Vishay OPTO Squad

SSRs, a Solid Alternative to an Age Old Challenge

Electrical relays have been around since the civil war. It is safe to say that they are not “leading edge” technology.  Yet, they are ubiquitous.  It is doubtful that you can go a day without activating several electrical relays.  If you drive a car, wash clothes, brew coffee, open a refrigerator door….   Relays make modern life possible, safe, and comfortable.

Though relays have developed significantly in the century in a half since “The War of Northern Aggression”, the fundamental operating principle of mechanical relays has not.  Basically, a current is applied to a coil, which generates a magnetic field. This magnetic field attracts a piece of ferrous metal, the armature, which in turns closes a circuit.  Thus a relatively small current can control a much larger current.  This control feature is as crucial to modern electrical systems today as it was when Abe Lincoln contemplated a political career.


To this day, this mechanical solution to electrical power control dominates electrical process control.  Yet, this solution is fraught with significant limitations.  Mechanical relays involve moving mechanical parts, and moving parts are subject to wear and failure.  Mechanical relays generally operate in air, where the opening and closing of an electrical circuit leads to arcing, which causes additional wear and generates electrical noise.  Mechanical relays do not simply open and close nicely, they “bounce” open and close, generating ‘more’ electrical noise and wear.  The list goes on.

With the advent of semiconductors after WWII and especially with the invention transistors and MOSFETs in the 1950’s and 1960’s, a new solid state alternative to the clickety Clack of the venerable mechanical relay came into the technical horizon.  No one embraced this requirement more than Ma Bell. With millions of phone circuits spread out throughout the country, and with the high cost of servicing these phone systems, the telephone company needed a more reliable solution. Thus AT&T pioneered some of the first practical solid state relays. AT&T was acquired by Siemens and later this division of Siemens became part of Vishay Optoelectronics.

ssrchallege2Thus today designers are presented with a wide range of solid state relay solutions which feature the exact functional analog of traditional mechanical relays, but without the reliability concerns.  The concept behind the Solid State Relay is deceptively simple. An LED generates photons which cross an electrical barrier and activate a photovoltaic stack. This PV stack in turn drives a circuit which turns on a pair of MOSFETs.  Instead of driving a coil with current, an LED is driven with a very small current, and instead of a mechanical contact closure a solid-state “contact closure” takes its place.

Today a wide variety of SSRs are available off the shelf in a wide range of small packages, load voltages, and current.  In a single package SSRs can be obtained capable of amps of current, and up to hundreds of volts.  They are available in NO (normally Open, or Form A), NC (normally closed or Form B), as well as “make before break”, and other common relay configurations.   Today, a designer has a large array of simple and easily implemented solid state relay solutions in small packages limited only by the imagination of the designer.


Article by Jose Espina – copyright 2016

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This entry was posted on October 21, 2016 by in Articles, Optocoupler, Solid-State Relay (SSRs).
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