at full rating. There are two general categories of power resis- The maximum permissible operating temperature for a given tors for which the 350°C nominal temperature limit does not apply. resistor is basically determined by the temperature limitations One is that class of power-precision resistors where high stability is a imposed by the materials used in its construction. Generally speak- salient feature, in which case the operating temperature is nominally ing, these limits cannot be sharply defined in terms of temperature limited to 275°C. The other category includes all exposed ribbon alone. Other factors such as resistance stability versus time, dete- resistors (see description of Corrib® and Powr-Rib®) which are rated rioration rates of insulation and moisture-resistance characteristics, for 375°C (675°F) maximum temperature rise when measured on the type and size of resistance wire, all enter into consideration of wire per NEMA standards. “acceptable service life.” Temperature Distribution on a Resistor For these reasons, the precise temperature limits corresponding to 100% rated wattage are somewhat arbitrary and serve primarily as The temperature rise varies (following a curve) along the length design targets. In the last analysis, once a wattage rating has been of the resistor with the hot spot at the center-top (of a horizontal assigned on the basis of an empirical hot spot limit, the verification of tube) and the ends at approximately 60% of the maximum tem- its correctness must be established through long term load-life tests perature rise. The terminals themselves are still cooler. When the based on performance and stability standards rather than the mea- resistor is vertical, the hot spot shifts upwards a little and the top surement of hot spot temperature. Maximum limits are stipulated for end is hotter than the bottom. The standard “Free Air Watt Rating,” parameter changes as a result of various tests, including a 2000 however, is used regardless of .

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Steps 3 Select a Resistor

Choose the most suitable resistor meeting the requirements of the application. Standard resistors carried in stock should be considered first. If a suitable resistor cannot be found in the standard sizes or resistance values, then select a non-standard resistor from the range on available sizes (consult factory).

Application Watt Rating

To allow for the differences between the actual service condi- 5. Pulse Operation tions and the “Free Air Watt Rating” it is a general engineering This is not an environmental condition but a circuit condition. practice to operate resistors at more or less than the nominal As a pulse of power, when averaged over the total on and off rating. The details by which such ratings can be estimated are time, results in less heat per unit time than for continuous duty, given in the following pages. Most thermal calculations, how- the temperature rise is affected. This may permit higher power ever, involve so many factors which are usually not accurately during the pulses. The conditions must be expertly considered known, that at best they are only approximations. for conservative rating. The open-wound “Powr-Rib®” resistor The most accurate method of determining or checking the construction is most suitable. rating is to measure the temperature rise in a trial installation. A thermocouple (made of #30 B & S gage wire) is recommended 6. Cooling Air for the measuring element. Even made with a Forced circulation of air over a resistor removes more heat per thermocouple will vary slightly with different samples and tech- unit time than natural convection does and therefore permits niques. The factors which affect the temperature rise act inde- an increased watt dissipation. Liquid cooling and special con- pendently of each other and are summarized as follows: duction mountings also can increase the rating. 7. Limited Temperature Rise 1. Ambient Temperature It is sometimes desirable to operate a resistor at a fraction of As the maximum permissible operating temperature is a set the Free Air Watt Rating in order to keep the temperature rise amount, any increase in the ambient temperature subtracts low. This may be to protect adjacent heat sensitive apparatus, from the permissible temperature rise and therefore reduces to hold the resistance value very precisely both with changing the permissible watt load. load and over long periods of time and to insure maximum life. 2. Enclosure 8. Other Considerations Enclosure limits the removal of heat by convection currents in High Resistance. High resistance units, which require the the air and by radiation. The walls of the enclosure also intro- use of very small diameter wire, generally should operate at duce a thermal barrier between the air contacting the resistor reduced temperature for maximum reliability. and the outside cooling air. Hence, size, shape, orientation, amount of ventilating openings, wall thickness, material and fin- High Voltage ish all affect the temperature rise of the enclosed resistor. A maximum voltage gradient of 500 volts R.M.S. (705 volts peak) per inch of winding length is recommended under nor- 3. Grouping mal conditions. For higher gradients in pulse applications or When resistors are close to each other they will show an for other special conditions such as oil immersion, consult fac- increased hot spot temperature rise for a given wattage tory. because of the heat received by radiation from each other and the increased heat per unit volume of air available for convec- High Frequency tion cooling. Non-inductively wound resistors are generally required for use at high frequencies. 4. Altitude The amount of heat which air will absorb varies with the densi- Military and Other Specifications ty, and therefore with the altitude above sea level. At altitudes The special physical operating and test requirements of the above 100,000 feet, the air is so rare that the resistor loses applicable industrial or military specification must be consid- heat practically only by radiation. ered. Military specification resistors should be ordered by their MIL numbers.

