IC and DMM Make a "Direct Readout" Temperature Probe

The simple temperature-measurement probe in Figure 1 can serve as an indispensable tool for troubleshooting and debugging electronic circuits. If the temperature at several points needs to be measured, a probe can be provided for IC ₁ (Maxim MAX6610), or one or more devices can be permanently integrated onto a printed circuit board or connected to various components. Resistors R ₁ , R ₂ , and R ₃ set the circuit's temperature-scaled voltage output to various values ​​(Table 1). Figure 2 shows representative outputs of the circuit at various temperatures.

The circuit's DC output voltage, which is proportional to temperature, can be displayed on any DVM (digital voltmeter) or handheld DMM (digital multimeter). The circuit runs on a nominal 3V source, such as a pair of AA alkaline batteries, and consumes only 200mA. If the circuit is equipped with a normally open momentary contact push button switch, a CR2016 lithium coin cell battery can keep the circuit running for hundreds of hours or years.

To generate the error curve in Figure 3, immerse the circuit and a platinum resistance thermometer standard in a temperature-controlled oil bath. The circuit's relative error with respect to the standard thermometer varies only 4°C over a range of -40°C to +125°C. The MAX6610 data sheet contains more information on temperature measurement error and output range (see Reference 1).

To use this circuit as a temperature probe, solder a 5mm-long, 1mm-diameter piece of uninsulated copper wire directly to the small copper pad on IC ₁ 's GND pin . The wire should make thermal and electrical contact with the GND pin, providing a low-resistance path from the sensing IC to the probe point. For added mechanical support, bond the wire to the pc board. Heat losses affect the accuracy of temperature measurements, and to minimize heat losses from the probe to the pc board, use long, thin copper traces to make the electrical connection from IC ₁ to its supporting elements.

Using the MAX6610 as a pc-board temperature sensor is somewhat different from using it as a temperature probe. For board-temperature sensing, IC ₁ must reside in close thermal contact with the board, connect large copper areas directly to the IC pins, and use short, thick traces or no traces at all between the copper areas and the IC pins. The copper areas provide thermal contact with the board and good heat transfer between the board and the sensor, thereby ensuring accurate temperature readings.

Reference
1. "Precision, Low-Power, 6-pin SOT23 Temperature Sensors and Voltage References," MAX6610/6611 data sheet, Maxim Integrated Products, November 2003, http://pdfserv.maxim-ic.com/en/ds /MAX6610-MAX6611.pdf.