What is a solid-state control?

When an external voltage (AC or DC) is applied across the terminals of an electronic switching device that switches on or off is called solid-state relay (SSR) control. The solid-state relay control has no moving parts, and it serves the same function as an electromechanical relay that results in a lifetime operation. The solid-state relay responds to an appropriate input or control signal because it consists of a sensor. The solid-state relay also responds to a type of solid-state switching device that switches power to the load circuitry thereby enabling the control signal to activate this switch it consists of a coupling mechanism. The SSR are designed to switch either AC or DC load. The packaged SSR use power semiconductor devices such as transistors (either BJT or MOS) and thyristors to switch currents up to around a hundred amperes that are maximum current ratings. As compared to other electromechanical relays these SSR have fast switching speeds and have no physical contacts to wear out. The way an electromagnetic relay can withstand a large momentary overload, the SSR does not have such ability and the user of the solid-state relay must consider this, as well as their higher "ON" state resistance.

Operation

For DC loads, a single state relay is based on a single MOSFET or multiple numbers of MOSFETs in a paralleled array in stock can work well. A single MOSFET can not block current in both directions because MOSFET's have an inherent substrate diode that conducts in the reverse direction. With the source pins tied together, two MOSFET's are used back-to-back for AC operation. To either side of the output, the drain pins are connected. When the relay is off to block current the substrate diodes are alternately reversed biased. The common source is always riding on the signal that is instantaneous signal level when the solid-state relay is ON. By the photodiode, both the gates are biased positive. So that the multiple MOSFETs in stock can be wired in parallel, it is common to provide access to the common source while switching a DC load. When the control input is removed a network is provided to speed the turn-off of the MOSFET. At the point of zero load currents, SCR (silicon-controlled rectifier) or TRIAC relays inherently switch off in AC circuits. The large transient voltages that might occur due to sudden collapse of the magnetic field-effect around the inductance would be prevented by not interrupting the circuit in the middle of the sine wave peak. The individual SCR's can be switched back on at the start of a new wave with the addition of a zero-point detector and this feature is known as zero-crossover switching.

Coupling

In a way that provides galvanic isolation between the two circuits, the control signal must be coupled to the control circuit. Optical coupling or photocoupler is used by many solid-state relay. The solid-state relay control voltage tries to energize an internal light-emitting diode (LED) that illuminates and turn-on a photo-sensitive diode. To switch the load the diode current turns back-to-back TRIAC, SCR, or MOSFET. To electrically isolate the control circuit from the load, the optical coupling is used. The opto-isolator is designed to switch a small amount of current and a solid-state relay consists of an opto-isolator to switch a large amount of current in response to a small current.

Parameters

Solid-state relays have parameters such as current, input voltage, output voltage, thermal and electrical parameters for operating area to be safe, thermal resistance, etcetera. When the AC voltage is zero, the SSR can include zero crossing hardware to only turn the voltage instant on or instant off. The proportional solid state relay SSR can delay the onset of voltage, after the zero-crossing to lower the current input. A SSR can use a bridge rectifier as a load. Please view all the SSR before using whether it is working well or not.

Advantages and Disadvantages

The advantages of solid-state relays over electromechanical relays are common to all the solid-state devices when they are compared with electromechanical devices. One must view all the advantages before using a solid-state relay for more benefit.

• Then mechanical relays, SSR have an inherently slimmer and smaller profile. It also allows tight packaging.
• They are silent in operation.
• The SSR switch faster than electromechanical relays. The optically coupled SSR depends on the time needed to power the LED on or off that happens in the micro to milliseconds.
• As there are no moving parts to wear and no contacts to build up or pit up carbon, it has increased lifetime even if it is activated many times.
• Regardless of the amount of use the output resistance remains constant.
• They have a bounceless and clean operation.
• As no spark is generated during switching, it can be allowed to be used in explosive environments.
• They are much less sensitive to operating environment factors and storage such as vibration, mechanical shock, humidity, etcetera.

The disadvantages of solid state relay as compared to mechanical relays are given below:

• Rather than the mechanical contacts, the voltage or current characteristics of the semiconductor has the following effects :
  • The resistance becomes higher and electrical noise increases when closed.
  • The resistance is lower and the leakage current is reversed, when opened.
  • The switched waveforms are distorting to some extent and the voltage or current characteristics are not linear. When activated, the electromechanical relay has the low ohmic resistance of the associated switch which is mechanical and also has extremely high resistance of the air gap and insulating materials when open.
  • Some SSR have polarity-sensitive output circuits. By polarity, the electromechanical relays are not affected.
• Due to voltage transients, there is a possibility of spurious switching.
• Required for gate charge circuit they have isolated bias supply.
• Due to the presence of the body diode, they have a higher transient reverse recovery time (T_rr).
• While electromechanical relay contacts tend to fail "open" then there is a tendency to fail "shorted" on their outputs.
• All kind of solid-state relays produces heat and this heat will cause a lowering (or derating) of the current flowing in the load.

Context and Applications

This topic is significant in the professional exam for undergraduate, graduate, and postgraduate courses like:

• Bachelors of Technology in Electrical Engineering
• Masters of Technology in Electrical Engineering

Practice Problems

1. Among the below-listed device relays, which relays function the same as solid-state relays?
a. Electromechanical device relay
b. Electrochemical device relay
c. Electrical device relay
d. None of the above

Answer- a
Explanation: The solid-state relay control has no moving parts and it serves the same function as an electromechanical relay and so results in longer lifetime operations.

2. What is the maximum rating of current in an SSR?
a. Around a hundred of amperes
b. Around a thousand of amperes
c. Around millions of amperes
d. None of the above

Answer- a
Explanation: To switch currents up to around a hundred amperes that are maximum ratings of current, the packaged solid-state relays uses power semiconductor devices such as transistors (either BJT or MOS) and thyristors.

3. What kind of profile does SSR have?
a. Larger and thicker profile
b. Smaller and slimmer profile
c. Smaller and thicker profile
d. Larger and slimmer profile

Answer- b
Explanation: Than mechanical relays, the solid-state relays have an inherently slimmer and smaller profile.

4. How does an SSR switch in comparison to electromechanical relays?
a. Slower
b. Faster
c. Same
d. None of the above

Answer- b
Explanation: The solid-state relays switch faster than electromechanical relays. The time needed to power the LED ON or OFF is micro to milliseconds in optically coupled solid-state relay.

5. How much is the reverse recovery time in the case of an SSR in comparison to an electromechanical relay?
a. Lesser
b. Higher
c. Same
d. None of the above
Answer- b
Explanation: Due to the presence of the body diode, the SSR has a higher transient reverse recovery time (Trr).

Related concepts

• Opto-isolator
• Opto-coupler