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All the components of an electrical apparatus — resistors, rheostats, , , chokes, wiring, terminal °F °C boards, , , electronic tubes, etc.—have their 540 300 Resistor in own limitations as to the maximum temperature at which they 500 Small Box can reliably operate. The attained temperature in service is the Resistor Resistor in in Free Air sum of the ambient temperature plus the temperature rise due Large Box to the heat dissipated in the apparatus. 400 200 The temperature rise of a component is affected by a num- Resistor: 100 watt, 0.75 x 6.5" ber of factors. The graphs and discussions which follow, ampli- (19.05 x 165.10 mm) 300 In unpainted 0.032" (0.813 mm) thick fy and supplement the factors on the previous page. upainted sheet steel box, no vents Small box: 3.375 x 3.375 x 8" Note that the Multiplying Factors given on the Short Cut (85.725 x 85.725 x 203.20 mm) Chart, on page 96 are the reciprocals of the “Percent Load mperature Rise Large box: 5.813 x 5.813 x 12.75" Te 200 (147.65 x 147.65 x 323.85 mm) Ratings” shown on the graphs in this section. The percent fig- 100 ures are, of course, expressed as decimals before finding the Box Temp., Small reciprocals. 100 Box Temp., Large Ambient Temperature Derating 0 0 Fig. 4 shows the percent of full load which power resistors can 025750 5 100 dissipate for various high ambient temperatures. Percent Rated Load Fig.5: Example of Effect of Size of Enclosure on Temperature Rise of An Enclosed 100 Resistor.

Ind. & Comm. Std. Mil-R-26: Char. V. 80 EIA: Char. E, H & V 25°40°

atts U.L.-NEMA Std. 100 60 for Resistors Mil-R-26: Char. U EIA: Char. G 80 40 2" (50.8 mm) space

Percent Rated W 1" (25.4 mm) space

attage Rating 0.5" (12.7 mm) space 20 60 275° 340°

0 0 50 100 150 200 250 300 350 40 Ambient Temperature, °C Fig. 4: Derating of Resistors for High Ambient Temperatures. 20

Derating Due to Enclosure Percent of Single Unit W The amount of derating required, if any, because of enclosure 0 1352467 8910 11 12 is affected by a number of factors, most of which are hard to Number of Resistors in Group determine accurately. The watts per square inch of surface, size, shape, orientation, wall thickness, material, finish and Fig.6: Derating of Resistors to Allow for Grouping amount and location of ventilating openings all play a part. Fig. 5 serves to indicate for a particular set of conditions how the temperatures varied with the size of enclosure for a moderate 100 size power resistor. 5000 ft. 80 Derating Due to Grouping

The temperature rise of a component is affected by the nearby atts 60 presence of other heat-producing units, such as resistors, elec- tronic tubes, etc. The curves in Fig. 6 show the power rating for groups of resistors with various spacings between the closest 40 points of the resistors, assuming operation at maximum per- missible hot spot temperature. If resistors are to be operated at Percent Rated W lower hot spot temperatures, the amount of derating for group- 20 ing can be reduced. 0 Derating for Altitude 0210 0430 0650 0 70 80 90 100 Altitude in Thousands of Feet The curve in Fig. 7 shows the proportional watts for various altitudes, assuming standard atmospheric conditions. Fig. 7: Derating for Altitude

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Pulse Operation “On” Time in to Reach Rated Temperature Unlike the environmental factors, which result in reduction of 0.1 0.2 0.3 0.4 0.5 15213 4 0 20 30 the watt rating, pulse operation may permit higher power in the 1000 pulses than the continuous duty rating. 200 watt vitreous enamel 000 1500 watt Corrib 0.125" (3.175 mm), ribbon The NEMA has set up certain standard duty cycles for motor Low-resistance Powr-rib, edgewound control resistors and the resistor ratings for some of these con- 800 300 watt Corrib 0.125" (3.175 mm), ribbon ditions are shown in Fig. 8. 10 & 50 watt vitreous enamel

The curves in Figures 10,11,12 and 13 illustrate the more attage Rating 700 High-resistance Powr-rib, roundwire general case of various combinations of on and off time for specified loads up to 1000% for a continuous series of pulses. 600 Intermediate loads can be approximated by interpolation. The “on-time” at which each curve flattens out also indicates the 500 maximum on-time for single pulses (with enough off-time for cooling to ambient). Additional data on single pulses is given by 400 Fig. 9. Resistors will reach about 75% of the rated maximum temperature rise in approximately 5 to 8 pulses and level off at 300

maximum rise in another 10 to 20 cycles, depending on per- Percent of Continuous-duty W 200 cent load, size, type, etc. Any curve passing above the inter- section of the designated on and off-times indicates a percent 100 load which can be used. A resistor operated at the rating of an 5210 0 30 50 100 200 300 500 1000 2000 interpolated curve through the point of intersection would oper- “On” Time in to Reach Rated Temperature ate at maximum rated temperature rise. Fig. 9: Time Required for Typical Resistors to Reach Rated Operating Temperatures at The exact temperature rise, of course, varies with each resis- Various Watt Loads. tor, depending on size, ohms winding, etc. The curves shown indicate the approximate rise for typical units only, as a band or range of values actually exists for each percent load. Ratings at over 1000% are not recommended except for Powr-Rib® resistors. Curves for intermediate size resistors can be roughly estimated by comparison with the sizes given. Ratings for single pulses in the milli- range (and up to 1 to 2 seconds) require individual calculation. This is because the ratings vary greatly with the resistance, or more specifically with the actual weight and specific heat of the resistance alloy used. Calculation is based on the assumption that all of the heat generated in the pulse goes to raise the temperature of the resistance wire.

