Nuts & Volts

Constant Current Sources
Version (B) senses the voltage
drop across the connector with a
moveable probe you can easily make
yourself. This method requires no
moving parts. Typical currents you use
range from 1 mA to 100 mA; however,
be careful with upper end currents
causing heating problems if a poorly
aged connector has heavy surface film.
High test currents may actually break
down the film and destroy the resistance
value you are trying to measure.
72 April 2006
Temperature Measurement Through ΔR
You can use a CCS to measure
temperature by observing changes in
resistance. Apply a constant current
through a thermistor and monitor the
voltage across it via the meter terminal
on your CCS. Thermistors, classified
according to their resistance at 25°C,
are available in a wide range of resistances
and tolerances from 0.5 to 10 %.
Thermistors with 0.5% tolerances have
manufacturer supplied characteristic
Thermistor Defined
A thermistor is a semiconductor temperature
sensor with a repeatable change in electrical
resistance as a function of temperature.
Thermistors usually possess a negative
temperature coefficient, with resistance
decreasing as temperature rises.
curves or tables of their values. Figure 8
is a typical curve for a 3 kΩ thermistor.
(See Thermistor Defined sidebar.)
You must consider self-heating of
the thermistor when you apply current.
Most thermistor data sheets list the
dissipation constant (the amount of
power in milliwatts required to raise
the thermistor 1°C above the surrounding
temperature). Typical values vary
from 1 to 10 mW/°C. You should limit
the power dissipated in the thermistor
(I 2 R, where I is the applied current and
R is the resistance at the measurement
temperature) to one tenth of this value
for best results. Indirect temperature
measurements also use this method.
You might want to measure the
operating temperature of a winding of
a small shaded-pole motor to ensure
compliance with an Underwriter's
Laboratory safety specification.
First, measure the resistance of the
winding at room temperature. Next,
heat the motor by running it at full
load, stalled rotor, multiplied by the
number of hours of operation. Now
remove the power from the winding and
measure the change in resistance of the
winding at set intervals (typically five
seconds) until it has partially cooled.
A DVM with a "hold" control is
ideal here. Extrapolate this data to
yield the resistance of the winding at
the instant you remove the power.
Fourth, use the equation below to
determine the operating temperature:
T hot = 1/K (R hot /R cold ) + (KT cold - 1)
where T hot is the operating temperature
in °C, T cold is the room temperature
in °C, R hot is the resistance in
ohms at the operating temperature,
R cold is the resistance in ohms at room
temperature, and K is the temperature
coefficient of resistivity of the winding
at room temperature. For both copper
and aluminum (the two most commonly
used materials), K = 0.0039.
You can find values for other materials
in any engineering handbook. NV