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December 1, 2011 / Aiman Kiwan

How a PhotoMOS Solid State Relay Works

An Introduction to PhotoMOS Relays

A relay is an electrical component whose output is controlled in an open or closed position by applying or removing drive voltage from an electrically isolated circuit.   For many years electromechanical relays were the only choice to perform such a function.  During the last two decades, however, new technologies have emerged enabling semiconductors to switch output circuits with an electrical control signal via electronic, magnetic, optical or other means that require no mechanical movement.

Panasonic offers Semiconductor relays in two types, Solid State Relays (SSR) and PhotoMOS Relays.  The main difference between the two types can be found in the semiconductor device being used to switch the output: SSRs utilize Triacs and PhotoMOS relays employ two MOSFETs.  PhotoMOS relays are used in telecommunication, measurement, security and industrial control.

Figure 1: PhotoMOS relay construction

Construction:

 The construction of the PhotoMOS relay is illustrated in figure 1.  The input pins are connected to a Light Emitting Diode (LED).  The LED is mounted on the upper part of the relay at least 0.4mm across from a photo-cell.  The LED and the detector are molded in a translucent resin that allows the light to pass, but provides a dielectric barrier between input and output.  Dual MOSFETs are integrated on the output side to serve as a switch for the load circuit

 Advantages:

There are many advantages to using PhotoMOS relays over Electromechanical or other Solid State relays.  These advantages include:

  • Low power consumption
  • Low leakage current
  • Stable on-resistance over lifetime
  • High reliability with extremely long life
  • Small size
  • Fast switching speed
  • High vibration and shock resistance
  • No contact bounce and no switching noise

Principle of Operation:

The input side of a PhotoMOS relay consists of a light emitting diode.  If current flows through the LED, it emits light.  The light passes through translucent silicon resin and is detected by an array of photo sensors.  This results in a voltage drop across the photo sensors, which is used to drive the MOSFETs switching the load circuit.

Figure 2: PhotoMOS relay basic input circuit

In the next posting, we’ll discuss how to design the circuit around the PhotoMOS.

Please post your questions in the comments section, and we’ll respond, and also include them in future blogs.

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