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A Solid-State Relay uses a Power Semiconductor like a TRIAC or MOSFET as a switch, eliminating any moving parts, which makes is more reliable compared to it's mechanical counterpart. A mechanical relay may wear out after, say, a hundred thousand switching cycles; a Solid-State Relay can switch any number of times without wear, making it more reliable. In addition to wear due to switching, a mechanical relay can spark at it's contacts; a Solid-State Relay, on the other hand, does not.
This circuit, unlike other MOSFET Relay topologies, offers true input isolation through an optocoupler due to the fact that the required gate voltage is derived from it's own single-diode rectifier power supply and not from the triggering circuit's power supply. I've seen some topologies in which the MOSFET is switched on using an optocoupler, but the power comes from the power supply of the microcontroller. That's not really isolation, in my opinion. This circuit provides true isolation and can be controlled safely using a microcontroller or any other triggering circuit with which a human might interact.
The circuit is simple. Two IRF740N High-Voltage MOSFETs (from kiani's FET library http://everycircuit.com/circuit/5282567524450304) are connected together in this arrangement to switch the load on and off. A 10V zener is used to protect the gates from an over-voltage. The red LED, VCVS and the NPN BJT together form an optocoupler. The transistor of the optocoupler is connected to the gates and sources of the MOSFETs. Form here on, I shall refer to the MOSFETs and their common terminals in singular, since they are connected together.
When the AC input turns on, the 1uF capacitor starts charging up. For a 0V input to the optocoupler, the NPN transistor stays off, so the MOSFET switch turns on when Vgs(th) is reached and then the zener starts conducting. When the input to the optocoupler is 5V, the optocoupler turns on, bringing the MOSFET's gate voltage very close to the source voltage, turning it off.
Advantages over a TRIAC switch:
-This switch is voltage controlled.
-Since there is no "holding" and "latching" current in a MOSFET, so the switch doesn't have to be repeatedly fired.
-A MOSFET has better on resistance than a TRIAC
The absence of a zero crossover circuit can be seen both as an advantage and disadvantage. The disadvantage is obvious: sharp rise in current causes problems. The advantage is that phase angle control (power control) is possible by sensing the mains voltage and firing the switch according.
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