12V

System Fan

Sanyo Denki

109R0612T4H12

LM26

V+

VTEMP

GND

HYST

OS

10k

0.12f

+5V

LM26

SNIS115S – MAY 2001 – REVISED SEPTEMBER 2015

www.ti.com

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LM26 thermostat (temperature switch) can be used in applications such as microprocessor thermal

management, appliances, fan control, industrial process control, power supplies for system protection, fan speed

adjust or plain temperature monitoring.

9.2 Typical Application

Figure 8. Two-Speed Fan Speed Control

9.2.1 Design Requirements

The requirement is to change speed fo a fan to maximum at 45°C with an accuracy of

Table 2. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Min Fan Speed

1900 RPM

Max Fan Speed

3800 RPM

Temperature Threshold To Switch From Min Speed to Max Speed

45°C

Threshold accuracy

±3°C

9.2.2 Detailed Design Procedure

The design procedure is simple. A fan was selected that has the capability to be controlled by an external NTC

thermistor. The recommended NTC thermistor adjusts the fan speed to maximum at 40°C. The LM26 meets the

threshold accuracy requirements for temperature control of the fan speed and allows setting the max speed

temperature threshold higher as required to 45°C. The resistance of the thermistor for the min fan speed is 6.8

k

Ω. Since thermistors have a negative temperature coefficient (NTC), 10 kΩ was chosen to ensure that the fan is

at min speed when the LM26 OS is off. When the OS output goes low at 45°C it simulates the low thermistor

resistance at higher temperatures thus setting fan to max speed.

10

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