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IC and DMM form direct-read-out temperature probe

The simple temperature-measurement probe in Figure 1 can serve as an indispensable tool for troubleshooting and debugging electronic circuits. To measure temperature at several points, you can equip IC₁, a Maxim (www.maxim-ic.com) MAX6610, with a probe, or you can permanently integrate one or more devices into a pc board or attach them to components. Resistors R₁, R₂, and R₃ set the circuit’s temperature- scaled voltage output to various values (Table 1). Figure 2 shows the circuit’s representative output versus temperature.

You can display the circuit’s temperature- proportional dc output voltage on any DVM (digital voltmeter) or handheld DMM (digital multimeter). The circuit draws only 200 uA from a nominal 3V power supply, such as a pair of AA alkaline cells. A CR2016 lithium- coin cell can operate the circuit continuously for several hundred hours or for several years if you equip the circuit with a normally open, momentarycontact pushbutton switch.

To produce the error curve in Figure 3, immerse the circuit and a platinum-resistance standard thermometer in a temperature-controlled oil bath. The circuit’s relative error with respect to the standard thermometer varies only 4^oC over -40 to +125^oC. The MAX6610’s data sheet includes additional information on temperaturemeasurement error and output range.

To apply the circuit as a temperature probe, solder a 5-mm length of 1-mmdiameter, uninsulated copper wire directly to a small copper pad at IC₁’s GND pin. The wire should make thermal and electrical contact with the GND pin and thus provide a path of low thermal resistance from the sensor IC to the point of probing. Glue the wire to the pc board to add mechanical support. Heat loss affects the temperature measurement’s accuracy, and, to minimize heat loss from the probe through the pc board, use long and thin copper traces to make electrical connections from IC₁ to its supporting components.

Applying the MAX6610 as a pc-board temperature sensor differs somewhat from using it as a temperature probe. For board-temperature sensing, IC₁ must reside in intimate thermal contact with the board. Connect large copper areas immediately to the IC’s pins and use short, thick traces—or none at all—between the copper areas and the IC’s pins. The copper areas guarantee accurate temperature readings by providing thermal contact with the board and good heat transfer between the board and the sensor.

Reference

“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.