onsemi MCR106-6G SCR: Key Features and Application Circuit Design Guide

The MCR106-6G from onsemi is a robust, industry-standard Silicon Controlled Rectifier (SCR) designed for AC power control and switching applications. This sensitive gate SCR is part of the popular MCR106 series, offering a reliable and cost-effective solution for a wide range of consumer and industrial electronics. Its ability to handle high surge currents and its sensitive triggering characteristics make it a preferred choice for designers.

Key Features of the MCR106-6G

The MCR106-6G stands out due to several critical attributes tailored for efficient power management:

High Surge Current Capability (`ITSM`): It can withstand non-repetitive peak surge currents up to 25 A, providing excellent resilience against unexpected current spikes and inrush currents commonly found in motor or lamp loads.

Sensitive Gate Triggering: The device features a very low Gate Trigger Current (`IGT`) of just 200 µA (max), allowing it to be driven directly from low-power sources like logic circuits, microcontrollers, or timing capacitors without needing a pre-amplification stage.

Repetitive Peak Off-State Voltage (`VDRM`): With a voltage rating of 400 V, it is well-suited for 120 VAC and 240 VAC mains applications, offering a sufficient safety margin for reliable operation.

Low Holding Current (`IH`): A low holding current of 5 mA (max) ensures the SCR remains latched on even when the load current drops to a very low value, which is crucial for maintaining conduction in various circuits.

TO-92 Package: The familiar and compact through-hole package makes it easy to prototype and integrate into a wide array of designs.

Application Circuit Design Guide

A primary application for the MCR106-6G is in AC phase-control circuits, such as light dimmers and motor speed controllers. Below is a guide for designing a basic AC lamp dimmer.

Basic AC Dimmer Circuit:

Component Selection and Design Considerations:

1. Main Switching Element: The MCR106-6G (SCR1) is the core component that switches power to the load (lamp). Its 400 V rating makes it suitable for this application.

2. DIAC Trigger Device: The DIAC (D1) is a bidirectional trigger device essential for generating a sharp gate pulse to reliably fire the SCR. A common DB3 or ST2 DIAC, with a breakover voltage of around 30 V, is typically used. It ensures the SCR triggers at the same phase angle for both halves of the AC cycle, providing symmetrical control.

3. Timing Circuit: The potentiometer (R\_pot) and capacitor (C1) form the RC timing network that determines the firing angle.

How it works: As the AC input voltage rises from zero each half-cycle, the capacitor (C1) charges through the resistor network (R\_pot + R1). The larger the resistance of R\_pot, the slower the capacitor charges.

Triggering: When the voltage across C1 exceeds the breakover voltage of the DIAC, it rapidly discharges into the gate of the SCR, turning it on. The SCR then conducts for the remainder of that half-cycle, supplying power to the load.

Adjustment: Rotating the potentiometer varies the RC time constant, which changes the point in the AC phase (the firing angle) at which the SCR triggers. This directly controls the amount of power delivered to the lamp, thereby adjusting its brightness.

4. Gate Protection Resistor (R2): This small series resistor (often 100-500 Ω) is critical for limiting the peak gate current during the discharge of the DIAC, protecting the sensitive gate-cathode junction of the SCR.

5. Snubber Network (Optional - R3, C2): For inductive loads (e.g., small motors), a simple RC snubber network across the SCR may be necessary to suppress voltage transients that could cause false triggering or damage the device.

Design Tips:

Ensure all components, especially the potentiometer, are rated for the AC line voltage.

Always implement proper electrical isolation and safety measures when working with mains voltage.

For microcontroller-based control, the sensitive gate allows the SCR to be driven via an opto-isolator (e.g., MOC3021) for safe isolation from the high-voltage AC side.

ICGOOODFIND

The onsemi MCR106-6G is an exceptionally versatile and robust SCR, ideal for a multitude of power control tasks. Its combination of high surge immunity, sensitive gate operation, and standard voltage rating provides designers with a perfect balance of performance, reliability, and ease of use. Whether in simple dimmer switches, over-voltage protection circuits, or motor controllers, the MCR106-6G remains a fundamental component in the power electronics landscape.

Keywords: Silicon Controlled Rectifier (SCR), Gate Trigger Current, AC Phase Control, Surge Current, Sensitive Gate