TCA785 circuit, datasheet & 3 phase control

How to use TCA 785 IC #Triac #SCR

TCA 785 Application Circuits

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This circuit uses the TCA 785 IC to drive two reverse parallel thyristors for AC power control applications. This configuration is particularly effective for controlling high power loads such as heaters or industrial motors with power ratings up to 15 kW.

How the TCA 785 Works in This Circuit

The TCA 785 is a highly integrated phase control IC designed for use in triggering thyristors or triacs. In this setup, it provides dual gate trigger pulses that are precisely synchronized with the AC line voltage. These pulses are sent to the gates of two antiparallel-connected thyristors (shown as THY00UL / MTT40A08N in the diagram), enabling bidirectional control of AC power.

Key Functional Stages of the Circuit:

Phase Angle Control:

1. A 10 kΩ potentiometer is used to adjust the firing angle from 0° to 180°, allowing smooth variation of power delivered to the load.

2. The reference voltage for phase control is stabilized using resistors and a zener diode.

Pulse Generation:

1. The IC’s pins 14 and 15 provide the trigger pulses:

1.1. Pin 14 activates during the negative half-cycle.

1.2. Pin 15 activates during the positive half-cycle.

2. These pulses are fed to pulse transformers (IT1 and IT2) to electrically isolate and transmit signals to the thyristor gates.

Pulse Amplification:

1. Transistors BD875 are used to amplify gate trigger signals.

2. Diodes and zener diodes (e.g., 1N4001 and BZX83C20) protect the transistors and clamp voltages to safe levels.

Snubber Network:

1. Each thyristor is protected by a snubber circuit comprising a capacitor and resistor to suppress voltage spikes during switching.

Load Control:

1. The output controls a high-power AC load (15 kW) by modulating the conduction angle of the thyristors, thus varying the effective RMS voltage across the load.

Pulse Rejection and Safety:

1. A dedicated pulse rejection switch is provided on pin 8 to block triggering under unwanted conditions.

Features

1. Continuous phase angle adjustment from 0° to 180° for precise power control

2. Dual thyristor drive with galvanically isolated triggering via pulse transformers

3. High-load capability: suitable for applications up to 15 kW

4. Robust protection through snubber circuits and zener voltage clamping

5. Compact design with minimal external components

6. Reliable triggering during both AC half-cycles with synchronized pulses

7. Integrated pulse suppression for added safety and noise immunity

FAQ

1. How does the TCA 785 control power?

It delays the firing angle of the thyristor in each AC cycle, allowing control over the portion of the waveform delivered to the load, effectively adjusting the power.

2. What kind of loads can the TCA 785 control?

It can manage both resistive and inductive loads, making it suitable for heating systems, AC motors, and industrial dimming.

3. What are typical companion components for the TCA 785 circuit?

Common supporting parts include BD875 transistors, 1N4001 diodes, BZX83 zener diodes, and isolated gate drive transformers.

4. Can the TCA 785 be used in a single-phase or three-phase system?

It is mainly designed for single-phase systems; controlling three-phase loads would require additional synchronization and circuitry.