Test Ideas: Controller keeps circuits cool
Use a microcontroller’s PWM output to adjust temperature based on measurements.
By Chien-Hung Chen, Mao-Chiao Weng, and Tai-Shan Liao, Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan -- Test & Measurement World, 11/1/2009 1:00:00 AM
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In many industry applications, you often need to operate an instrument or a device under test at low temperature. When this need arises, you can build a small TE (thermoelectric) cooling controller for your instrument that can control temperature to within 1°C.
Cooling an instrument or a device can improve the SNR (signal-to-noise ratio), and it can also prolong product life. For example, the dark-and-noise signal of an infrared detector's output at room temperature can be an order of magnitude smaller when cooled.
We used the cooling device we built as part of a study of semiconductors. Figure 1 shows the block diagram of the system; Figure 2 (view Figure 2) shows the circuit schematic. The design uses the temperature feedback signal to optimize the system.
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Figure 1. A temperature controller uses feedback from a thermistor to monitor temperature. |
The heart of the design is an ATmega8535 microcontroller. It produces a PWM (pulse-width modulation) signal based on a temperature measurement. A thermistor-to-digital converter produces a digital output on its three-wire SPI (serial peripheral interface), sending the data to the microcontroller. A TLC7524 DAC (digital-to-analog converter) generates a control voltage from the PWM signal. The voltage controls the cooling device through an op amp and MOSFET (Q1), which can produce up to 2 A of control current.
We divided the code into three parts: Main, Timer1, and Timer2. The main code sets system parameters, cooling temperature, and the timers.
The Timer1 code reads the temperature data and computes an average value from 50 measurements. The Timer2 code compares the measured temperature to the temperature setting and adjusts the current to the cooling controller through the microcontroller’s PWM output signal.
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Figure 2. A microcontroller produces a PWM signal that an op amp converts to a control signal for a cooler. |
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