Ohmite Resistor Selection Guide

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Application Notes

100

400

Percent Rated Watts

Percent Current

or Voltage

300

200

100

0

2000

Resistor Selection

R E S I S T O R F A C T S A N D F A C T O R S

A resistor is a device connected into an electrical circuit to
introduce a specified resistance. The resistance is measured in
ohms. As stated by Ohm’s Law, the current through the resistor
will be directly proportional to the voltage across it and inversely
proportional to the resistance.

The passage of current through the resistance produces heat.
The heat produces a rise in temperature of the resistor above
the ambient temperature. The physical ability of the resistor to

withstand, without deterioration, the temperature attained, limits
the operating temperature which can be permitted. Resistors are
rated to dissipate a given wattage without exceeding a specified
standard “hot spot” temperature and the physical size is made
large enough to accomplish this.

Deviations from the standard conditions (“Free Air Watt
Rating”) affect the temperature rise and therefore affect the watt­age at which the resistor may be used in a specific application.

S E L E C T I O N R E Q U I R E S 3 S T E P S

Simple short-cut graphs and charts in this catalog permit rapid
determination of electrical parameters. Calculation of each
parameter is also explained. To select a resistor for a specific
application, the following steps are recommended:

1 . (a) Determine the Resistance.

(b) Determine the Watts to be dissipated by the Resistor.

2 . Determine the proper “Watt Size” (physical size) as controlled

by watts, volts, permissible temperatures, mounting conditions and
circuit conditions.

3 . Choose the most suitable kind of unit, including type, terminals

and mounting.

S T E P 1 D E T E R M I N E R E S I S T A N C E A N D W A T T S

Ohm’s Law

(a) R = V or I = V or V = IR

Ohm’s Law, shown in formula form above, enables determina­tion of the resistance when the required voltage and current are
known. When the current and voltage are unknown, or the best
values not decided on, at least two of the three terms in Ohm’s
Law must be measured in a trial circuit.

(b) P = I2R or P = VI or P =

Power in watts, can be determined from the formulas above,
which stem from Ohm’s Law. R is measured in ohms, V in volts,
I in amperes and P in watts.

I R

2

V
R

Why Watts Must Be Accurately Known

Stated non-technically, any change in current or voltage pro­duces a much larger change in the wattage (heat to be dissi­pated by the resistor). Therefore, the effect of apparently small
increases in current or voltage must be investigated because
the increase in wattage may be large enough to be significant.
Mathematically, the wattage varies as the square of the cur­rent, or voltage, as stated in the formulas (b). For example, an
increase of 20% in current or voltage will increase the wattage
44%. Figure 1 below graphically illustrates the square law rela­tion. Hence, the actual current must be used in figuring the watt­age and the increase in wattage due to apparently small chang­es, then determined in order to select the proper size resistor.
Allowance should be made for maximum possible line voltage.

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Fig. 1: Rapid increase of wattage with current or voltage.

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S T E P 2 P O W E R R A T I N G O R P H Y S I C A L S I Z E O F R E S I S T O R

100

400

Temperature Rise above Ambient Temperature

Resistor Load — Percent Rated Watts

300

200

100

0

0 20 40 60 80 9

010 30 50 70

350

250

150

50

700

600

500

400

300

200

100

0

°F

°C

A

375°C 675°F

B 325°C 585°F

C 300°C 540°F

D 250°C 450°F

Bare Resistor—
NEMA;
Corrib & Powr-rib

U.L.-NEMA Std.
for Resistors

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

Mil-R-26: Char. U
EIA: Char. G

350

100

Percent Rated Watts

Ambient Temperature, °C

60

20

0

0 100 200 30050 150 250

80

40

U.L.-NEMA Std.
for Resistors

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

Mil-R-26: Char. U
EIA: Char. G

25°40°

275° 340°

A resistor operated at a constant wattage will attain a steady tempera­ture which is determined largely by the ratio between the size (surface
area) and the wattage dissipated, The temperature stabilizes when the
sum of the heat loss rates (by radiation, convection and conduction)
equals the heat input rate (proportional to wattage). The greater the
resistor area per watt to be dissipated, the greater the heat loss rate
and therefore the lower the temperature rise. The relation between the
losses varies for different resistors.