Duty Cycle in Percent 025210 15 045430 35 0 5 50 1400 375 Powr-rib (low resistance) 1200 350

325 1000 attage Rating 0.125 x 8.5" Corrib 300 (28.575 x 215.90 mm) 800 275 Powr-rib (high resistance) 250 600 225 400 200

175 Percent of Continuous-duty Current Rating Percent of Continuous-duty W 150 200 125 100 100 seconds on: 5110 5115 5 15 seconds off: 75 70 75 45 30 15 NEMA Duty Cycles

Fig. 8: Percent of Continuous Duty Rating for Resistors for Typical NEMA Duty Cycles.

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100 100 70 70 50 Small Vitreous Enamel Resistors 50 Corrib Resistors 40 40 10 watt 300 watt 30 30 50 watt 1500 watt 20 20 200% 125% 150% 100 100 200% 70 70 50 50 500% 40 40 30 500% 30

ime in Seconds 20 20 1000% 1000% 10 10 me in Seconds “On” Ti “On” T 7 7 5 5 4 4 3 3 2 2

1 1 113 5 0 30 50 100 300 500 1K 3K 5K 10K 113 5 0 30 50 100 300 500 1K 3K 5K 10K 247 20 40 70 200 400 700 2K 4K 7K 247 20 40 70 200 400 700 2K 4K 7K “Off” Time in Seconds “Off” Time in Seconds Fig. 10: 10 Percent of Continuous Duty Rating for Pulse Operation of small to Medium Fig.12: Percent of Continuous Duty Rating for Pulse Operation of CORRIB®, Corrugated Size Vitreous Enameled Resistors. Ribbon Resistors.

100 100 70 70 50 Large Vitreous Enamel Resistors 50 Powr-ribs 40 40 160 watt shown round wire 30 125% 30 150% edgewound 20 20 edgewound 200% 200%

100 100 300% 70 70 50 500% 50 500% 40 40 200% 30 30 800% 20 20 1000% 1000% 300% 500% 10 10 me in Seconds “On” Ti me in Seconds “On” Ti 800% 7 7 1000% 5 5 4 4 3 3 2 2 round wire 1 1 113 5 0 30 50 100 300 500 1K 3K 5K 10K 113 5 0 30 50 100 300 500 1K 3K 5K 10K 247 20 40 70 200 400 700 2K 4K 7K 247 20 40 70 200 400 700 2K 4K 7K “Off” Time in Seconds “Off” Time in Seconds

Fig. 11: Percent of Continuous Duty Rating for Pulse Operation of Large Vitreous Fig. 13: Percent of Continuous Duty Rating for Pulse Operation of Powr-Rib®, Bare Enameled Resistors. Resistors

400 Cooling Air 350 Resistors can be operated at higher than rated wattage when cooled by forced circulation of air. A typical curve is illustrated

atts 300 in Fig 14. The curve tends to level off at higher as excessive hot spots develop where the air flow does not reach all parts uniformly. 250 Limited Temperature Rise 200

When it is desired to operate a resistor at less than maximum Percent Rated W temperature rise, the percent watts for a given rise can be 150 read from “Temperature Rise vs. Resistor Load” Fig 2 graph on page 91. 100 0 500 1000 1500 Air , ft./min. Fig. 14: Percent of Free Air Rating for Typical Resistor Cooled by Forced Air Circulation.

200 1-866-9-OHMITE • Int’l 1-847-258-0300 • Fax 1-847-574-7522 • www.ohmite.com • [email protected] Resistor Selection Application Notes Short-Cut Chart Method To Find Required Size (as affected by application conditions) 1. For each Condition, locate the relevant value on the scales below 2. Multiply the Factors together. and record the corresponding Factor (F1 to F7). Note: The Standard 3. Multiply the Watts by the product obtained from 2 above. Free Air Condition Factor is always 1.

Watts Application Conditions

Ambient Pulse Limited Temperature Enclosure Grouping Altitude Operation Cooling Air Temp. Rise

°CF1 %F2 no. F3 ft. F4 %F5 fpm F6 °CF7 300 6.6 100 2.0 100 1000 0.10 1500 0.27 40 50 13.0 5.0 3 1.4 1400 10.0 2 1.3 0.28 90 1.9 90 4.1 std. brackets 900 0.11 8.0 1.5 1300 0.29 7.0 3.2 12 1.6 0.12 80 1.8 80 1200 6.0 800 0.30 100 8 1.5 5.0 2.7 0.13 1100 0.32 70 1.7 70 1.5 0.14 4.0 0.5" space 4 1.4 700 1000 200 2.2 2 1.3 0.15 60 1.6 60 900 0.16 0.35 3.0 1.9 600 Record the 12 1.5 0.17 800 1.4 0.18 watts to be 50 1.5 50 2.5 dissipated as 8 1.4 0.19 700 0.40 1.6 500 0.20

set by your 1" space 2.0

40 1.4 40 percent load 600 circuit 4 1.3 1.3 200 1.4 thousands of feet 1.75 conditions. 2 1.2 400 0.25 500

