Applications Note AN36
Issue1 - JULY 2002
NTC Thermistor
The ZXBM200x series are designed to be
used in conjunction with an NTC type
thermistor with a value of 100k at 25 C. The
thermistor is connected directly between the
SPD pin and Ground. The thermistor can
either be local i.e. part of the Fan or Blower
itself and positioned where it can easily
detect the air temperature, or it can be remote
to the device, perhaps positioned on a
motherboard or in an air duct.
An NTC thermistor has a non-linear
characteristic whereby there is a larger
change in resistance between 25 C and 30 C
than there is between 55 C and 60 C. See
Figure 3. However from Ohms Law when two
resistors are in parallel and one changes in a
linear fashion it will produce a resultant
non-linear resistance characteristic but with
the opposite curve to that of the thermistor.
See Figure 4. Now when these two effects are
put together the resultant characteristic seen
in Figure 3 is achieved.
Figure 4
The SPD pin on the ZXBM200x series
contains an internal resistor network of
19.5k to an internal 2V rail and 52. 5k to
Ground. The purpose of this is to set the
speed range for a 100k NTC thermistor and
to help cancel out the thermistor’s
non-linearity against temperature.
The manner in which the non-linearity is
removed can be seen in Figure 14 where the
SPD voltage is linear with any slight non-
linearity in the speed response more likely
due to other motor characteristics.
It is also possible to use the ZXBM200x series
in conjunction with other lower values of NTC
thermistor, for example one with a value of
10k at 25 C. In this case an external resistive
divider as shown in Figure 5 below is used to
set the control speed range. The ratio of R1
and R2 in this figure should be chosen to give
the desired speed range against temperature.
With careful selection of the R1 and R2, it is
possible for a linear response within ±1% to
be achievable.
If needed R1 and R2 can also be added when
using a 100k thermistor in order to adjust
the speed range to match the particular
thermistor’s characteristics.
Figure 3
AN 36 - 4
ZETEX [ ZETEX SEMICONDUCTORS ]
