AC Power Control with Thyristor

Phase Angle Control using TRIAC

Introduction to 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.

Principle of Phase Angle Control

Top - Output Voltage

Bottom - Gate Drive Signal

In the above example 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 here is the high voltage AC is on the switch itself. This can also be a big problem for solid state controllers unless they use a small relay, which some microwave do just that.

Above is a practical TRIAC test circuit. Press either switch and the lamp will come on at half brightness. Press both together full brightness. This allows testing of both SCR sides individually. The brightness should be the same for both sides or the TRIAC is defective. With no switch pressed, lamp should be totally off. R1 and R2 should be in the range of 100 to 470 ohms.

Basic TRIAC circuit with potentiometer.

Better triac circuit with capacitor.

Best response triac circuit with a diac.

The key to successfully triggering a triac is to make sure the gate receives its triggering voltage 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.

The DIAC, or 'diode for alternating current', is a trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the DIAC abruptly decreases, leading to a sharp decrease in the voltage drop across the DIAC itself thus producing a sharp increase in current flow through the triac gate.

This assures a fast, clean cut on of the TRIAC. The DIAC remains in its conduction mode until the voltage drops to a very low value far below the trigger voltage. This is called the holding current. Below this value, the diac switches back to its high-resistance (off) state. This behavior is bidirectional, meaning typically the same for both the positive and negative half cycles.

Most DIACs have a breakdown voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.

DIACs have no gate electrode, unlike some other thyristors. Some TRIACs contain a built-in DIAC in series (I've never seen one in the field) with the TRIAC's "gate" terminal for this purpose. DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or Mt1 ("Main Terminal") and Mt2. Most specification sheets don't bother to label A1/A2 or Mt1/Mt2.

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MPPT is usually considered in the field of Solar Power.

It is a technique used in solar charge controllers for the storage batter. It monitors battery voltage as well as output voltage of solar panel and regulates the charging current.

Solar panels deliver maximum power at peak power voltage (Vpp). The Vpp depends on intensity of solar radiation and surface temperature of the solar panel. Solar panel voltage is not constant throughout the day. Sometimes the solar panel produces output voltage will be more than the Vpp and other times, less. The additional voltage of the solar panel must be converted into additional current for the benefit of the battery. The MPPTs is used for it.

To charge a battery, the output voltage of PV module should be higher than that of the battery. If the PV module's Vpp is just below the battery voltage, then the current drops nearly to zero. So, the typical PV modules are made with a Vpp of around 17V when the cell temperature of 25º C. It will drop to around 15V on a very hot day. On the other side, in a very cold day, it can rise to 18V. Such kind of module is ideal for a standard 12V battery.

The MPPT varies the ratio between the voltage and current delivered to the battery, in order to deliver maximum power. If there is excess voltage available from the PV, then it converts that into additional current to the battery. As the Vpp of the PV array varies with temperature and other conditions, it "tracks" this variance and adjusts the ratio accordingly. Thus it is called a Maximum Power Point Tracker.

A block diagram to demonstrate the concept of MPPT is shown below.