Free Air Watt Rating

The wattage rating of resistors, as established under specified stan­dard conditions, is defined as the “Free Air Rating” (“Full Rating” or
“Maximum Power Rating”). Several standard methods of rating are
in use based on different service conditions. The method of both
the “National Electrical Manufacturers Association” (NEMA) and the
“Underwriters’ Laboratories, Inc.” (UL) can be described as follows:

The relation of the “Free Air Watt Rating” of tubular type, vitre­ous enameled resistors to the physical size, is to be set at such a
figure that when operated at their rated watts, the temperature rise
of the hottest spot shall not exceed 300°C (540°F) as measured by a
thermocouple when the temperature of the surrounding air does not
exceed 40°C (104°F). The temperature is to be measured at the hot­test point of a two-terminal resistor suspended in free still air space
with at least one foot of clearance to the nearest object, and with
unrestricted circulation of air.

A slightly different definition of temperature limit used as a basis
for wattage rating, and which results in a slightly higher attained tem­perature, was originally established in military specification MIL-R-26
for wirewound resistors.

Characteristic V resistors are required to dissipate rated wattage in
an ambient of 25°C without exceeding a maximum operating tempera­ture of 350°C at the hottest spot. This corresponds to a temperature
rise of 325°C in a 25°C ambient. Although MIL-R-26 permits a 25°C
greater temperature rise than NEMA or UL, the reference ambient for
the latter two is 15°C higher. Consequently, the difference in attained
temperature between the two systems is only 10°C. The curves in Fig.
2 show the relation between temperature rise and wattage for various
specifications. Note the differences in the permissible rise for each
specification.

Fig. 2: Approximate hot spot temperature rise of a resistor in free air for various speci­fications.

The absolute temperature rise for a specific resistor is roughly
related to the area of its radiating surface. It is also dependent upon a
number of other factors, however, such as thermal conductivity of the
core and coating materials, emissivity factor of the outer surfaces, ratio
of length to diameter, heat-sink effect of mountings, and other minor
factors.

The maximum permissible operating temperature for a given resis­tor is basically determined by the temperature limitations imposed by

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Application Notes

Resistor Selection

the materials used in its construction. Generally speaking, these limits
cannot be sharply defined in terms of temperature alone. Other factors
such as resistance stability versus time, deterioration rates of insula­tion and moisture-resistance characteristics, type and size of resis­tance wire, all enter into consideration of “acceptable service life.”

For these reasons, the precise temperature limits corresponding to

100% rated wattage are somewhat arbitrary and serve primarily as
design targets. In the last analysis, once a wattage rating has been
assigned on the basis of an empirical hot spot limit, the verification of
its correctness must be established through long term load-life tests
based on performance and stability standards rather than the mea­surement of hot spot temperature. Maximum limits are stipulated for
parameter changes as a result of various tests, including a 2000 hour
load-life test.

It is also assumed that the temperature rise at a given wattage
is independent of the ambient temperature in which this wattage is
being dissipated. Therefore, for high ambient temperatures, the oper­ating wattage should be limited in accordance with the curves of Fig.

3. Although the assumption that temperature rise is independent of
ambient is not exactly true, the approximation is sufficiently close for
all practical purposes and, therefore, has been adopted for derating
purposes.

Fig. 3: Derating for ambient temperature.

Despite the above variables, figures may be cited in terms of
“watts dissipated per square inch of winding surface” for a given tem­perature rise. For power type resistors operating at 300°C rise above
ambient, this figure varies between approximately 6.3 watts per
square inch for large resistors (175 watt) to about 9 watts per square
inch for smaller resistors (12 watt). It should also be observed from
Fig. 2 that temperature rise is not directly proportional to wattage dis­sipated. Note, for example, that at 50% rated wattage, the temperature
rise still remains about 70% of that at full rating.

