hat determine their accuracy during use. The
performance indices affect the accuracy to a greater or lesser extent depending on the application.
Some of these indices are: Tolerance at DC, Temperature Coefficient of Resistance (TCR),
Voltage Coefficient of Resistance (VCR), Noise, Stability with respect to Time and Load, Power
Rating, Physical Size, and Mounting Characteristics. Resistor networks typically require
temperature and voltage tracking performance.

Please refer to:
www.ohmcraft.com/pdf/rpdictionary.pdf
for an expanded explanation of resistor
terminology.

Selection Requirements
_______________________________________________________________________________________________________________________________

1.
Determine the resistance in ohms and watts.
2.
Determine the proper physical case size as controlled by voltage, watts, mounting
conditions, and circuit design requirements.
3.
Select the resistor that meets your needs for type, termination and mounting.


Step 1
_______________________________________________________________________________________________________________________________

Ohms Law

E=IR or I=E/R or R=E/I

Ohms Law, as shown in the above formula, enables one to define the voltage (E), current (I), or
resistance (R) when two of the three terms are known. When current and voltage are unknown
they must be measured in the model circuit.

Power Law

W=I
2
R or W=EI or W=E
2
/R


Watts (power) can be determined from the above formulas that are derived from Ohms Law. R is
measured in Ohms, E in volts, I in amperes, and W in watts.

Contact us:
sales@ohmcraft.com
585-624-2610
Contact us:
sales@ohmcraft.com
585-624-2610
2
Watts must be accurately determined before resistor selection. Simply stated any change in
voltage or current produces a much larger change in wattage (heat dissipated by the resistor). The
effects of relatively small increases in voltage or current must be determined because the increase
in wattage may be significant enough to influence resistor selection. As stated in the above
formulas the wattage varies as the square of the current or voltage. Allowances should be made
for maximum possible voltage.


Step 2
_______________________________________________________________________________________________________________________________

Power Rating and Physical Size

A resistor operated at a constant wattage will reach a steady temperature that is determined
largely upon the ratio between the substrate size (surface area) and the wattage dissipated.
Temperature stabilizes when the sum of the heat loss rates (by radiation, convection, and
conduction) equals heat input rate (wattage). The larger the resistor surface area per watt to be
dissipated, the greater the heat loss rate and therefore the lower the temperature rise.

Free Air Wattage Rating (Maximum Power Rating) is defined as the wattage rating of resistors as
established under specified standard conditions. 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 such as thermal conductivity, ratio of length to width, heat-sink effects of mounting,
and other minor factors.

The precise temperature limits corresponding to 100% rated wattage are somewhat arbitrary and
serve primarily as design targets. 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 test (see Application Note: Life Test Data High Voltage Chip Resistors) based on
performance and stability standards rather than the measurement of hot spot temperature.


Step 3
_______________________________________________________________________________________________________________________________


Resistor Selection

Select the most suitable resistor that meets the requirements of the application. OhmCraft
resistors are made to your specification. Refer to the appropriate data sheet to determine part
number or call OhmCraft for assistance.

Wattage Rating

To allow for the differences between actual operating conditions and the Free Air Wattage Rating
it is a general engineering practice to operate resistors at less than the nominal rating.

Voltage Rating

Determine maximum applied (working) voltage that the resistor will be exposed to and select the
appropriate package size.
3
Power Derating Characteristics
0
20
40
60
80
100
120
0
25
50
75
100
125
150
Ambient Temperature
% Rated Power
Rated Power
Pulse Operation

When a resistor is operated in a pulse application, the total power dissipated by the resistor is a
function of the pulses duty cycle. Typically, one will define the number of joules of energy the
resistor must dissipate and choose a resistor accordingly. For additional information refer to our
Pulse Resistor white paper or contact OhmCraft.

High Frequency

OhmCraft resistors, due to their design and construction, have very low capacitance and are
inherently a non-inductive design. For additional information refer to our High Frequency Attributes
Application Note.

Military and Other Specification

The special physical operating and test requirements of the applicable industrial or military
specification must be considered. Contact OhmCraft for additional information.


Effect of the power ratings on components
_______________________________________________________________________________________________________________________________

All the components of an electrical apparatus including resistors, capacitors, rectifiers, and semi-
conductors have their own limitations as to the maximum temperature at which they can reliably
operate. The attained temperature in operation is the sum of the ambient temperature plus the
temperature rise due to the heat dissipation in the equipment.


Ambient Temperature Derating, below defines the percent of full load that power resistors can
dissipate as a function of ambient temperature.







4
Temperature Coefficient of Resistance
_______________________________________________________________________________________________________________________________


Temperature Coefficient of Resistance (TCR) is expressed as the change in resistance in ppm
(0.0001%) with each degree of change in temperature Celsius (
°
C). MIL STD 202 Method 304 is
often referenced as a standard for measuring TCR. This change is not linear with temperature.
TCR is typically referenced at +25
°
C and changes as the temperature increases or decreases. It
can be either a bell or S shaped curve. It is treated as being linear unless very accurate
measurements are required, then a temperature correction chart is used. A resistor with a TCR of
100 ppm will change 0.1% over a 10-degree change and 1% over a 100-degree change.

The following formula expresses the rate of change in resistance value per 1
°
C in a prescribed
temperature range.

TCR (ppm/
°
C) = (R-R
0
)/R
0
X 1/(T-T
0
) X 10
6

R:
Measured
resistance
( ) at T
°
C R
0:
Measured resistance ( ) at T
0

°
C T: Measured test temperature
°
C T
0
: Measured test temperature
°
C

In the context of a resistor network, this TCR value is called absolute TCR in that it defines the
TCR of a specific resistor element. The term TCR tracking refers to the difference in TCR
between each specific resistor in the network.


Voltage Coefficient of Resistance
_______________________________________________________________________________________________________________________________


The Voltage Coefficient of Resistance is the change in resistance with applied voltage. This is
entirely different and in addition to the effects of self-heating when power is applied. A resistor with
a VCR of 100 ppm/V will change 0.1% over a 10 Volt change and 1% over a 100 Volt change.
VCR becomes very important in high Ohmic value resistor (100 Ohm and above) where typical
VCRs can be greater than 1000 ppm/
°
C to specify the voltage that will be applied. Failing to do
this may result in a resistor that will not meet your specification.

The rate of change in resistance value per 1 volt in the prescribed voltage range is expressed by
the following formula:

VCR (ppm/V) = (R
0
-R)/ R
0
X 1/(V
0
-V) X 10
6

R:
Measured
resistance
( ) at base voltage R
0:
Measured resistance ( ) at upper voltage V: Base voltage V
0
: Upper voltage

In the context of a resistor network, this VCR value is called the absolute VCR in that it defines the
VCR of a specific resistor element. The term VCR tracking refers to the difference in VCR
between each specific resistor network. Please refer to the application note: Voltage Ratio
Tracking and Voltage Coefficient of Resistance.
5

Summary
_______________________________________________________________________________________________________________________________


When specifying a resistor, the following parameters MAY be of interest. Please use this chart to
help you define the operating characteristics for your specific application. All of them may not
important for your specific application. Also, please do not hesitate to contact Ohmcraft f