Even robotic systems occasionally need a cooling fan. When they do, the cooling circuit must be an intelligent one that makes minimal demands on power consumption and space. The circuit described here is such a cooling fan driver for an active heatsink assembly. The driver uses readily available and inexpensive components, and it runs on a 12-V dc supply (see the figure).
The circuit uses an LM35 temperature sensor chip (U1). The chip’s output voltage varies linearly with temperature changes in Celsius degrees and operates over a wide temperature range. It exhibits very low self-heating in still air and has a low current consumption.
The circuit’s operation is straightforward. U1’s output feeds directly to the input of a fairly complex and versatile linear-scale dot/bar graph LED driver, LM3914 (U2). The driver contains 10 voltage comparators, each with its non-inverting terminal connected to a specific tap on a floating precision multistage potential divider. All of the inverting terminals are connected in parallel and brought to the input through a unity-gain buffer.
The output of each comparator is externally available and can sink up to 30 mA. The current is internally limited, but can be externally preset from 2 to 30 mA via a single resistor. The inputs operate down to ground, and the device runs on a single supply of less than 3 V.
In this application, only two of the 10 outputs are needed to control the cooling fan’s driver relay (RL1). When the temperature increases, U1’s output goes high and U2 senses that analog voltage level at pin 5. When the temperature reaches 33°C, pin 13 on U2 goes low and forward-biases transistor Q1, energizing RL1. As a result, the 12-V dc supply is extended to the cooling fan through surge limiter resistor R3, activating the fan. Zener diode D1 suppresses the counter electromagnetic field (EMF) generated during the relay switching operation.
You can calibrate the circuit to activate at 33°C by adjusting the trim potentiometer (P1) and monitoring U2’s output at pin 13. After checking the calibration to ensure it’s correct, paint the pot’s control to lock it in place. Jumper JP1 can be used to select between two preset fan switching modes—for example, 33°C and 36°C.