Simple Solid State Relay (SSR) Circuit

A Solid State Relay SSR circuit is like an electronic ON/OFF switch, but way cooler.

Unlike regular switches with clicky parts, this one uses special electronics transistors or triacs to turn things on and off.

This makes it super reliable, quiet and lasts a long time, perfect for all sorts of electronic projects.

Circuit Working:

Parts List:

The ICs MOC3040 or MOC3041 function similarly to standard opto couplers but instead of a phototransistor they use a phototriac 100 mA 400 V at 25°C.

One key advantage of these ICs is their ability to work with a wide range of silicon controlled rectifiers SCRs and triacs, which is often not possible with opto couplers based on phototransistors.

For designing a soli state relay SSR to operate at 220V the choice of triac depends on the type of load.

For resistive loads the TIC226D 400 V triac is suitable.

However, for inductive loads a 630 V triac like the TIC226M may be necessary.

It’s important to ensure that the working voltage of capacitor C1 matches the specifications of the triac being used.

The value of the input side resistor R1 can be calculated based on the input voltage, Vin and the current through the LED in the MOC opto coupler, Ioc using the below formula.

For example, if vin is 12V and Ioc is 30mA standard for the MOC 3040, R1 calculates to approximately 356 ohms which can be rounded to a practical value like 330 ohms.

For the MOC3041 with Ioc = 15mA a suitable R1 value could be around 680 ohms.

The maximum current that the 220V SSR can handle is about 8 Amps.

For higher power applications a different triac should be selected accordingly.

Formula:

Comprehending the R1 Formula on a basic SSR Circuit.

The Formula: R1 = 1000 × I_oc * (V_in – 1.3)

here,

• R1 is the input side resistors value expressed in ohms Ω.
• I_oc is the milliampere mA current flowing through the optocouplers LED.
• V_in the input voltage in volts V.
• 1.3 is a steady voltage drop, usually expressed in volts V, across the optocouplers LED.

Explanation:

In a simple solid state relay SSR circuit that makes use of an optocoupler, the value of the input resistor R1 is determined using this formula.

Limiting the current flowing through the LED in the optocoupler requires the resistor.

This is how the formula operates:

• Ioc: The specs of the optocoupler are used to determine the required current flowing through the LED.
• The optocoupler is reliably triggered by this current.
• Vin – 1.3: This section determines the voltage drop across R1.
• To compensate for the voltage drop across the LED, 1.3V is deducted from the input voltage.
• 1000: For uniformity in the computation, this factor is used to convert the current from milliamperes (mA) to amperes A.

How to Build:

To build a Simple Solid State Relay SSR Circuit you need to follow the below mentioned steps:

Circuit Construction:

• Gather the components as per the diagram.
• Connect one end of the load to the 220V power supply.
• Connect the other end of the load to MT1 terminal of the triac.
• Connect the MT2 terminal of the triac to the phase of the 220V AC.
• Connect the positive input voltage to the anode of the MOC3041 LED through a series resistor R1.
• Connect the negative input supply to the cathode of the MOC3041 led.

Testing:

• Apply the input voltage to the MOC3041.
• Verify that the load resistive or inductive turns on and off as expected based on the input voltage.

Safety Precautions:

• Ensure that all connections are secure and insulated.
• Use appropriate safety gear gloves, goggles when working with electrical components.
• Be cautious of high voltages present in the circuit.

Note:

• This basic setup can be expanded or modified based on specific requirements and applications.
• Always refer to the datasheets of the components used and follow best practices for circuit design and assembly.

Conclusion:

A Solid State Relay SSR circuit is an electronic switch that uses solid state components to perform switching operations without any mechanical parts.

It offers advantages such as reliability, durability and fast switching speeds, making it suitable for various applications in industrial control systems and automation.

References:

Datasheet MOC3041

The solid-state relays (SSR) circuit diagram.  

Datasheet TIC226