Relay contact protection is needed as the switching of inductive or capacitive loads produces switching sparks due to transient voltage, which can influence the electrical service life of the relays. The following protective circuits for the contacts reduce contact wear-
The concept is that this protective devices shows very low resistance for specific high voltage range. Thus when there is high enough voltage then range, then this device shows low resistance to pass transient current through them, protecting the circuit or device that is connected in parallel to them.
US = Voltage progression, 1 = Closing, 2 = Opening
Relay contact protection by diode from transient voltage
Advantage: Can be used for all capacities, low over voltage,minimum space required, low price .
Disadvantage: Very long release delay.
Diode and Z-diode protection from transient voltage- relay contact protection
Advantage: Low overvoltage (defined by Z-diode),short release delay
Disadvantage: Cannot be used for large capacities
RC combination protection from transient voltage- relay contact protection
Advantage: Low overvoltage, short release delay.
Disadvantage: High current load on the contacts when switching on; more complicated and expensive at greater capacities
Varistor protection from transient voltage- relay contact protection
Advantage: Short release delay, low price
Disadvantage: Cannot be used for all operating voltages and capacities
Short URL to this post http://goo.gl/RND8lb