What is the basic principle of a temperature transmitter?

The temperature transmitter consists of a measuring unit and an amplifying unit. The range step element is composed of a circuit board composed of an input circuit and a feedback circuit. The range unit is different depending on the input signal. There are three range units that match the three input modes of DC millivolt, thermocouple and RTD, and the amplification unit is common to the three inputs.

The DC millivolt signal can be provided by any sensor or sensitive component. The DC millivolt range unit is relatively simple. Before the DC millivolt signal is amplified 5 times, there is a tuning circuit so that when the input signal Ui=0, the output signal Uo=1v ( According to the standard signal protocol, a feedback circuit is designed to meet this requirement, so that the output voltage Uo has a certain relationship with the feedback voltage Uf: Uo=5Uf. It is necessary to repeatedly adjust the zeroing potentiometer W1 and the feedback potentiometer Wf to achieve accuracy. Degree requirements.
The thermoelectric range unit must have cold junction compensation and linearization. The bridge method for cold junction compensation is basically the same as the foregoing. The linearization function is because the relationship between thermoelectric potential and temperature is nonlinear. A negative feedback circuit is designed to have a polygonal line characteristic. The slope of each segment is different, but it can be approximated as a curve, so that the closed-loop amplification factor is related to the input thermoelectric potential. The size of the curve changes, which essentially offsets the nonlinearity of the thermocouple, and the output voltage is substantially proportional to temperature.

The thermal resistance range unit introduces a thermal resistor in a two-wire system. For platinum resistance, the resistance-temperature curve is a monotonically convex curve, and the resistance increment is getting smaller as the temperature increases. A positive feedback circuit is designed, so that the growth of the output signal becomes more and more significant as the input resistance value increases, that is, the output electrical specific resistance curve is a concave curve, thereby realizing the linearization function. The copper resistor itself has good linearity and does not require a linear ratio.

The amplifying unit uses the basic circuit of an integrated operational amplifier plus a power amplifier.
When the above temperature transmitter measures DC millivolts, the range is 3-100mv, the zero migration is -50-50mv; the range matched with the thermocouple is 3-60mV; and the range with the platinum thermal resistance is -100-500°C. The accuracy levels are all 0.5.


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Thermocouple compensation wire use precautions