air velocity: feet per 0.50 30 1.3 30 100 1.3 12 1.4 0.30 400 1.5 1.2 1.4 300 0.60 20 1.2 20 0.35 300 1.3 1.2 8 1.3 0.40 1.2 200 0.70 Standard 2" space 1.1 4 1.2 200 0.50 free air 50 1.1 10 1.1 10 0.80 1.1 2 1.1 0.60 100 5 1.0 0.90 conditions 0.75 1.0 25 1.0 None 1.0 1 1.0 0 100 1.0 Still 1.0 300 1.0

watts to be factor factor factor factor factor factor factor dissipated Temperature at Factors apply Factors apply to Factors apply to Percent load for Factors are Low temperatures installation includes approximately for uniformly spaced altitudes show. No pulse operation approximations may be desired room temperature average sheet banks of derating is required must first be only. Effectiveness because of plus temperature metal boxes of resistors with for altitudes to determined from of cooling varies adjacent rise due to adjacent dimensions such spacing as shown. 5000 ft. above sea graphs and data on with installation. apparatus, heat sources. that watts per sq. level. page xx. increased stability in. of surface are in or maximum the range of 0.2 to reliability. 0.4.

Example Four resistors, each dissipating 115 watts, are to be Operation (1) On Ambient Temperature scale locate 50°C. Note and mounted in a group. Spacing is to be 2” surface to sur- record F1 = 1.1 as shown. Locate and record the other factors. face. Ambient to be 50°C (122°F). Enclosure to be total. F1 F2 F3 F4 F5 F6 F7 Other factors standard. Determine Watt Size required. 50° 100% 4@2” Standard Conditions 1.1 x 2.0 x 1.2 x 1 x 1 x 1 1 Operation (2) Multiply the factors together = 2.64 Operation (3) 115 Watts x 2.64 = 304 Watts Free Air Watt Size Rating required for each resistor.

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The resistance alloys used for all except the lowest ohmic Both of these wire classes are rated by the wire manu- values show such little change with temperature that in most facturers as having a TCR of 0±20ppm/°C. The expression power circuits the resistance is considered constant. Actually “0±20ppm/°C” implies that, although the nominal value of the there may be changes at full load of -4% to +8% of the initial TCR is zero, the actual value may lie anywhere within the toler- resistance. The change is attributed in most part to the “tem- ance range of –20ppm/°C to +20ppm/°C. perature coefficient of resistance” (TCR) which is the change For other resistance such as the widely used nickel- in resistance expressed as “parts per million per centi- -iron, for example, a nominal value of +140ppm/°C is grade of temperature” (ppm/°C). given. Actually, however, a tolerance of ±30ppm is applicable For special applications which require very constant resis- so that the TCR may range between the limits of +110 to tance, it may be necessary to specify the maximum permissible +170ppm/°C. TCR for the range of temperature involved. This would limit Unfortunately, the TCR of a completed power resistor is the choice of wire to only certain types of resistance alloys. generally somewhat different from that of the original wire. The commonly known low TCR alloys in the 800 ohms per This is because the TCR may be affected by such factors as circular-mil-foot class consist largely of nickel and chromium heat treatment during processing, and materials and methods alloyed with small amounts of aluminum and either copper or of construction. Without special controls and precautions, the iron. Other low resistivity alloys, 294 ohms per circular-mil-foot, TCR over the range of 25°C to 300°C rise may increase to as consist primarily of nickel and copper with only traces of other much as metals. 0±80ppm from the original 0±20ppm for certain types of Temperature Attained, °C wire on vitreous enameled resistors. Theoretical changes in -50 0 +25 +100 +200 +300 resistance with temperature are shown in Fig. 15. +8% The circuit designer should carefully consider the actual 90 ohms +7% per C.M.F. wire needs of the circuit before specifying limits on the TCR of a +380 PPM/ϒC desired resistor. Wherever possible it is best to select a resis- +6% tor for a critical application so that it operates at a low tem- +5% perature rise. This will also provide the maximum stability over 675 ohms a long period. For low TCR (and other) applications, Ohmite +4% per C.M.F. wire +140 PPM/ϒC can provide resistors with an “Ohmicone” (silicone-) +3% coating. “Ohmicone” is processed at much lower temperatures than vitreous enamel and therefore makes control of TCR and +2% tolerance easier. Data on the TCR and other properties of vari- +1% ous alloys is given on page 98. Range for 0 294 or 800 ohms per C.M.F. wire Calculated Change in Resistance ± ϒ -1% 20 PPM/ C

-2% Ambient Temp. -3% -100 0 +100 +200 +300 Temperature Rise, °C Fig. 15: Calculated change in resistance with nominal TC assumed constant.

