What is a solid state relay?

A solid state relay 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.
An alternative to Electro-mechanical relay offering enhanced electrical performance and reliability. A switch that has no moving mechanical parts - Solid State

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Why use a solid state relay?

  • 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/li>
  • Low Power Consumption
  • Reduced Electromagnetic Interference

Why do some solid state relays have off state leakage current?

Leakage current comes from the snubber network. The amount of leakage current( typically 10-15 ma ) is dependent on the size of the capacitor used in the snubber network. Crydom uses inductive snubber network instead of resistor capacitor networks in most of their relays, which will have atypical leakage current.

What is meant by instantaneous or random turn-on?

When the control voltage is given the relay output will turn-on regardless of what the line voltage is.

I have an a solid state relay that turns on but will not turn off. Why?

First measure the control voltage. The input of a DC input solid state relay (SSR) must go below one volt to guarantee that the relay will shut off (10Vrms for AC input SSR). If there is less than one volt across the input (10Vrms for an AC SSR) or if the input is completely disconnected, then the problem most likely resides in the output of the relay.

The two most likely reasons for an SSR failing to turn off are: the SCR's have been damaged, or there is an over-temperature condition. When SCR damage occurs, the relay will typically half-wave. This is due to the fact that usually only one of the SCR dies will become damaged. This can result from excessive surge currents, transients on the AC mains, or other electrical phenomena. To determine if this has occurred, allow the relay to adequately cool down (approximately 15 minutes) and connect to a small load, such as a 100W bulb. If the bulb is illuminated without the control signal, then the relay's output is shorted. Likewise, if the bulb is only partially illuminated when the control signal is applied, then the relay is half-waving.

The relay will also fail to turn off when the output SCRs exceeds their maximum rated temperature of 125"C. To determine whether a relay is "running" hot, measure the temperature of the base plate near the mounting screw. The temperature should not exceed 80"C to 90"C. If the relay exceeds 80"C, then the heat sink of the application must be reevaluated. Also, ensure that there is an adequate amount of thermal compound between the base plate and the heat sink. However, it is also important to note that too much heat sink compound will also impede the relays ability to adequately transfer heat to the heat sink.

I am using an AC output SSR for use with a DC load, but the relay will not shut off. Why?

AC output SSR's are zero current turn-off relays and thus must cycle through the zero current before it can turn off. You must use a DC output relay to control a DC load

When would you use a SCR SSR, Triac SSR?

Triac output SSR's are general-purpose relays typically used for resistive type loads only. The SCR output solid state relays are used to switch resistive or inductive loads, especially those with high inrush currents

What is minimum Off-State DV/DT?

The rate of rise of applied voltage across the output terminals that the SSR (AC) can withstand without turning on in the absence of a turn-on control signal. Usually expressed as a minimum value at maximum rated voltage in terms of "volts per microsecond" (V/us)

What is the difference between a solid state relay & electromechanical relay?

SOLID STATE RELAY:

Solid state relay have a long lifespan and superior reliability when exposed to shock and vibration. They are small-sized, require less control power, and have very short response times. Loads can be switched with no bounce and no switching noise is generated. Neighbouring components cannot be disturbed during the switching process by the electromagnetic radiation typically caused by coils or switching sparks. However, when compared to mechanical relays, the solid-state relays experience more power loss on the load circuit. For this reason they are often mounted on a heat sink.

ELECTROMECHANICAL RELAY:

Electromechanical relay: can be used for many purposes because they can function in AC and DC load circuits. When their contact is open, the relays do not allow any leakage current to flow through. The closed contact has a low resistance and creates only minimal power loss in the load circuit. Multiple contacts are also possible so that a control signal can simultaneously switch multiple contacts and thus multiple load circuits. Electromagnetic relays are also very insusceptible to transients.
However, in contrast to solid-state relays, the moveable contacts and reset springs found in mechanical relays are susceptible to wear and tear