Designing an analog circuit is usually a irritating expertise, as famous by Nick Cornford in his Design Thought (DI), “DIY RTD for a DMM.” With a fortnight’s wrestle, I accomplished the design of 4 mA to 20mA, two-wire temperature transmitter utilizing a platinum resistance temperature detector (RTD) sensor, as proven in Determine 1.
Determine 1 A two-wire transmitter with a platinum RTD sensor, R4. R1 and R4 will decide the currents by means of the respective limbs; these currents have to be stored to 0.5 mA. The circuit is designed to function from 0 °C to 300 °C, the place R5 might be adjusted to alter this temperature vary.
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In industrial processing purposes, a two-wire topology is used to attach the temperature sensor within the area to the management room gear, akin to a distributed management system (DCS), programmable logic controller (PLC), or indicator. A 24-V DC provide is fed from the management room, and the present drawn is proportional to the temperature. Because the energy and sign journey in the identical pair of wires, this association presents cable financial savings.
In Determine 1’s circuit, R4 is an RTD. As per the platinum RTD’s temperature versus resistance desk (DIN EN 60751), R4 is 100 Ω at 0 °C and 212.2 Ω at 300 °C.
This circuit is designed for the vary of 0 °C to 300 °C, the place the load present can be 4 mA for 0 °C and 20 mA for 300 °C (chances are you’ll change R5 to realize different ranges). The present is proportional to the resistance of the RTD, which is barely non-linear with respect to temperature. The accuracy claimed on this circuit is ±1%, which is satisfactory for a lot of purposes.
To keep away from self-heating of the RTD, solely 0.5 mA is shipped by means of it. R1 and R2 have to be adjusted to go 0.5 mA in every limb. U1 and U4 are wired as zero tempco present sources. The distinction in voltage between RTD (R4) and R3 is amplified within the instrumentation amplifier U3.
The RTD at 300 °C (or R4 at 212.2 Ω) sends 160 µA by means of R8. R10 units the present by means of it as 40 µA. Therefore, a complete of 200 µA is shipped because the enter to the transmitter IC, U5.
U5 then amplifies this present by 100 to twenty mA and converts it right into a two-wire format. The U2 circuit generates -3.3 VDC to feed the unfavourable provide voltage pin of U3. Correct outcomes have been solely achieved when working U3 with a twin provide. The RTD at 0 °C provides a 4-mA present on the output (LOAD).
Jayapal Ramalingam has over three many years of expertise in designing electronics methods for energy & course of industries and is presently a contract automation guide.
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