202 1-866-9-OHMITE • Int’l 1-847-258-0300 • Fax 1-847-574-7522 • www.ohmite.com • [email protected] Resistor Selection Application Notes Resistance Alloys and Uses A number of different resistance alloys are used in winding Other Alloys resistors and rheostats as shown in Fig. 16. The general use In to the alloys tabulated which show small changes for each alloy is indicated by the column headed, “Resistance in resistance with temperature, there are others which some- Range for Which Used.” Whether a particular alloy can be used times have to be used for very low resistance units. These on a specific resistor can be estimated by dividing the given alloys have higher temperature coefficients, which limit their resistance by the area of the given winding space and deter- use to applications where the change in resistance with load mining whether the quotient falls within the limits given hereaf- is not important. An example is No. 60 alloy, which has a ter. The “high resistance” alloys cover the range from approxi- resistance of 60 ohms per circular-mil-foot and a temperature mately 10 to 25,000 ohms per square inch of winding area, the coefficient of +700ppm/°C. “low to medium” type from 5 to 400 ohms and the “very low Ballast Wire resistance” alloys from less than an ohm to 250 ohms. It should There are other alloys which are selected especially for be noted that the “Ohms per Square Inch” ranges overlap their high temperature coefficient of resistance. These are considerably, indicating that in many instances a given resis- used for so-called “ballast” resistors where a large change in tor could use any of several alloys. Both the upper and lower resistance is desired with a change in load. A typical ballast limits of the ranges are only approximate and in general can be wire is Nickel, which has 58 ohms/cmf and a temperature coef- extended somewhat when necessary. ficient of +4800ppm/°C. Others are “Hytemco” and “Balco” at The actual temperature coefficient of a complete resistor 120 ohms/CMF and a TC of +4500pp /°C. is generally greater than the nominal for the wire alone. The approximate change in overall resistance at full load is shown in the table. ASTM Ohms Mean Temp Temperature Resistance Average Resis- Alloy Alloy Composition per Trade Coeff. of Res. Range for Range for tance Change Class* (Approximate) CMF Names ppm/°C TCR °C Which Used at Full Load** Nickel base, Evanohm 1a non-magnetic 800 Karma 0 ± 20 -65 to + 250 Very high, Medi Under ± 1% Ni 75%, Cr 20% Moleculoy um and up, for to ± 2% 1b plus Al, Cu, Fe, etc. 800 Nikrothal L 0 ± 10 -65 to + 150 low temp. coeff. Iron base, magnetic Alloy 815-R 2a 800 0 ± 20 -65 to + 200 Alternate Under ± 1% Fe 73%, Cr 22.5%, Kanthall Dr sometimes to ± 2% Al 4.5% (plus Co Mesaloy 2b in one alloy) 800 0 ± 10 0 to + 150 for Class 1 Chromel A 3a 650 Nichrome V + 80 ± 20 Nickel-Chromium Nikrothal B -65 to + 250 High and + 4 to + 5% 80% — 20% Protoloy A + 60 ± 20 medium 3b 675 Tophet C Chromel C Electroloy 4 Nickel-Chromium-Iron 675 Nichrome + 140 ± 30 -65 to + 200 High and + 5 to + 8% 60%—16%—24% Nikrothal 6 medium Tophet C Advance 5a Copel 0 ± 20 Low and low Under Copper-Nickel 300 Cupron -65 to + 150 to medium for ± 1% to ± 2% 5b 55% — 45% Cuprothal 294 0 ± 40 low temp. coeff. Neutroloy Low and low Under 6 290 Manganin 0 ± 15 + 15 to + 35 to medium for ± 1% 13% Mn, 87% Cu low TC near 25°C to ± 2%**

Copper-Nickel 180 Alloy 7 77% — 23% 180 Cuprothal 180 + 180 ± 30 -65 to + 150 Very low + 5% to + 8% Midohm

Copper-Nickel 90 Alloy 9 90% — 10% 90 95 Alloy + 450 ± 50 -65 to + 150 Very low + 5% to + 10% Cuprothal 90 *American Society for Testing Materials. Tentative Specification B267-68. **For resistor with 300°C hot spot rise from 25°C ambient except 54°C rise for Manganin. Fig. 16: Table of Resistance Alloys Generally Used for Resistors and Rheostats. 1-866-9-OHMITE • Int’l 1-847-258-0300 • Fax 1-847-574-7522 • www.ohmite.com • [email protected] 203 Resistance Values Preferred Standard Resistance Values

The resistance values listed below and their decimal multiples is represented. The omission of a resistance value does not have been designated as standard by the International Elec- necessarily mean that Ohmite cannot manufacture the desired trotechnical Commission (IEC). This listing ensures that every value. possible resistance value within its respective tolerance range Please contact Ohmite at 866-964-6483 or sales@ohmite. com for resistance values not shown in this table. 1% Tol. 5% Tol. 10% Tol. 20% Tol. 1% Tol. 5% Tol. 10% Tol. 20% Tol. 1% Tol. 5% Tol. 10% Tol. 20% Tol. E96 Values E24 Values E12 Values E6 Values E96 Values E24 Values E12 Values E6 Values E96 Values E24 Values E12 Values E6 Values (Plus 250Ω (Plus 25Ω (Plus 25Ω (Plus 25Ω (Plus 250Ω (Plus 25Ω (Plus 25Ω (Plus 25Ω (Plus 250Ω (Plus 25Ω (Plus 25Ω (Plus 25Ω and 500Ω) and 50Ω) and 50Ω) and 50Ω) and 500Ω) and 50Ω) and 50Ω) and 50Ω) and 500Ω) and 50Ω) and 50Ω) and 50Ω) 100 10 10 10 255 523 102 261 536 105 267 549 107 27 27 56 56 110 11 274 562 113 280 576 115 287 590 118 294 604 12 12 30 619 121 301 62 124 309 634 127 316 649 130 13 324 665 133 33 33 33 68 68 68 137 332 681 140 340 698 143 348 715 147 357 732 150 15 15 15 36 750 75 154 365 768 158 374 787 16 383 806 162 39 39 82 82 165 392 825 169 402 845 174 412 866 178 422 887 18 18 43 909 182 432 91 187 442 931 191 453 953 196 464 976 200 20 47 47 47 205 475 210 487 215 499 22 22 22 500 50 50 50 221 51 226 511 232 237 24 243 249 250 25 25 25

