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IC manages room temperature

02 Jan 2013  | Javier Solorzano

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Room temperature can be uncomfortable when it rises to a certain level. A fan can help, but we do not want the fan on at all times, only when the temperature rises above a particular threshold. By building a simple circuit with an op amp, a comparator, and a voltage reference in one IC package, a simple temperature-controlled fan circuit can be built with minimal external component count.

Measurement set-up
A temperature-controlled fan application circuit as shown in figure 1 has the TS12011 IC powered from a 2.5V voltage from a buck converter that can also be used to drive other ICs such as a microcontroller.

The buck converter input voltage is 6V and is generated by four 1.5V AA batteries. Furthermore, the batteries power any LM35 temperature sensor and a silicon-controlled rectifier (SCR) to control fan operation. The temperature sensor provides an output voltage that is linearly proportional to ambient temperature in Celsius. For every one (1) degree Celsius increase/decrease in temperature, the temperature sensor's output voltage increases/decreases by 10mV.

In this application circuit, the fan is set to turn on when the temperature rises to 26.5°C. For instance, at a temperature of 26.5°C, the temperature sensor will generate an output voltage of 265mV that will in turn generate a HIGH state at the output of the comparator and the FAN will turn on. For testing purposes, an external power supply was substituted for the temperature sensors. Figure 2 shows the complete circuit where a demo board is used along with the IC.

Results
When the power supply voltage applied to the input of the op amp reaches approximately 265mV, the fan turns on successfully. A 265mV voltage corresponds to a temperature of 26.5°C.

Considerations
In this IC, the LHDET pin is useful for capturing one-time events. While this function was not explicitly used in this application, the LHDET pin was connected to VIN thereby disabling one-time-latch operation. For a detailed explanation of this function, please refer to the video referenced below.

Conclusion
In summary, the application circuit described here achieves three goals: a) reducing the total number of analogue ICs used from 4 to 2, thereby saving significant pcb area; b) low-supply-voltage operation from 0.8V to 2.5V; and c) minimising the supply current consumed by the op amp/reference/comparator functions to 1.5µA.

About the author
Javier Solorzano is Senior Applications Engineer for Touchstone Semiconductor Inc.

To download the PDF version of this article, click here.




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