SAFETY ISOLATING SOLID STATE RELAY
Original Publication Date: 1980-Dec-31
Included in the Prior Art Database: 2004-Apr-02
Xerox Disclosure Journal
Machine power distribution is commonly limited to electromechanical relay switching because of the safety requirement that the circuit controlled be disconnected when in the OFF state. Electromechanical relays suffer limitations because they transmit switching noise, draw large surge currents when switched on, suffer loss of life with increased switching frequency, experience contamination of contacts under high load and generate a switching arc. Solid state devices, usually triacs or SCR's are now in widespread use as replacements for electromechanical relays but from the standpoint of providing operational safety for machines, they too suffer limitations which include a basic failure mode of the thyristor being short-circuited, a shorted phototransistor or pre-driver SCR which prevents the thyristor from assuming the OFF state regardless of input signal and the OFF state resistance of the composite device is generally less than 50,000 ohms.