Ohm’s Law

P = Watts I = Amperes Ohm’s Law defines the relationships between (P) power, (V) voltage, (I) cur- 2 rent, and (R) resistance. One ohm is the resistance value through which one Watts = Volts Amperes = Volts Ohms Ohms will maintain a current of one . Watts = Amperes2 s Ohms Amperes = Watts Volts Watts = Volts s Amperes I Current is what flows on a wire or R Resistance determines how Amperes = Watts conductor like flowing down a much current will flow through a V•I V/R Ohms river. Current flows from negative to component. Resistors are used to 2• P/V I R positive on the surface of a conductor. control voltage and current levels. A 2 P I Current is measured in (A) amperes very high resistance allows a small V /R Watts Amps √P/R or amps. amount of current to flow. A very low √P•R V R P/I2 resistance allows a large amount of Volts Ohms V Voltage is the difference in electri- current to flow. Resistance is mea- P/I V2/P cal potential between two points in sured in ohms. I•R V/I a circuit. It’s the push or pressure V = Volts R = Ohms behind current flow through a circuit, P Power is the amount of current Volts = Watts s Ohms Ohms = Volts and is measured in (V) volts. times the voltage level at a given Watts Amperes Volts = 2 point measured in wattage or watts. Amperes Ohms = Volts Watts Volts = Amperes s Ohms Ohms = Watts Amperes2

204 1-866-9-OHMITE • Int’l 1-847-258-0300 • Fax 1-847-574-7522 • www.ohmite.com • [email protected] Resistor Terminology

