Solid State Relay

Solid State Relay

Solid State Relays are semiconductor equivalents of the electromechanical relay and can be used to control electrical loads without the use of moving parts

Solid State Relay Example

Lets assume we want a micro-controller with a digital output port signal of only +5 volts to control a 120V AC, 600 watt load (heating element or lamp). For this we could use the MOC3020 or MOC 3041 opto-triac isolator, but the internal triac can only pass a maximum current (Itsm) of 1 Amps peak at the peak of a 120V AC supply so an additional switching triac must also be used.

First lets consider the input characteristics of the MOC 3020 opto-isolator (other opto-triac are available). The opto-isolaters datasheet tells us that the forward voltage, (Vf) drop of the input light emitting diode is 1.2 Volts and the maximum forward current,(If) is 50mA.

The LED needs about 10mA to light reasonably bright up to its maximum value of 50mA. However the digital output port of the Micro Controller can only supply a maximum of 30mA. Then the value of current required lies somewhere between 10 and 30 mA. Therefore:

Rmax = 380ꭥ and Rmin = 126ꭥ acording to ohms low. (We can use LED Calculator).

Thus a series current limiting resistor with a value between 126ꭥ and 380ꭥ’s can be used. As the digital output port always switches +5 volts and to reduce the power dissipation through the opto-coupler LED, we will choose a preferred resistive value of 240ꭥ’s. This gives an LED forward current of less than 16mA. In this example,any preferred resistor value between 150ꭥ and 330ꭥ’s would do.

The heating element load is 600 watts resistive. Using a 120V AC supply would give us a load current of 5 amperes (I=P/V). As we want to control this load current in both half cycles (all 4 quadrants) of the AC waveform,we would require a mains switching triac.

The BTA06 is a 6 amps (It(rms)) 600 volt triac suitable for general purpose ON/OFF switching of AC loads,but any similar 6 to 8 amp rated triac would do. Also this switching triac requires only 50mA of gate drive to start conduction which is far less than the 1 amp maximum rating of the MOC 3020 opto-isolator.

Consider that the output triac of the opto-isolator has switch ON at the peak value (90 degree) of the 120Vrms AC supply voltage. This peak voltage has a value of: 120 x 1.414 = 170Vpk. If the opto-triacs maximum current (Itsm) is 1 ampere peak, then the minimum value of series resistance require is 170/1=170ꭥ’s or 180ꭥ’s to the nearest preferred value. This value of 180ꭥ’s will protect the opto-coupler output triac, as well as the gate of the BTA06 triac on a 120VAC supply.

If the triac of the opto-isolator switches ON at the zero crossover value (0 degree) of the 120Vrms AC supply voltage, then the minimum voltage required to supply the required 50mA gate drive current forcing the switching triac into conduction will be: 180ꭥ x 50mA = 9.0 volts. Then the triac fires into conduction when the sinusoidal Gate-to-MT1 voltage is greater than 9 volts.

Thus the minimum voltage required after the zero crossover point of the AC waveform would be 9 volts peak with the power dissipation in this series gate resistor being very small so an 180ꭥ/0.5 watt rated resistor could safely be used. Consider the circuit below.

Schematic shown is for 220V AC supply. So R1 & R2 value changed for 220V AC supply.

This type of opto coupler configuration forms the basis of a very simple solid state relay application which can be used to control any AC mains powered load such as lamps and motors. Here we have used the MOC 3020 which is a random switching isolater. The MOC 3041 opto-triac isolator has the same characteristics but with built-in zero-crossing detection allowing the load to receive full power without the heavy inrush currents when switching inductive loads.

Diode D1 prevents damage due to reverse connection of the input voltage, while the 56ꭥ resistor R3 shunts any di/dt currents when triac is OFF eliminating false triggering. It also ties the gate terminal to MT1 ensuring the triac turns-off fully.

If used with pulse width modulated, PWM input signal, the ON/OFF switching frequency should be set to less than 10Hz maximum for an AC load otherwise the output switching of this solid state relay circuit may not be able to keep up.

Note: This article not written by me. All credits goes to original author of this article.