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An early switch operating by this principle was invented by Chester I. Hall of the General Electric Company, with a patent filed in 1921 and approved in 1924.Like the modern infinite switch, Hall's invention used a bi-metallic strip, heated by a constant current, to break a connection after a given period of time.
Another type of thermal switch is a PTC (Positive Temperature Coefficient) thermistor; these thermistors have a "cutting off" temperature at which the resistance suddenly rises rapidly, limiting the current through the circuit. When used in conjunction with a thermistor relay, the PTC will switch off an electrical system at a desired temperature.
Relay logic diagrams represent the physical interconnection of devices. Each rung would have a unique identifying reference number and the individual wires on that rung would have wire numbers as a derivative of the rung number. Thus, if a rung was labelled as 105, the first independent wire would be 1051, the second as 1052, and so forth. A ...
A limit switch with a roller-lever operator; this is installed on a gate on a canal lock, and indicates the position of a gate to a control system A limit switch mounted on a moving part of a bridge In electrical engineering , a limit switch is a switch operated by the motion of a machine part or the presence of an object.
A relay Electromechanical relay principle Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off. A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple ...
Ladder notation is best suited to control problems where only binary variables are required and where interlocking and sequencing of binary is the primary control problem. Like all parallel programming languages , the sequential order of operations may be undefined or obscure; logic race conditions are possible which may produce unexpected results.
Electromechanical protective relays operate by either magnetic attraction, or magnetic induction. [9]: 14 Unlike switching type electromechanical relays with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays have well-established, selectable, and adjustable time and current (or other operating parameter) operating characteristics.
Communication is digital and bi-directional between the controller (primary) and the boiler (secondary). Various commands and kinds of information can be transferred; however, the most basic command is to set the boiler's target water temperature. OpenTherm makes use of a traditional untwisted 2-wire cable between controller and boiler.