Adjustable Resistor: A resistor so constructed Current-limiting Resistor: A resistor inserted in the portion under consideration. The unit that its resistance can be readily changed.* into an electric circuit to limit the flow of current of impedance is the ohm and is represented : A periodic current the to some predetermined value. Note: A current- by Z. average value of which over a period is zero. limiting resistor, usually in series with a or Intermittent Duty: A requirement of service The equation for alternating current is the same circuit breaker, may be employed to limit the that demands operation for alternate intervals flow of circuit or system at the time of a as that for a periodic current except that I0=O* of (1) load and no-load; or (2) load and rest; or fault or short-circuit.* Ambient Temperature: The temperature of (3) load, no-load and rest; such alternate inter- Dielectric Strength: the surrounding coiling medium, such as gas The dielectric strength of vals being definitely specified.* or liquid, which comes into contact with heated an insulating material is the maximum potential Intermittent-Duty Resistor: A resistor capable parts of the apparatus.* gradient that the material can withstand without of carrying for a short period of time the high rupture.* It is usually specified in volts per unit Ampere: The unit of constant current which, overload current for which it is designed without thickness. maintained in two parallel rectilinear conduc- exceeding the specified temperature rise. Dielectric Test: tors of infinite length separated by a distance of A test which consists of the -Duty Resistor: A resistor for use in one meter, produces between these conductors application of a voltage higher than the rated the armature or rotor circuit of a motor in which a equal to 2x10-7 mks (meter-- voltage for a specified time for the purpose of the armature current is almost constant. second) units of force per meter of length. determining the adequacy against breakdown Mega Ohm: A unit of resistance equal to one of insulating materials and spacings under nor- Armature Resistor: A resistor connected in million ohms. mal conditions.* series with the armature of a motor either to MIL Resistor: : A resistor built in accordance limit the on starting, the gradual A unidirectional current in with Joint Army-Navy specifications. short circuiting of which brings the motor to which the changes in value are either zero or Multi-Section Resistor: so small that they may be neglected. A given A resistor having two normal , or to regulate the speed by or more electrically independent sections. armature-voltage control. current would be considered a direct current in † some applications, but would not necessarily NEC: The National Electrical Code is the stan- Axiohm : Centohm® Coated axial terminal be so considered in other applications.* dard of the National Board of Fire Underwriters wirewound resistor. ®† Dividohm : A resistor with a bare side and for electric wiring and apparatus as recom- Bracket Terminal Resistor: A resistor mended by the National Fire Protection equipped with slotted metal end j brackets that clamp for adjustment. Edgeohm†: Association and approved by the American serve as a means of mounting and connecting A high-current resistor made of an Standards Association. to the resistor. alloy resistance ribbon wound on edge form- NEDA: ing an oval-shaped coil supported by grooved National Electronic Distributors : That property of a system of Association. conductors and dielectrics which permits the insulators which space adjacent turns and insulate them from the support bars. Support NEMA: The National Electrical Manufacturers storage of when potential differ- Association, a non-profit trade association, ences exist between the conductors. Its value is bars are secured to steel end pieces forming a sturdy resistor suitable for continuous-and- supported by the manufacturers of electrical expressed as the ratio of a quantity of electricity apparatus and supplies. NEMA is engaged to a potential difference. A capacitance value is intermittent-duty applications. EIA: in standardization to facilitate understanding always positive.* Electronic Industries Alliance. : between the manufacturers and users of elec- : A device, the primary purpose of The electromotive force trical products. is the agency causing the flow of current in which is to introduce capacitance into an elec- Nominal Diameter: As applied to tubular resis- tric circuit. Capacitors are usually classified, a circuit. It is the electrical pressure (or drop) measured in volts. tors, this is the diameter of the ceramic tube according to their dielectrics, as air capacitors, expressed in inches and/or fractions thereof. mica capacitors, paper capacitors, etc.* : The unit of capacitance of an electric Nominal Length: As applied to tubular resis- Clearance: The shortest distance through condenser in which a charge of one produces a difference of potential of one volt tors, this is the length of the resistor base space between two live parts, between live or core expressed in inches and/or fractions parts and supports or other objects, or between between the poles of the capacitor. Ferrule Resistor: thereof. any live part and grounded part. A resistor supplied with fer- Non-Inductive Resistor: Conduction: rule terminals for mounting in standard fuse A non-inductive The transmission of heat or elec- power resistor is one in which the tricity through, or by means of, a conductor. clips. Field Discharge : and distributed capacitance are reduced to an Conductor: A body so constructed from con- A switch usually of absolute minimum. the knife blade type having auxiliary contacts ducting material that it may be used as a carrier Ohm: A unit of resistance defined as the resis- of .* for connecting the field of a generator or motor across a resistor (field discharge) at the instant tance at 0°C of a column of mercury of uniform Continuous Duty: A requirement of service cross-section having a length of 106.3 centime- that demands operation at a substantially con- preceding the opening of the switch. Fixed Resistor: ters and a of 14.4 grams. stant load for un indefinitely long time.* A resistor designed to intro- : duce only one set amount of resistance into an An instrument for measuring elec- Continuous-Duty Resistor: A resistor that is tric resistance that is provided with a scale capable of carrying continuously the current electrical circuit. : graduated in ohms. for which it is designed without exceeding the The unit of inductance of a closed cir- Periodic Duty: cuit in which an electromotive force of one volt A type of intermittent duty in specified temperature rise. which the load conditions are regularly recur- Continuous Rating: Continuous rating is the is produced when the electric current traversing the circuit varies uniformly at the rate of one rent.* rating that defines the load which can be car- Periodic Rating: The rating which defines ried for an indefinitely long time.* ampere per second. Hot Spot: The point or location of maximum the load which can be carried for the alternate Convection: Convection is the motion resulting periods of load and rest specified in the rating, in a fluid owing to differences of density and the temperature on the external surface of a resis- tor. the apparatus starting cold and for the total time of . specified in the rating without causing any of ®† Inductance: The (scalar) property of an elec- Corrib : A tubular resistor consisting of an the specified limitations to be exceeded.* tric circuit or of two neighboring circuits which alloy resistance ribbon, crimped and edge- Power: determines the electromotive force induced in The time rate of transferring or trans- wound on a ceramic core, the ribbon being one of the circuits by a change of current in forming energy; the rate of doing or securely and permanently fastened to the core either of them.* expending energy. by vitreous enamel or cement. Power Resistor: Impedance: The apparent resistance of an A resistor capable of dissipat- Creepage Distance: The shortest distance AC circuit, being the combination of both the ing 5 watts or more. between conductors of opposite polarity or resistance and reactance. It is equal to the ratio Rating: A designated limit of operating char- between a live part and ground as measured of the value of the EMF between the terminals acteristics of a machine, apparatus or device, over the surface of the supporting material. to the current, there being no source of power based on definite conditions.

