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Triac Circuits

Optimizing the Triacs

Modern power control systems utilize electronic devices like Thyristors for power switching, phase control, chopper etc. These devices also find applications in inverter design, brilliance control in lamps, speed control of motors etc.

Triacs are the most common semiconductor devices used in power control and switching applications. The electronic power control circuits are designed to control the distribution or levels of AC or DC power sources. Such power control circuits can be used to manually switch power to electrical devices or to switch power automatically when parameters such as temperature or light intensities go beyond preset level Triac or Triode for alternating current is an electronic device equivalent to two silicon controlled rectifiers joined in inverse parallel (but with polarity reversed) with their gates connected together.

This results in a ‘bi-directional electronic switch’, which can conduct current in either direction when triggered. Like SCR, Triac is also a three terminal device. The MT1 and MT2 (Main Terminals 1 and 2) terminals are used to pass current in either direction while the third terminal G ( gate ) is used to send trigger pulse to the device.

Triac can be triggered by either a positive or negative voltage applied to its gate electrode. When the voltage on the MT2 terminal is positive with respect to MT2 and a positive voltage is applied to the gate, the ‘Left SCR’ in the triac conducts. If the voltage is reversed and a negative voltage is applied to the gate, the’ Right SCR’ conducts. Minimum holding current ‘Ih’ must be maintained to keep the triac conducting.

AC or DC pulses can trigger Triac and four modes of triggering are possible:

  1. Positive voltage to MT2 and positive pulse to gate
  2. Positive voltage to MT2 and negative pulse to gate
  3. Negative voltage to MT2 and positive voltage to gate
  4. Negative voltage to MT2 and negative voltage to gate

Triacs are borne with some inherent drawbacks, which will reflect in their working. Careful designing of triac based circuits give better performance in their working. The important drawbacks of Triacs are Rate effect, RF interference Backlash effect etc.

Triac Rate Effect

Between the MT1 terminal and gate of a triac, an’ Internal capacitance’ exists. If the MT1 terminal is supplied with a sharply increasing voltage, it causes enough gate voltage break through to trigger the triac. This condition is referred to as ‘Rate Effect’, an unwanted effect caused mainly by the high transients in the AC line. Rate effect also occurs when the load is switched on due to high ‘inrush voltage’.

Rate effect is severe particularly in driving inductive loads such as motor because the load current and voltage are ‘out of phase’. An R-C Snubber network will minimize the rate effect and makes the switching clean. The R-C Snubber network is connected between the MT2 and MT1 terminals of triac as shown in the figure.

Radio Frequency Interference (RFI)

Unwanted RF generation is another major problem encountered in triac switching. Each time the triac is gated on its load, the load current switches sharply from zero to a high value depending on the load resistance and supply voltage. This switching action (in a few microseconds) generates a pulse of RF1. It is least when the triac is triggered close to 00 and 1800 zero crossing points but maximum in 90 0 wave form. This is because at 00 and 1800 zero crossing points, ‘switch on current’ is minimum.

Switch on current is maximum at 900 producing very high RFI. The strength of RFI is proportional to the length of the wire connecting the load with the triac. The RFI is annoying particularly in lamp dimmer circuits and can be eliminated using a simple L-C- RFI suppression network.

Backlash Effect

A serious ‘Control Hysteresis’ or ‘Backlash’ develops in triac controlled lamp dimmer circuits, when the gate current is controlled by a variable potentiometer. When the resistance of pot meter increases to maximum, the brightness of the lamp reduces to minimum. After this, the lamp never turns on till the resistance of the pot meter is reduced to a few ohms, say 50 to 70 ohms. This occurs due to the discharging of capacitor connected to the Diac.

When the triac fires, capacitor discharges by the Diac and generate the ‘backlash effect’. This problem can be easily rectified by connecting a 47 to 100 ohms resistor in series with the Diac or adding a capacitor (C2) to the gate of the triac. This capacitor (C2) will slow down the backlash effect and the full turn effect can be obtained. The connection of capacitor is shown in figure.

Common Triacs

Triac Circuits

(Front view)

BT 136 600V – 4A MT1 MT2 G
BT 138 600V – 12A MT1 MT2 G
BT 139 600V 16A MT1 MT2 G
BTA 23 800V – 12A MT1 MT2 G
BTA 22 800V – 10A MT1 MT2 G
BTA 40 800V – 40A A1 A2 G
BTA 41 800V – 40A A1 A2 G

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