As shown in figures "A", "B", "C" and "D" of the illustration pertaining to various diagrams specific to a PLC controlled temperature control system using a bimetallic thermostat, which statement is true? See illustration EL-0253.
• Bimetallic thermostat behavior – does it act like a continuously varying sensor or a simple on/off switch? • Difference between analog (variable) and digital/discrete (on/off) PLC signals. • Role of the contactor for the heater circuit shown in the diagram – is the heater controlled proportionally or just switched on/off?
• Look at figure A: when the bimetallic strip bends and reaches the magnet/contact, does the PLC see many levels of voltage or just a change between two states? • In figure D, examine the timing diagram: do the lines for "Thermostat" and "Heater" ramp smoothly or simply change state between two levels? • Consider typical PLC inputs and outputs for simple thermostatic control: are most basic thermostats wired to discrete input cards or analog input cards?
• From the illustration, confirm whether the thermostat input to the PLC is shown as a switch contact or as a continuously varying signal. • Check whether the PLC output is shown feeding a contactor coil (on/off device) or an analog output module (4–20 mA, 0–10 V, etc.). • Verify that the timing diagram in figure D only shows two states (open/closed or on/off) for both the thermostat and heater rather than a continuously variable level.
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