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AXBM2001 Datasheet(PDF) 4 Page - Zetex Semiconductors |
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AXBM2001 Datasheet(HTML) 4 Page - Zetex Semiconductors |
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4 / 14 page  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. 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. AN 36 - 4 Applications Note AN36 Issue1 - JULY 2002 Figure 3 Figure 4 |
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