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Part 1: Basic Triacs and SCRs

This page will discuss basic triacs and SCRs. A bidirectional, three-terminal thyristor (SCR) is called a triac. This device can switch the current in either direction by applying a small current of either polarity between the gate and one of the two main terminals. The triac is fabricated by integrating two thyristors in an inverse parallel connection. It is used in AC applications such as light dimming, motor-speed control, etc.

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Some Basic AC


The AC (alternating current) we get from a household wall socket is called a sin wave. If one could fold one-half of the sin wave under/over the other, we would have a circle of 360 degrees. This power is generated at a power plant by spinning an electromagnet within a coil of wire. (Same thing as an auto alternator.) The rotor speed is 3600 rotations per minute (RPM) or 60 cycles per second. The 120 volts from the outlet is the RMS value, which basically means the same power value as DC. (Note "peak" is amplitude.)

AC constantly changes voltage and polarity. The voltage starts at zero degrees at zero volts. It goes to the positive peak at 90 degrees, back to zero volts at 180 degrees, goes to the negative peak at 270 degrees, and back to zero at 360 degrees, and starts again. That is one cycle. Every cycle the sin wave passes what is called the zero-crossing point at 180 and 360 degrees. That is important to know as we will see below. Another way to think of RMS is an average.

In the above example at 120 Volts AC RMS the peak would be 170 volts (120 X 1.414), peak-to-peak would be 340 volts, and the period (1/60) equals 16.7 milliseconds.

Half-wave rectification

Many devices, in particular electronics, must use DC or direct current. A diode is a solid-state device that conducts in one direction only. When the anode (A) is positive and the cathode (K) is negative (though the load) current (I'm assuming electron flow from negative to positive) will flow through the load, through the diode and back to the power supply. Thus current will flow only for the positive half-cycle (0 to 180 degrees) and the diode will shut-off during the negative half-cycle from 180 degrees to 360 degrees.

What is power? Voltage (in volts) is the "push" and the current (in Amperes) is what is being pushed. (Electric charges) Power is voltage times current. Power is measured in watts. So one amp at one volt equals one watt. (I'm not going into all of Ohm's Law here. See your text.) We must have voltage and current together to get power, so an open switch, broken wire, or a shut-off diode delivers no power. In the case above, we get very poor power transfer with the diode off half the cycle and the positive voltage level changing constantly.

Let's say the AC in is 12 volts RMS. To get peak we multiply 12 by 1.414, which equals about 17 volts. But the average voltage DC is peak times .3185 equals about 5.4 volts. This is what is called pulsating DC. Pure DC, such as from a 12 volt auto battery, has none of the "ripple" and will be a real 12 volts. Put a DC voltmeter across the load above, one will read 5.4 volts. Switch the meter to AC, one will still read a voltage of some value. This is normal as one is reading the "ripple" riding the DC. Connect the same AC voltmeter across a clean DC source such as a car battery, one will read zero volts AC.

Full-wave rectification

Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each. Four rectifiers arranged this way are called a diode bridge or bridge rectifier.

Note the frequency has doubled! 60 hertz in will be 120 hertz out.

Power delivered here is much greater than half-wave rectification because we are using both half-cycles. Using 12 volts AC again, we have 12 X 1.414 or 17 volts peak. But now to get the average we multiply peak (17 volts) by 0.637 which equals 10.83 volts, double that of half-wave.



In the above example the capacitor (C) acts as a voltage storage device and partially discharges between half-cycles. In this case we have a pulsating DC with a much smaller ripple component. Note that because the capacitor never discharges back to zero, we can't use this in a triac or SCR circuit as explained below.

In our final example we use a center-tapped transformer for full wave output. In this case the voltage will be half. So a 24 VAC secondary will produce produce 12 volts X .9 = 10.83 volts pulsating DC. T in this case would be half of 16.7 milliseconds or about 8.35 milliseconds.

Pictures:

3 phase rectifier
Bridge Rectifiers
Diodes
"Stud" rectifier (high current)
Transformers, four salvaged from microwave ovens (PC board mount), one from Radio Shack

Turning a Diode On/Off

Pictured above is a silicon controlled rectifier (SCR) or thyrister. It's a diode with a "gate." An SCR not only conducts in one direction like any other diode, but the gate allows the conduction itself to be cut on and off. When the ON switch is pressed, the SCR is turned on, and current flows from negative to positive through the SCR and Load to the positive terminal. Once turned on, the SCR will remain on until the Off switch is pressed, breaking the current path.

Note that the ON switch is referred to as 'normally open' (N.O.) and makes (closes) a connection when pressed. The OFF switch is referred to as 'normally closed' (N.C.) breaks (opens) the connection when pressed. Both of these are push button switches. In electrical terms an 'open' is an undesired broken connection while a 'short' is an undesired connection.

In the circuit above the Load is a DC motor. Press the ON switch the motor runs and will run until the Off switch is pressed. Note the direction of a DC motor depends on polarity. Reverse the leads on the motor, it will run in the opposite direction.

In this example we have placed a diode in series with the gate on/off switch. When one presses the ON switch, the motor will run, the light will come on, etc. When the switch is released, the power is killed without use of an OFF switch. This is because the AC input goes back to zero volts at 180 and 360 degrees shutting off the SCR. And as a diode, the SCR only conducts one-half the cycle.

In this circuit example we have placed variable resistor (potentiometer) in series with the gate diode. (This was also known as an old style volume control knob.) By "turning the knob" we in alter the trip point in turning on the SCR only part of the half-cycle or if enough resistance, turn the SCR off. For a picture of a variable resistors click here.

Triacs

A triac is a solid state AC switch. A small current on the gate terminal can switch very large AC currents. Think of a triac as two back-to-back SCRs where the cathode of one SCR is connected to the anode of the other and vise-versa. The gates are connected together. Because we have the two SCRs type configuration allows the switching of both half-cycles.

Closing the switch will cut on the triac. The idea is to use a small low-power switch to control high power devices such as motors or heaters. The danger is here is the high voltage AC is on the switch.

Refer to page 398-401 in your textbook for lamp dimmers.


Basic circuit


Better


Best response with a diac.

The key to successfully triggering a TRIAC is to make sure the gate receives its triggering current from the main terminal 2 side of the circuit (the main terminal on the opposite side of the TRIAC symbol from the gate terminal). Identification of the MT1 and MT2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.


Commercial lamp dimmer in 220 volt countries. BR100 is a diac.

A DIAC provides cleaner switching for the triac. DIACS are specialized Shockley diodes connected back-to-back.

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 (pdf file)





Note: pdf files require Acrobat reader.

Posted February 15, 2009. KIM-1 my first computer

PDF files and spec sheets

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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.

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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