Triacs Part 2: Solid State AC Relays
A solid state relay (SSR) consists of four main parts:- An opto-isolator or opto-coupler to isolate the low voltage DC control, often from a microcomputer, from the high voltage AC. The input is often one or two light emitting diodes while the output is often a photo transistor.
- A zero-crossing detector circuit to switch the triac on during the time the sin wave is a little past at zero or 180 degrees. This helps prevent damage the load and needless power surges.
- A triac acting as an AC switch.
- A snubber circuit(s) to prevent false firing of the triac.
Basic transistor optocoupler.
In the above basic opto-coupler (or opto-isolator) the voltage at Vo is at Vcc with S1 open. When S1 is closed the infra-red light from light emitting diode D1 falling on the base of Q1 causes it to conduct to ground like a switch. The voltage at Vo drops to zero. Q1 is an NPN bipolar photo transistor and both the LED and transistor are often one part. Also see Using Opto-Couplers An important note is the output has no electrical connection to the input and can hold up to several thousand volts. Also see More sample circuits.
The most basic solid state relay
Opto-Isolators with Diacs
An opto-isolator is a solid state device designed to provide electrical isolation between input and output. The input consists of a light emitting diode (LED) in a six or eight pin dip (IC) package depending on type. The output can be a photo transistor, photo diac, etc. There is no electrical contact between input and output. When the LED is turned on, the diac, transistor, etc. will conduct from the light emitted from the diode thus turning on the triac like a switch. The MOC3011 series is made to connect to triacs, the MOC301x types for 110 volts, and the MOC302x types for 240 volts.
Note the above output using this configuration is often used in PLCs.
(above) MOC3042
Other optocouplers have have a built in zero-crossing detector.
Snubbers
A snubber circuit (usually of the RC type) is often used between MT1 and MT2. Snubber circuits are used to prevent premature triggering caused for example by voltage spikes in the AC supply or those produced by inductive loads such as motors. Also, a gate resistor or capacitor (or both in parallel) may be connected between gate and MT1 to further prevent false triggering. That could increase the required trigger current and perhaps a delay in turnoff as the capacitor discharges.
In this circuit above the "hot" side of the line is switched and the load connected to the cold or ground side. The 39-ohm resistor and 0.01µF capacitor are for snubbing of the triac, and the 470 ohm resistor and 0.05 µF capacitor are for snubbing the coupler. These components may or may not be necessary depending upon the particular and load used.
For more on the above optocoupler see moc30xx series opto-isolator and MOC3042 with zero-crossing circuits. (both pdf files)
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- Tutorials updated/added August 2009:
- Basic Triacs and SCRs
- Solid State AC Relays with Triacs
- Basic Transistor Driver Circuits for Arduino Micro-Controllers
- Opto-Isolated Transistor Drivers for Arduino Micro-Controllers
- Multi-use demo board built in class.
- Using the ATMEGA168/Arduino with the TA8050 Motor Controller
Demonstrates use of pulse-width modulation for motor speed control.
- Basic Capacitors
- Basic Diodes and Rectifiers
- Series batteries
- Parallel batteries
- Potato battery
- Thermoelectricity
- Reed Switches
- Bi-Metallic Thermostats
Added February 2009: Using a CdS Photo resistor. How to use photocells and touches on comparators, thermistors, relays, etc. Includes circuits to build and test.
- Voltage Comparator Information And Circuits
- Photocell with a Comparator
- Photocell with relay and Triac
- Basic Thermistor
Arduino demos April 30, 2009:
Using the ATMEGA168/Arduino with a 24LC08 Serial EEPROM
Using the ATMEGA168/Arduino with a DS1307 Real Time Clock
More to come. this will include using the MCP23016 I2C I/O Expander and several simple demo projects
for robotics and power control.
Atmega168/Arduino features:
14k flash program storage
1k RAM for program memory
6 PWM outputs
6 A/D inputs
UART and SPI interfaces
2 Hardware interrupts
20 general purpose I/O pins (shared with PWM and Analog pins)
16 MHz RISC microcontroller
Open-source hardware, IDE, bootloader
Easy upgrade to more powerful hardware (Wiring)
Easy to use and learn.
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March 2002