The temperature transmitter is composed of a measurement unit and an amplification unit. The range adjustment element consists of a circuit board that includes an input circuit and a feedback circuit. The design of the range unit varies depending on the type of input signal. There are three distinct range units tailored for three different input modes: DC millivolt, thermocouple, and RTD. However, the amplification unit remains consistent across all three input types.
For the DC millivolt signal, which can originate from various sensors or sensitive components, the range unit is relatively straightforward. Before the signal is amplified by a factor of 5, a tuning circuit ensures that when the input voltage (Ui) is zero, the output voltage (Uo) is set to 1 volt. To achieve this, a feedback circuit is also implemented, maintaining a specific relationship between the output voltage (Uo) and the feedback voltage (Uf), where Uo = 5 × Uf. This requires careful adjustment of the zeroing potentiometer (W1) and the feedback potentiometer (Wf) to meet the desired accuracy.
In the case of the thermocouple input, the range unit must include cold junction compensation and linearization. The cold junction compensation typically uses a bridge method, similar to what has been described earlier. Linearization is necessary because the relationship between the thermoelectric potential and temperature is nonlinear. A negative feedback circuit is used to create a piecewise linear characteristic, with each segment having a different slope. This allows the system to approximate a smooth curve, making the closed-loop gain dependent on the input thermoelectric potential. As a result, the nonlinearity of the thermocouple is effectively compensated, and the output voltage becomes approximately proportional to the temperature.
For the resistance temperature detector (RTD) input, a two-wire system is used, where a thermal resistor is integrated. Platinum resistors exhibit a monotonic convex curve in their resistance-temperature relationship, with the resistance increasing more rapidly as the temperature rises. A positive feedback circuit is incorporated so that the output signal increases as the input resistance value increases. This results in an output curve that is concave, thereby achieving a linearization effect. In contrast, copper resistors inherently have good linearity and do not require additional linearization.
The amplification unit is based on a standard operational amplifier configuration combined with a power amplifier stage to ensure sufficient output drive capability.
When measuring DC millivolts, the transmitter operates within a range of 3 to 100 mV, with a zero migration range of -50 to +50 mV. It is compatible with thermocouples over a 3 to 60 mV range and with platinum resistance temperature detectors from -100°C to 500°C. All models maintain an accuracy level of 0.5%.
Optical Lens,Biconcave Spherical Lens,Meniscus Lens,Biconvex Spherical Lens
Danyang Horse Optical Co., Ltd , https://www.dyhorseoptical.com