Triac relay-contact protection
Posted on Jan 20, 2013 5160
This circuit can be used to prevent relay contact arcing for loads up to 50 amperes. There is some delay between the time a relay coil is energized and the time the contacts close. There is also a delay between the time the coil is de-energized and the time the contacts open. For the relay used in this circuit both times are about 15 ms. The TRIAC across the relay contacts will turn on as soon as sufficient gate current is present to fire it. This occurs after switch SI is closed but before the relay contacts close.
When the contacts close, the load current passes through them, rather than through the TRIAC, even though the TRIAC is receiving gate current. If SI should be closed during the negative half cycle of the ac line, the TRIAC will not turn on immediately but will wait until the voltage begins to go positive, at which time diode Dl conducts providing gate current through Rl. The maximum time that could elapse before the TRIAC turns on is S-Vi ms for the 60 Hz supply. This is adequate to ensure that the TRIAC will be on before the relay contact closes.