Thermocouple-to-DMM interface

Thermocouples are economical and rugged devices for temperature measurements. Because of their small size, they respond quickly and are good choices where fast response to temperature changes is important. Type K thermocouples have a wide temperature range. “and are used from cryogenics to jet engine exhaust analysis. The present circuit converts thermocouple outputs into a direct voltage with a gradient of 10 mV °C-1, which is just about all that is needed to enable a digital multimeter (DMM) to be used as a readout. Alternatively, the interface output signal may be fed to a computer system for more advanced temperature recording applications.

The AD595A from Analog Devices is a complete instrumentation amplifier and thermocouple cold junction compensator on a monolithic chip. It combines an ice point reference with a precalibrated amplifier to produce an output of 10 mV ‘C-1 directly from a thermocouple signal. The AD595A is laser trimmed for type K (chromel-alumel) thermocouples.
The application of the AD595A shown here could not be simpler. The Type K thermocouple is connected to a special socket. Resistor R1 is included to ensure that common-mode voltages induced in the thermocouple loop are not converted to normal mode. The AD595A features an alarm output. +ALM (pin 12). which is used here to drive a low-current LED. The +ALM output goes low when one or both of the thermocouple leads are interrupted. It should be noted that the cold junction compensation provided by the AD595A will be affected whenever the alarm circuit is actuated. This means that readings taken when the alarm output is actuated are invalid.
Because a thermocouple output voltage is non-linear with respect to temperature (see Table). and the AD595A linearly amplifies the compensated signal, the following transfer function must be used to determine the actual output voltage:
Uo=247.3(Uth+0.011)

where Uo is the output of the AD595A and Uth is the thermocouple output in mV. Since ANSI Type K and DIN NiCr-Ni thermocouples are composed of identical alloys. both may be used with the present interface.
Construction of the interface is straightforward since very few parts are involved. Note the copper area under the converter chip to improve the thermal contact between the thermocouple socket and the IC. A low thermal resistance is important here to ensure that the on-chip ice point reference operates correctly.

The interface may be powered by a symmetrical or an asymmetrical power supply. In the first case, the ‘0’ and ‘ground. terminals are interconnected and taken to the terminal of the battery. Note, however, that temperatures below 0 °C cannot be measured if a single-ended supply is used. When a symmetrical, supply is used, the full temperature range becomes available. A symmetrical supply is best provided by two 9-V batteries.

The current drain of the circuit is not greater than 1 mA with the thermocouple connected, and not greater than 10 mA with the thermocouple disconnected (alarm LED lights).

parts list
Resistors:
R1 = 10 kΩ
R2 = 2.2 kΩ
Capacitors:
C1 : C3 = 4.7 μF, 25 V, radial
C2 : C4 = 100 nF
Semiconductors:
D1 = LS3369EH (low-current LED: red)
D2: D3 = 1N400
Integrated Circuits:
IC1 = AD595A (Analog Devices)
Miscellaneous:
K1 = special thermocouple type-K socket, e.g RS Electronics 473-127.
Plastic enclosure with battery compart-
ment; approx. size 80x60x20mm
Further reading:
“Temperature measurement techniques”
Elector Electronics, December 1991.
“A fast, precise thermometer”, Elector Electronics, January 1992.