Onsemi MCR100-6 Thyristor: Key Specifications and Application Circuit Design
The Onsemi MCR100-6 is a widely used sensitive gate silicon controlled rectifier (SCR) designed for low-power AC switching applications. Its compact plastic package and sensitive triggering characteristics make it a popular choice for consumer electronics, industrial control systems, and various household appliances. Understanding its key specifications and how to implement it in a circuit is crucial for effective design.
Key Specifications
The MCR100-6 is part of a series of sensitive gate SCRs that require very low gate current to trigger. Its primary specifications include:
Repetitive Peak Off-State Voltage (VDRM): 600 V. This defines the maximum forward and reverse voltage the device can block repeatedly.
On-State Current (IT(RMS)): 0.8 A RMS. This is the maximum continuous RMS current it can conduct in the on-state.
Gate Trigger Current (IGT): Max 200 µA. A key feature, this extremely low current requirement allows it to be triggered directly from high-impedance sources like CMOS logic chips or microcontroller pins.
Gate Trigger Voltage (VGT): Max 0.8 V.
Holding Current (IH): Max 5 mA. This is the minimum anode current required to keep the thyristor conducting after it has been triggered.
Package: TO-92. This common through-hole package makes it easy to prototype and use on standard breadboards and PCBs.
Basic Application Circuit Design
A classic application for the MCR100-6 is in AC power control, such as a simple light dimmer or motor speed controller. The circuit below illustrates a basic half-wave control setup.
Circuit Components:
1. Onsemi MCR100-6 Thyristor
2. AC Source (e.g., 120VAC, 60Hz)
3. Load (e.g., a small lamp or motor, rated for the AC source and less than 0.8A)
4. Diode (D1) - A general-purpose diode (e.g., 1N4007) to protect the SCR from reverse voltage transients.
5. Variable Resistor (R1) - Acts as a timing/potential adjuster.
6. Capacitor (C1) - Works with R1 to form the RC timing network that controls the firing angle.
7. Trigger Diac (D2) - A bidirectional trigger device (e.g., DB3) that provides a sharp pulse to the SCR's gate when the capacitor voltage reaches its breakover voltage.
How the Circuit Works:
During each positive half-cycle of the AC input, the capacitor C1 charges through the variable resistor R1. The rate of charging is determined by the resistance of R1. When the voltage across C1 reaches the breakover voltage of the Diac (typically around 30V), the Diac fires and rapidly discharges the capacitor into the gate of the MCR100-6. This low-energy pulse triggers the SCR into conduction. The SCR then latches on for the remainder of that positive half-cycle, delivering power to the load.
The moment the AC voltage crosses zero, the SCR turns off (commutates) and remains off until triggered again in the next cycle. By adjusting R1, you change the time it takes for the capacitor to charge. This, in turn, changes the firing angle—the point in the AC phase where the SCR is triggered. A longer delay results in less power delivered to the load (dimmer light, slower motor), while a shorter delay delivers more power.
Design Considerations:
Gate Protection: Although the sensitive gate is an advantage, it is also susceptible to electrostatic discharge (ESD) and voltage transients. Avoid static discharge during handling.
Heat Sinking: For currents approaching the 0.8A maximum, a small heat sink may be necessary to prevent thermal overstress.
Snubber Circuit: In circuits with inductive loads (like motors), an RC snubber network across the SCR is often required to suppress voltage spikes that could cause false triggering or damage.
The Onsemi MCR100-6 is an excellent choice for designers seeking a reliable, cost-effective, and easy-to-use thyristor for low-power AC switching. Its extremely low gate drive requirement allows for simple interfacing with control logic, making it a fundamental component in a vast array of electronic control applications. Understanding its parameters and the principles of phase-angle control is key to a successful design.
Keywords:
1. Thyristor/SCR
2. Gate Trigger Current (IGT)
3. Firing Angle
4. AC Phase Control
5. Sensitive Gate
