What is a Light Activated Silicon Controlled Rectifier? (LASCR)

by Lewis Loflin

A light activated silicon controlled rectifier (LASCR) is a silicon controlled rectifier (Thyristor) that conducts when the gate is exposed to light. The gate still operates as a normal gate in a SCR, but is in many cases left disconnected. LASCR is a unidirectional device that conducts current in only direction. The basic LASCR circuit is illustrated above. Like all SCRs and triacs one must use A.C. or pulsating D.C. for proper operation. Also see Basic SCRs/Triacs.

While the light source to activate a LASCR can be a simple flashlight or light bulb, the source is often a light emitting diode or LED as illustrated above.

Illustrated above a LED and LASCR in the same package. This often six-pin device is called an opto-coupler. These devices provide electrical isolation between the input and output circuits. In the case of the of the H11Cx series illustrated above this will provide isolation of up to 5300 volts RMS. Also H11C6 SCR opto-Coupler (PDF file)

In the above circuit we are using a H11C6 opto-coupler to directly drive a small-watt light bulb. The SCR section of the opto-coupler is rated at 400 volts at 300 milliamps. (mA) Note that being a unidirectional device, the SCR will provide only half-wave rectification and the bulb will only be about half as bright. The input can be controlled from 5-volt computer logic.

Finally in the above circuit we are using a H11C6 through a diode bridge to switch on a triac. This circuit operates as full wave and delivers max power to the load.

LASCR opto-coupler with SCR controlling pulsating DC with a microcontroller.

Also see Simple DIAC Based Relaxation Oscillator Pulse Generator