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000126-UK NTC
TEST &MEASUREMENT
Measuring with an NTC
temperature measurement using a multimeter
By Bob Stuurman
It is often desirable to know how hot a part is.
The usual method is to use a thermometer
to measure the temperature, but this may
not always be practicable.
However, with the help of a small NTC and a
multimeter it is just as easy. During the devel-
opment of a speed controller with a final
stage consisting of MOSFETs, it was neces-
sary to measure the temperature of these
devices. A normal thermometer is unsuitable
for this purpose, so a search was made for a
sensor that could be secured to a MOSFET.
We found an NTC resistor, type 46720055,
in the Conrad Electronics catalogue that
appeared eminently suitable. The NTC has a
flange with a hole with which it can be
secured using a screw. This was exactly what
we needed. But how could the measured resis-
tance be easily converted to temperature?
It was hoped that a graph could
be generated showing the linear
relationship between temperature
and resistance. But this was not so,
the relationship appeared to be a
logarithmic one. The graph shown
here is the result. The function is a
straight line when plotted using a
logarithmic resistance scale. Not all
measured data points lie exactly on
a straight line. Also, differences
between individual NTCs can cause
variations. However, we may
assume that the error will be smaller
than 5%. The nominal value of the
NTC is 10 k
Ω
it becomes very simple to measure
the temperature of a part. Because the
NTC is small, it has very little thermal
inertia. It quickly settles to the tem-
perature of its environment and con-
tributes little heat. It is easy to attach
to a TO-220 MOSFET, an electric
motor, or something similar. The ther-
mal conductivity is very good because
the mounting flange is made from
brass. The NTC is small and light. As
a consequence, vibrations — such as
from an electric motor — have no
influence on the measurement.
The horizontal axis of the graph is
the measured resistance value.
Going straight up from this value
until you hit the curve allows you to
read the corresponding temperature
on the left.
Examples: 1000 Ω corresponds to
a temperature of 90 °C, 2000
Applications
corre-
sponds to a temperature of 70 °C.
(000126)
Ω
Using the NTC, a DVM with an appro-
priate resistance range and the graph
Graph
120
First, two flexible leads, each 30 cm long, were
soldered to the NTC terminals. Next, the NTC
was securely fastened to a normal glass ther-
mometer with a measuring range from 0 to
110 °C. This whole assembly was submerged
in an electric kettle filled with water. After
switching on, the measured resistance was
recorded at every 5 °C temperature rise, until
the water boiled. At this point the kettle was
switched off. While the water cooled down, the
resistance was recorded again at every 5 °C fall
in temperature. It was noticed that the tem-
perature indicated by the thermometer fell
steadily but that the resistance fluctuated.
Apparently, the NTC reacts faster to turbulence
in the water than to the thermometer does.
110
100
temperature
( ° C)
70
60
50
40
30
20
10
1
2
3 4 5 6 78910 2
2
3 4 5 6 78910 3
2
3
4 5 6
78910 4
resistance (
Ω
)
000126 - 11
Figure 1. The resistance characteristic of an NTC forms a straight line if we use a
logarithmic resistance scale.
11/2000
Elektor Electronics
59
at 25 °C. From the
graph, at 10 kΩ, we read a value of
approximately 23 °C. This is suffi-
ciently accurate for our purposes.
90
80
0
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