The temperature transmitter is composed of a measuring unit and an amplifying unit, with the range step element built from a circuit board that includes both an input circuit and a feedback circuit. The specific design of the range unit varies depending on the type of input signal. There are three distinct range units tailored for DC millivolt, thermocouple, and RTD (Resistance Temperature Detector) inputs, while the amplification unit remains consistent across all three input types.
For the DC millivolt signal, which can originate from any sensor or sensitive component, the range unit is relatively straightforward. Before the signal is amplified five times, a tuning circuit ensures that when the input voltage Ui is zero, the output voltage Uo is set to 1V. To achieve this, a feedback circuit is implemented where the output voltage Uo is five times the feedback voltage Uf. This requires careful adjustment of the zeroing potentiometer W1 and the feedback potentiometer Wf to meet the accuracy specifications.
In the case of the thermocouple input, cold junction compensation and linearization are essential. The bridge method used for cold junction compensation is similar to that described earlier. However, because the relationship between thermoelectric potential and temperature is nonlinear, a negative feedback circuit is introduced to create a piecewise linear characteristic. Each segment of the response has a different slope but collectively approximates a smooth curve, allowing the closed-loop gain to vary with the input thermoelectric potential. This effectively compensates for the nonlinearity of the thermocouple, resulting in an output voltage that is nearly proportional to temperature.
For the RTD input, a two-wire system is used, and a platinum resistor is incorporated. The resistance-temperature curve of platinum is monotonic and convex, meaning that the resistance increases at an accelerating rate as temperature rises. A positive feedback circuit is employed so that the output signal increases with the input resistance. This results in an output curve that is concave, thereby achieving a linearization effect. In contrast, copper resistors have naturally good linearity and do not require additional linearization.
The amplifying unit is based on a standard operational amplifier circuit combined with a power amplifier stage. This ensures that the signal is accurately amplified and conditioned for further processing or transmission.
When measuring DC millivolts, the transmitter operates within a range of 3–100 mV, with a zero migration range of –50 to +50 mV. For thermocouples, the matched range is 3–60 mV, and for platinum resistance, it covers a temperature range of –100 to +500°C. All models offer an accuracy level of 0.5%, making them suitable for a wide range of industrial applications.
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