Temperature monitor and fan controller reduce fan noise

David Hanrahan, Analog Devices Inc, Limerick, Ireland -- EDN, 5/10/2001

The scheme in Figure 1 reduces system acoustic noise by running system fans at their optimum speeds for a given temperature. IC₁ combines ±1°C-accurate temperature measurement of three temperatures with automatic fan-speed control of two channels. A two-wire serial interface allows you to oversee critical temperature and fan-speed data. NPN transistors, such as the 2N3904, can measure temperatures in remote locations. The icrocontrollerinterface allows you to connect an LCD to display all monitored parameters.

You program IC₁'s automatic-fan-speed-control function using T_MIN and T_RANGE (Figure 2). T_MIN is the temperature at which the fan automatically turns on and runs at minimum speed. T_RANGE is the temperature-to-fan speed-control slope, with options of 5, 10, 20, 40, and 80°C. Choosing one of the T_RANGE options allows you to define how the fan reacts to temperature variation.

An example of programming IC₁ follows. Setting Configuration Register 1 (Reg 0x00) to 0x99 starts the device in automatic-fan-speed-control mode, with the  function enabled. A setting of Remote Temp 1 T_MIN/T_RANGE (Reg 0x25)=0x63 sets T_MIN for fan 1 to 48°C and T_RANGE to 40°C. The fan reaches full speed at 88°C. A setting of Remote Temp 2 T_MIN/T_RANGE (Reg 0x26)=0x62 results in a T_MIN for Fan 1 of 48°C and a T_RANGE of 20°C. The fan reaches full speed at 68°C.

The fan runs at low speed until the system requires a higher level of cooling. The fan speed responds automatically to temperature variation, reducing acoustic noise (Figure 3). You can program additional features, such as fan spin-up time and minimum fan speed. The automatic-fan-speed-control mode allows flexibility over which temperature channel controls each fan. You can also decide that the maximum speed calculated for all temperature channels controls the fans.

Figure 4 shows the circuit diagram for the temperature-monitor/fan-control circuit. The PIC16C84 writes the configuration settings to IC₁ and optionally drives an LCD. The µC bit-bangs pins 2 and 3 to provide serial clock and data for IC₁. The µC reads and displays all temperatures and fan speeds. The LEDs that connect to the  and  outputs illuminate whenever an overemperature condition occurs or a fan fails. The  output is a fail-safe output that goes low if a preprogrammed overtemperature  limit is exceeded. In the event of an overtemperature condition, both fans automatically run at full speed. If some external device pulls the  pin low, the fans also run at full speed. The  pin signals catastrophic fan failure. If one fan fails, the second fan automatically spins at full speed to compensate for the loss of airflow. Should the failing fan recover or be replaced, both fans automatically return to their normal operating speeds.

You can extend this idea for multiple fans with redundant cooling using two ICs to monitor six temperature zones (Figure 5). This scheme cross-connects the  and  signals. The same pin drives fans A₁ and A₂, which will run at the same speed. Likewise, fans B₁ and B₂ connect in parallel to a common FET. Fans C and D are redundant coolers that the system uses only if it gets excessively hot or a fan fails. If either fan A₁ or A₂ fails, fans B₁ and B₂ automatically run at full speed. The  output of IC₁ asserts low, pulling  of IC₂ low, which causes redundant fans C and D to run at full speed. If either fan B₁ or B₂ fails, fans A₁ and A₂ automatically run at full speed. The  output of IC₂ pulls  of IC₁ low, causing fans A₁ and A₂ to run at full speed. Fans C and D also automatically run at full speed. If the faulty fan is replaced, all fans return to their normal operating speeds.  signals alert the system to fan failure.