Tiristor TRIAC Funcionamiento

Understanding the SCR and TRIAC

Introduction to SCR

• The discussion begins with a focus on the SCR (Silicon Controlled Rectifier), highlighting its function as a diode activated by a small current signal sent through its gate.

Activation and Functionality of SCR

• The SCR operates like a switch, closing when it receives a signal at the gate, allowing current to flow in one direction from anode to cathode.

• Once activated, the only way to deactivate the SCR is if the current flowing through it drops below a specific value known as "holding current."

Deactivation Methods

• To turn off the SCR, one can either reduce the circulating current below holding current or introduce an external switch that interrupts the circuit.

• Another method for deactivation involves removing power from the SCR entirely.

Triggering Mechanism

• Activation requires sending both a triggering current and voltage to the gate; these parameters are crucial for proper operation.

Transition to TRIAC

• Unlike an SCR which conducts in one direction, a TRIAC allows bidirectional conduction of current, functioning similarly but with added versatility.

TRIAC Characteristics

Internal Structure of TRIAC

• A TRIAC can be visualized as two inverted SCR devices connected in parallel, enabling it to conduct electricity in both directions.

Terminal Designations

• The terminals of a TRIAC are labeled T1 and T2 (or A1 and A2), with one terminal designated as the gate. This configuration facilitates activation similar to that of an SCR.

Activation Process for TRIAC

• Like an SCR, activation occurs via its gate; however, due to its design, it can handle currents flowing in either direction.

Key Specifications and Data Sheets

Understanding Specifications

• Important specifications such as trigger voltage and holding current must be referenced from data sheets for accurate application.

Example: Z0405 TRIAC

• The Z0405 model can operate up to 600 volts and handle peak currents up to 21 amperes briefly. These details are essential for practical applications.

Understanding Triacs and Their Operation

Current Specifications for Triac Activation

• The normal operating current for the triac is 4 amperes, with a maximum triggering current of 5 milliamps (mA) required to activate it.

• The gate trigger voltage can reach up to 1.3 volts; however, this value may vary based on testing conditions.

• It is essential to ensure that the gate receives at least 5 mA for reliable operation, although lower currents can also work within specified limits.

Maximum Current Handling

• The maximum gate current that the triac can handle is 1.2 amperes; exceeding this could damage the device.

• A resistor should be placed in series with the gate to limit current and prevent damage while ensuring proper activation.

Circuit Design Considerations

• The triac functions as a switch controlled by a signal through its gate terminal; it requires an additional circuit element (normally closed switch) to deactivate it.

• To turn off the triac, one method involves opening a normally closed switch in the circuit, which interrupts current flow.

Deactivation Methods

• Another way to deactivate the triac is by reducing the circulating current below its holding current threshold or temporarily cutting power from the circuit.

• This approach will effectively reset the triac until reactivated by sending another pulse through its gate.

Resistor Calculation for Gate Activation

• When designing circuits, care must be taken to include a resistor in series with the gate during activation to avoid exceeding safe limits.

• Using Kirchhoff's law helps determine necessary resistance values based on supply voltage and desired trigger conditions.

Practical Application Example

• In practical applications, such as using a Z45 component, it's crucial to monitor how much current flows through the gate when powered by a battery (e.g., 9V).

Understanding the Circuit and Triac Functionality

Circuit Analysis and Calculations

• The circuit is powered by a 9V battery, which supplies voltage to both the gate and other components.

• Using the formula discussed earlier, the voltage drop across the gate is calculated as 1.3V, leading to a current calculation of approximately 7.7mA through a 1kΩ resistor.

• This current (7.7mA) exceeds the minimum requirement of 5mA for triggering the triac, ensuring reliable operation without exceeding its maximum rating of 1.2A.

• The configuration includes a 1kΩ resistor connected to pin 3 of the triac, facilitating proper gate activation as per datasheet specifications.

• Two LEDs are arranged in an antiparallel configuration; one LED faces down while the other faces up to indicate circuit status.

Testing Circuit Functionality

• Initial tests involve applying positive voltage to terminal two and negative to terminal one, observing which LED activates based on gate signal input.

• When a negative signal is applied at the gate via a 1kΩ resistor, no LEDs light up; however, applying a positive signal illuminates the red LED.

• To deactivate the triac once activated by sending a positive signal, power must be removed temporarily from the circuit.

• The red LED remains lit after initial activation until power is cut off or another method is employed to turn off the triac.

Inverting Power Supply Polarity

• A test will be conducted by reversing polarity: connecting positive to what was previously negative and vice versa.

• Observations will focus on whether activating with positive or negative signals affects which LED lights up during this polarity change.

Understanding TRIAC Functionality

Basic Setup and LED Behavior

• The setup involves connecting the negative terminal to "laos" and the positive terminal to "uno." The speaker tests which connection (positive or negative) will light up an LED.

• Initially, the positive connection does not activate any LEDs. When the negative connection is applied, a green LED lights up, indicating that current flow is dependent on polarity.

Current Flow and TRIAC Operation

• Sending a negative signal activates the green LED, demonstrating how current flows through the internal structure of the TRIAC when activated by a negative input.

• The TRIAC can conduct current in both directions, making it suitable for alternating currents (AC). This dual-direction capability allows for effective control over AC signals.

Understanding Alternating Current Signals

• An alternating signal consists of both positive and negative phases. The speaker emphasizes that understanding this behavior is crucial for practical applications involving phase control with TRIACs.

• When a positive voltage reaches the gate of the TRIAC, it activates and allows current to flow in one direction; conversely, when a negative voltage arrives, it reverses direction.

Phase Control Applications

• The discussion transitions into phase control using TRIACs. When alternating current changes from positive to negative, different LEDs are activated based on polarity.

• While currently demonstrated with LEDs, these principles apply broadly in AC circuits where capacitors may also be used for phase control.

Key Differences Between TRIAC and SCR

• A critical distinction between TRIAC and SCR (Silicon Controlled Rectifier): TRIAC conducts in both directions while SCR only conducts in one direction. This feature makes TRIAC more versatile for AC applications.

• Important characteristics include maintenance current—if it drops below a certain threshold, the TRIAC turns off—and firing voltage requirements necessary for activation.

Voltage Drop Considerations

• Unlike an ideal switch that would show no voltage drop when closed, a small voltage drop occurs across a functioning TRIAC due to its inherent properties.

• For example, data sheets indicate that when activated, there could be up to 2V drop across the device. This value varies but serves as an important consideration during circuit design.