Wednesday, May 12, 2010

Ambient Light Is Everywhere! - The Effect of Ambient Light on High Voltage Opto-couplers - by D.Deschenes

The fact is, all of VMI's glass-body multi-junction silicon diodes are affected by ambient light to some small degree.

A condensed explanation of the physics of the interaction is this: A photon from ambient light strikes an electron in the silicon of the diode and causes movement of the electron, creating a current with a direction depending on the applied electric field on the diode. For VMI’s typical diodes, the silicon is passivated with a hermetic glass, which is opaque and keeps ambient light from significantly affecting the diode.

"Problem" Ambient Light in Opto-couplers

Our opto-couplers use that same principle to create a relationship between low voltage infrared LEDs and an exposed silicon high voltage diode. To make the opto-coupler sensitive to the infrared light of the LEDs embedded in the device, the entire opto-coupler is molded in an optically clear plastic. Because the part is molded in clear plastic, the exposed silicon of the high voltage diode can be excited by ambient light. Though the affect of ambient light on the leakage of the high voltage diode is small, it can be of particular annoyance when you desire the opto-coupler to be ‘off.’

The Solution

Typically, leakage caused in an opto-coupler by ambient light is solved by encapsulating the entire circuit in which the opto-coupler is being used. This is done usually for the primary reason of keeping high voltage from arcing and causing damage in the circuit. Over-encapsulation of high voltage sections within an assembly is quite common. Another possible solution to limiting ambient light would simply be to place the opto-coupler in an opaque box. Although I would refrain from using a metal box unless you have encapsulated the high voltage areas of the circuit. Because, you know…. arcing.  Just a thought.

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