RES Principle of Operation:
RES controllers are in principle, as the name already indicates, electrical resistance measuring and control devices. They use the property of certain heatseal band alloys of changing their specific resistance with temperature. This behavior of the heatseal band is expressed by its temperature coefficient Tk.
The advantage of this control method is the instant feed back of the heating element temperature (resistance) without and additional sensor that introduces thermal inertia. The resistance is constantly monitored with a high sampling rate by measuring the current through the element and the applied voltage; this information is fed back to the controller as the process variable (PV). It is also supplied at the analog output, for display on an external temperature meter or other instrument.
After comparing this information with the set point "SP" the controller adjusts the voltage output to the heating element so that it will always maintain the desired temperature (resistance).
Voltage control is accomplished by phase-angle-control of the primary current going to the transformer. The measurement or purely electrical variables together with the high sampling rate in a highly dramatic, thermo-electrical control loop. A wide range of secondary voltages can be selected for the system because control is exercised on the primary side of the transformer. This allows optimum 'fitting' of the transformer's secondary voltage to the application.