The wattage ratings used in this catalog, unless otherwise stated for
certain types, are on the basis of a nominal operating temperature of
350°C at full rating. There are two general categories of power resis­tors for which the 350°C nominal temperature limit does not apply. One
is that class of power-precision resistors where high stability is a salient
feature, in which case the operating temperature is nominally limited to
275°C. The other category includes all exposed ribbon wire resistors
(see description of Corrib® and Powr-Rib®) which are rated for 375°C
(675°F) maximum temperature rise when measured on the wire per
NEMA standards.

Temperature Distribution on a Resistor

The temperature rise varies (following a curve) along the length
of the resistor with the hot spot at the center-top (of a horizontal
tube) and the ends at approximately 60% of the maximum tempera­ture rise. The terminals themselves are still cooler. When the resistor
is vertical, the hot spot shifts upwards a little and the top end is hot­ter than the bottom. The standard “Free Air Watt Rating,” however, is
used regardless of position.

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Application Notes

Resistor Selection

S T E P S 3 S E L E C T A R E S I S T O R

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).

A P P L I C A T I O N W A T T R A T I N G

To allow for the differences between the actual service condi­tions and the “Free Air Watt Rating” it is a general engineering
practice to operate resistors at more or less than the nominal
rating. The details by which such ratings can be estimated are
given in the following pages. Most thermal calculations, however,
involve so many factors which are usually not accurately known,
that at best they are only approximations.

The most accurate method of determining or checking the
rating is to measure the temperature rise in a trial installation. A
thermocouple (made of #30 B & S gage wire) is recommended
for the measuring element. Even measurements made with a
thermocouple will vary slightly with different samples and tech­niques. The factors which affect the temperature rise act inde­pendently of each other and are summarized as follows:

1. Ambient Temperature

As the maximum permissible operating temperature is a set
amount, any increase in the ambient temperature subtracts from
the permissible temperature rise and therefore reduces the per­missible watt load.

2. Enclosure

Enclosure limits the removal of heat by convection currents in
the air and by radiation. The walls of the enclosure also intro­duce a thermal barrier between the air contacting the resistor
and the outside cooling air. Hence, size, shape, orientation,
amount of ventilating openings, wall thickness, material and fin­ish all affect the temperature rise of the enclosed resistor.

3. Grouping

When resistors are close to each other they will show an
increased hot spot temperature rise for a given wattage
because of the heat received by radiation from each other and
the increased heat per unit volume of air available for convec­tion cooling.

5. Pulse Operation

This is not an environmental condition but a circuit condition.
As a pulse of power, when averaged over the total on and off
time, results in less heat per unit time than for continuous duty,
the temperature rise is affected. This may permit higher power
during the pulses. The conditions must be expertly considered
for conservative rating. The open-wound “Powr-Rib®” resistor
construction is most suitable.

6. Cooling Air

Forced circulation of air over a resistor removes more heat per
unit time than natural convection does and therefore permits an
increased watt dissipation. Liquid cooling and special conduc­tion mountings also can increase the rating.

7. Limited Temperature Rise

It is sometimes desirable to operate a resistor at a fraction of
the Free Air Watt Rating in order to keep the temperature rise
low. This may be to protect adjacent heat sensitive apparatus,
to hold the resistance value very precisely both with changing
load and over long periods of time and to insure maximum life.

8. Other Considerations

High Resistance. High resistance units, which require the use of
very small diameter wire, generally should operate at reduced
temperature for maximum reliability.

High Voltage

A maximum voltage gradient of 500 volts R.M.S. (705 volts
peak) per inch of winding length is recommended under nor­mal conditions. For higher gradients in pulse applications or for
other special conditions such as oil immersion, consult factory.

High Frequency

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 den­sity, and therefore with the altitude above sea level. At altitudes
above 100,000 feet, the air is so rare that the resistor loses heat
practically only by radiation.

Military and Other Specifications

The special physical operating and test requirements of the
applicable industrial or military specification must be considered.
Military specification resistors should be ordered by their MIL
numbers.

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