Glossar / Resistor User's Handbook
Resistors are probably the most common and well known of all
electrical components. Applying a resistor to a circuit normally requires (1)
calculating the nominal values needed in the circuit application (resistance,
power rating, etc.) then (2) developing acceptable tolerances for the resistor
that ensure it will function properly in all extremes of the application. The
first design consideration is relatively simple, typically based on straightforward
theory and linear calculations. The second design task can be more difficult
because resistors have several characteristics that alter their nominal resistance
value when used in a practical circuit. Here is a brief review of the important
considerations and specifications. Resistors are often designated as “precision”
or “power”. Precision resistors are designed for applications where
tight resistance tolerance and stability are primary considerations. They generally
have restricted operating temperature limits and power dissipation ratings.
Power resistors can also have tight tolerance and be quite stable, but their
design emphasis is to optimize power dissipation. Power resistors generally
have extended operating temperature limits.
- Resistor tolerance is the deviation from the nominal
value. It is expressed as a ±%, measured at 25°C with no
load applied. Some resistor designs have extremely tight tolerances.
For example, precision wirewound resistors are made with tolerances
as close as ±0.005%. Film resistors typically have tolerances
of ±1% to ±5%. In applications like precision voltage
dividers and networks, the designer should consider resistor sets matched
for resistance or ratio tolerances. Often, these matched sets save cost
over buying individual resistors with very tight resistance tolerances.
Temperature Coefficient of Resistance
- Temperature Coefficient of Resistance (TCR) specifies
the maximum change in resistance with change in temperature and is expressed
as “parts per million per degree Centigrade” (ppm/°C).
A wide range of TCRs are available to the designer (typically from ±1
ppm/°C to ±6700 ppm/°C) for specific applications.
Specifying TCR is important in applications where the change in resistance
with temperature changes must be small. Equally important may be applications
where a specific TCR is required (temperature compensation circuits
for example). Typically, there are two contributors to temperature-related
resistance changes; the resistor’s temperature increases as it
dissipates power and also, the resistor’s temperature is affected
by the ambient temperature.
Often matching TCRs for pairs or sets of resistors is more important
than the actual TCR itself. In these cases, matched sets are available
which assure that resistance values of the set track in the same magnitude
and direction as operating temperature changes.
- Power ratings are normally specified at +25°C
and must be reduced as the resistor’s temperature increases. A
derating chart is often used. Since these parameters are application
dependent, power derating curves or charts should be considered general
rather than absolute. Power ratings are based on many factors. The safest
designs use the largest physical size operating at conservative temperatures
and power ratings.
- Temperature rating is usually the maximum operating
temperature of the resistor. An operating temperature range is often
specified: for example, -55°C to +275°C.
Frequency Response and Rise Time
- Frequency response relates to the change in impedance
with frequency, caused by reactive components from the resistors inductance
and capacitance. Rise time is an associated parameter, relating the
resistors response to a step or pulse input. Wirewound designs use special
winding techniques to minimize reactive components. Typical reactive
values for these special designs are less than 1µh for a 500 ohm
resistor, and less than 0.8 pf capacitance for a 1 megohm resistor.
A typical fast rise time resistor has a rise time of 20 nsec or less.
- In Aryton-Perry windings, a layer is first wound
in one direction. After a layer of insulation, the next winding is wound
in the opposite direction with the turns crossing every 180 degrees.
- Stability is defined as the repeatability of resistance
of a resistor when measured at a referenced temperature and subjected
to a variety of operating and environmental conditions over time. Stability
is difficult to specify and measure since it is application dependent.
Experience with practical circuits has given us some guidelines; Wirewound
and bulk metal designs are best, while designs using composition materials
are less stable. For highest resistance stability, it is best to operate
critical resistors with limited temperature rise.
- Voltage coefficient is the change in resistance
with applied voltage. It is associated with carbon composition and carbon
film resistors, and is a function of the resistor’s value and
- Noise does not effect the resistor’s value,
but can generate circuit errors in high gain and sensitive circuits.
Wirewound and metal film resistors are best: carbon composition and
film have high noise potential.
- The thermocouple effect generates a thermal emf
at the junction of two dissimilar metals. In resistors, it is caused
by the materials used in leads and the resistive element. It is normally
insignificant, but may be important in high gain or critically balanced
circuits and low ohm resistors. Thermal EMF is minimized by keeping
the resistor leads and body at the same temperature.
- Reliability is the statistical probability that
a resistor will perform its function. Normally, it is specified as Failure
Rate per 1,000 Hours of Operation. Various statistical studies are used
at arriving at these failure rates by testing large samples. Reliability
is seldom defined for commercial products, but is a common requirement
for critical designs such as in aerospace applications.
Low Value Resistors for Shunt & Current Sensing
- Special low ohm power resistors are often used for
measurement shunts, and for current sensing applications. The value
of these resistors is low, often less than 0.1 ohm. Some special considerations
Lead material should be of good conductivity to prevent the lead resistance
from becoming a significant portion of the total resistance. Measurement
points should be specified for critical applications; a point 3/8"
from the end of the resistor body is universally accepted.
Four-Terminal (Kelvin) Connections
- Four terminal leads are often specified for low
ohm current sensing applications where lead resistance is a significant
factor in total resistance. The Kelvin connection removes the voltage
drop error in the current leads, since the sensing leads are attached
at a fixed point and carry no large current.