Resistors, Capacitors, and Inductors 249Photocells are rated by specifying their resistance at
low and high light levels. These typically vary from
600 :–110 k: (bright), and from 100 k:–200 M:
(dark). Photocell power dissipation is between 0.005 W
and 0.75 W.
Thermistors. Thermistors, thermal-sensitive resistors,
may increase or decrease their resistance as tempera-
ture rises. If the coefficient of resistance is negative, the
resistance decreases as the temperature increases; if
positive, the resistance increases with an increase in
temperature. Thermistors are specified by how their
resistance changes for a 1°C change in temperature.
They are also rated by their resistance at 25°C and by
the ratio of resistance at 0°C and 50°C. Values vary
from 2.5:–1 M: at room temperature with power
ratings from 0.1–1 W.
Thermistors are normally used as temperature-sensing
devices or transducers. When used with a transistor, they
can be used to control transistor current with a change in
temperature. As the transistor heats up, the
emitter-to-collector current increases. If the power supply
voltage remains the same, the power dissipation in the
transistor increases until it destroys itself through thermal
runaway. The change in resistance due to temperature
change of the thermistor placed in the base circuit of a
transistor can be used to reduce base voltage and, there-
fore, reduce the transistor emitter to collector current. By
properly matching the temperature coefficients of the two
devices, the output current of the transistor can be held
fairly constant with temperature change.Varistors. Varistors (voltage-sensitive resistors) are
voltage-dependent, nonlinear resistors which have
symmetrical, sharp breakdown characteristics similar to
back-to-back Zener diodes. They are designed for tran-
sient suppression in electrical circuits. The transients
can result from the sudden release of previously stored
energy—i.e., electromagnetic pulse (EMP)—or from
extraneous sources beyond the control of the circuit
designer, such as lightning surges. Certain semiconduc-
tors are most susceptible to transients. For example, LSI
and VLSI circuits, which may have as many as 20,000
components in a 0.25 inch × 0.25 inch area, have
damage thresholds below 100μJ.
The varistor is mostly used to protect equipment
from power-line surges by limiting the peak voltage
across its terminals to a certain value. Above this
voltage, the resistance drops, which in turn tends to
reduce the terminal voltage. Voltage-variable resistors
or varistors are specified by power dissipation
(0.25 –1.5 W) and peak voltage (30–300 V).Thermocouples. While not truly a resistor, thermocou-
ples are used for temperature measurement. They
operate via the Seebeck Effect which states that two
dissimilar metals joined together at one end produce a
voltage at the open ends that varies as the temperature at
the junction varies. The voltage output increases as the
temperature increases. Thermocouples are rugged, accu-
rate, and have a wide temperature range. They don’t
require a exitation source and are highly responsive.
Thermoouples are tip sensitive so they measure the
temperature at a very small spot. Their output is very
small (tens to hundreds of microvolts, and is nonlinear,
requiring external linearization in the form of cold-junc-
tion compensation.
Never use copper wire to connect a thermocouple to
the measureing device as that constitutes another
thermocouple.Resistance Temperature Detectors. RTDs are very
accurate and stable. Most are made of platinum wire
wound around a small ceramic tube. They can be ther-
mally shocked by going from 100qC to 195 qC 50
times with a resulting error less than 0.02qC.
RTDs feature a low resistance-value change to
temperature (0.1 : 1 qC. RTDs can self heat, causingFigure 10-9. Various types of resistor networks.1
23
4
57 89101112131461
23
4
57 89101112131461234578910111213146