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Note 1: Such operating characteristics as load, Resistive Conductor: A resistive conductor is Still Air: Still air is considered air having no voltage, frequency, etc., may be given in the a conductor used primarily because it possess- circulation except that created by the heat of rating. es the property of high electric resistance.* the resistor which is being operated. Note 2: The rating of control apparatus in gener- Resistivity: The resistivity of a material is the Tapped Resistor: A resistor with two or more al is expressed in volts, amperes, resistance of a sample of the material having steps. or kilowatts as may be appropriate, except specified dimensions. Temperature Coefficient of Resistance: A that resistors are rated in ohms, amperes and Resistor: A device, the primary purpose of measure of the increase or decrease in resis- class of service.* which is to introduce resistance into an electric tance of a resistive conductor due to change in Reactor: A device used for introducing reac- circuit.* temperature in parts per million (ppm). tance into a circuit for purposes such as motor Resistor Core: The resistor core or base of RT = Rr + [Rr(αT – αTr)] starting, paralleling transformers and control of a power resistor is the insulating support on Where, current.* which the resistive conductor is wound. : A device which converts alternating Rheostat: RT = Resistance of conductor at temperature An adjustable resistor so construct- T current to unidirectional current by virtue of a ed that its resistance may be changed without characteristic permitting appreciable flow of opening the circuit in which it may be con- Rr = Resistance of conductor at reference current in only one direction.* nected.* temperature Tr Resistance: The (scalar) property of an electric Screw-Base Resistor: A power-type resistor α = Temperature coefficient of resistance at circuit or of any body which may be used as equipped with Edison-type screw-base termi- reference temperature Tr part of an electric circuit which determines for a nals for quick interchangeability. Temperature Rise: Temperature rise is the dif- given current the rate at which electric energy Short-Time Rating: The rating that defines the ference in temperature between the initial and is converted into heat or and load which can be carried for a short and defi- final temperature of a resistor. Temperature rise which has a value such that the product of the nitely specified time, the machine, apparatus or is expressed in degrees C or F, usually referred resistance and the square of the current gives device being at approximately room tempera- to an ambient temperature. Temperature rise the rate of conversion of energy. In the general ture at the time the load is applied.* equals the hot spot temperature minus the case, resistance is a function of the current, but ambient temperature. Silicone: A silicone coating meeting MIL-R-26 the term is most commonly used in connection Thermal Shock: used on power type wirewound resistors. Thermal shock consists of a with circuits where the resistance is indepen- sudden marked change in the temperature of Slim Mox A flat style resistor Ohmite manufac- dent of the current.* the medium in which the device operates. tures. They are available in a variety of sizes Resistance Tolerance: The resistance tol- Thermocouple: and values. A device for converting heat erance of a power resistor is the extent to energy into electrical energy consisting of a Single-Wound Resistor: A resistor that has which its resistance may be permitted to devi- pair of dissimilar conductors so joined as to only one layer of resistance wire or ribbon ate above or below the specified resistance. produce a thermo-electric effect. It is used with wound around the insulating base or core. Resistance tolerance is usually expressed in a millivoltmeter to measure temperature rise in Stackohm®†: percent. A resistor consisting of a hollow apparatus. Resistance Method of Temperature ceramic core, oval in shape, about which resis- Thermometer Method of Temperature Determination: tance wire is wound and completely embedded This method consists in the Determination: This method consists in the in an insulating and heat conducting coating. determination of temperature by comparison of determination of the temperature by a mercury the resistance of the winding at the temperature or alcohol thermometer, by a resistance ther- to be determined with the resistance at a known mometer, or by a thermocouple, any of these temperature.** instruments being applied to the hottest part Resistance Values of the apparatus accessible to a mercury or alcohol thermometer.** Abbreviations and Part Numbering Structure Tolerance (%) The tolerance is the allowable Part deviation from teh nominal resistance value. Numbering Scientific Varying Duty: A requirement of service that Prefix Abbreviation Structure Numeric Value Description Notation demands operation at loads, and for intervals Milli m Thousandth R001 0.001 1 Milli Ohm 1.0 x 10-3 of time, both of which may be subject to wide variation.* Centi c Hundredth R010 0.01 1 Centi Ohm 1.0 x 10-2 Voltage (V or E): -1 The unit of measure is Deci dTenth R100 0.1 1 Deci Ohm 1.0 x 10 the volt. A unit of electrical pressure, EMF or ——One 1R00 11Ohm 1.0 x 100 potential difference. Ohmite’s voltage rating is Deca, Deka da Ten 10R0 10 1 Deca Ohm 1.0 x 101 the voltage that can be applied to the resistor without arcing or degrading the resistor. Hecto h Hundred 1000* 100 1 Hecto Ohm 1.0 x 102 Voltage Coefficient (VCR) The unit of mea- 1001* 1,000 1 Kilo Ohm 1.0 x 103 sure is in parts per million (ppm). Voltage coef- Kilo k Thousand 1002* 10,000 10 Kilo Ohms 1.0 x 104 ficient defines the change in the value of the 5 1003* 100,000 100 Kilo Ohms 1.0 x 10 resistor that occurs as the voltage changes. 1004* 1,000,000 1 Mega Ohm 1.0 x 106 The resistor is measured at two and 1504* 1,500,000 1.5 Mega Ohms 1.5 x 106 the deviation is then calculated. VCR is usually Mega M Million 1005* 10,000,000 10 Mega Ohms 1.0 x 107 states as the change per volt (ex. 2ppm/v). 1006* 100,000,000 100 Mega Ohms 1.0 x 108 Watt: 1506* 150,000,000 150 Mega Ohms 1.5 x 108 A unit of . It is the power expended when one ampere of direct current 1007 1,000,000,000 1 Giga Ohm 1.0 x 109 flows through a resistor of one ohm. 1507 1,500,000,000 1.5 Giga Ohms 1.5 x 109 Winding Pitch: Giga G Billion 1008 10,000,000,000 10 Giga Ohms 1.0 x 1010 The distance from any point 1009 100,000,000,000 100 Giga Ohms 1.0 x 1011 on a turn of a resistive conductor to the cor- 1509 150,000,000,000 150 Giga Ohms 1.5 x 1011 responding point on an adjacent turn measured parallel to the long axis of the winding. 100A 1,000,000,000,000 1Tera Ohm 1.0 x 1012 Tera TTrillion 150A 1,500,000,000,000 1.5 Tera Ohms 1.5 x 1012 * ASA Standard 100B 10,000,000,000,000 10 Tera Ohms 1.0 x 1013 ** NEMA Standard *Part Numbering Structure may vary by product line † Ohmite trade name

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