Hints and Tips - Solid State Relays

Solid State relays no moving parts like a mechanical relay.

The Mechanical relay was invented in 1835 in the USA

Solid Sate relays were invented by Crydom engineers in 1972.

What is a Solid State Relay?

A Solid State Relay (SSR) is a totally electronic device that depends on the electrical, magnetic and optical properties of semiconductors to control the flow of current in a circuit. SSRs are an alternative to Electro-mechanical relays offering enhanced electrical performance and reliability. The main characteristic of a Solid State Relay is that it has no moving mechanical parts, which translates into a much higher life expectancy, among other benefits.

Which are the benefits of using a Solid State Relay?

Solid State Relays have several benefits against their electromechanical counterparts. The following are only a few of them:

• Extremely Long Life
• Silent Operation
• No Moving Parts Eliminates Arcing & Switch Bounce
• Resistant to Shock and Vibration
• Reliable Operation in Harsh Environments
• Operation is Not Dependent Upon Orientation
• Low Input Power Consumption
• Reduced Electromagnetic Interference

 How do I select a Solid State Relay (SSR)?

When selecting a Solid State Relay several aspects must be taken into consideration, such as:

• Load Voltage
• Load Current
• Input Voltage
• Ambient Temperature
• Mounting Style (PCB, Plug-in, Panel or DIN Rail mounting)
• International Approvals (UL, CSA, VDE, TUV, etc.)

 Which are the typical applications for Solid State Relays?

SSRs offer a high reliability alternative to electro-mechanical relays when switching a wide range of loads such as motors, heaters, solenoids, lamps, compressors, fans, among others, in applications as diverse as:

• Coffee machines
• Commercial / Industrial cooking equipment
• Packaging machinery
• Plastic machinery (extrusion/thermoforming)
• HVAC & R systems
• Lighting control systems
• Lifts and escalators
• Medical equipment

What is the Mean Time Between Failures (MTBF) of Crydom Products?

Unlike a typical electromechanical relay, there are several variables that will directly impact the life expectancy of a solid state relay in a given application. These variables primarily revolve around the electrical characteristics of the application, but some mechanical issues also impact the relay's life expectancy. To assist with evaluating the reliability of their Solid State Relays, Crydom commissioned an independent specialized company to analyze the designs of several different Crydom SSR models. The final result yielded a MTBF rating of between 5 million and 34 million hours, depending upon the product family.

When to use “Random Firing” or “Instantaneous Firing”

For inductive loads it has generally been proposed that a zero cross turn on relay is used. Depending on the application and the power factor, it may be more suitable to use an instantaneous turn on relay for inductive loads. There is no standard rule but in general an instantaneous turn on relay is the preferred type for inductive loads.

For resistive loads a “Zero Cross” relay is generally used as this will only switch to the “ON” state once the relay detects a “zero cross” of the voltage. As voltage and current are in phase this means that the current starts around zero and there are not high and sharp flanks of current to the load.

Why there is a “minimum load current” for an AC SSR.

The AC relays have thyristor output elements and for these to stay “ON” a small amount of current must flow through them. This is called the “holding current”. If the load current is less than this “holding current” then the relay may behave erratically or not switch on at all.

In some cases, if the relay is in the “on” state and the SCRs do not have enough holding current and switch off, then the load current may be diverted over the optocoupler which - depending on the current – could damage the optocoupler.