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What is a solid state relay? How does SSR relay work?

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Solid State Relay: An introduction of the solid state relay. All you questions about ssr relay will be answered in this article. Get to be an expert on the solid state relay and solid state module.

LOS ANGELES - Amzeal -- HUIMU Industrial (HUIMULTD):
Overview of solid state relays, following are the basics of SSR-- definition, types, working principle, structure, circuit diagram, instructions and etc.

§1 What is a solid state relay (SSR)?
Solid State Relay (also known as SSR, SS Relay, SSR relay or SSR switch) is an integrated contactless electronic switch device that is compactly assembled from an integrated circuit(IC) and discrete components. Depending on the switching characteristics of the electronic components (such as switching transistors, bi-directional thyristors and other semiconductor components), the SSRs are able to switch the "ON" and "OFF" state of the load very quickly through the electronic circuit, just like the function of traditional mechanical relays. Compared with the previous "coil-reed contact" relay, namely Electromechanical Relay(EMR), there is no movable mechanical part inside the SSR, and there is also no mechanical action during the switching process of the SSR. Therefore, the Solid-State Relay is also called "non-contact switch".

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The structural characteristics of the SSR switch make it superior to the EMR. The main advantages of solid state relays are as follows:

● The semiconductor component acts as a switch for the relay, which is small in size (compact size) and long in life (long lifetime).

● Better Electro-Magnetic Compatibility than EMR - immunity to Radio Frequency Interference (RFI) and Electro-Magnetic Interference (EMI), low electromagnetic interference, and low electromagnetic radiation.

● No moving parts, no mechanical wear, no action noise, no mechanical failure, and high reliability.

● No spark, no arc, no burning, no contact bounce, and no wear between contacts.

● With "zero voltage conduction, zero current shutdown" function, easy to achieve "zero voltage" switching.

● Fast switching speed (SSR switching speed is 100 times higher than general EMR), high operating frequency.

● High sensitivity, low electrical level control signals (SSR can directly drive large current loads through the small current control signals), compatible with logic circuit (TTL, CMOS, DTL, HTL circuits), easy to implement multiple functions.

● Generally packaged by insulation material, with good moisture resistance, mildew resistance, corrosion resistance, vibration resistance, mechanical shock resistance and explosion-proof performance.

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Click here to know more information about the comparation of SSR and EMR. (https://www.huimultd.com/INFORMATION/Relays-Comparation/)

§2 What Is The Structure Of A Solid-State Relay?
The solid-state relays are four-terminal active devices, two of the four terminals are input control terminals, and the other two terminals are output control terminals. Although the types and specifications of SSR switches are numerous, their structures are similar and consist mainly of three parts (as shown in Figure 2.1): Input Circuit (Control Circuit), Drive Circuit, and Output Circuit (Controlled Circuit).

§3 What Is The Symbol Of Solid-State Relays?
The symbol of the solid-state relay in the circuit schematic is shown below (Figure 3.1).

§4 What Are The Types Of Solid-State Relays?
The types of solid state relays are various and the classification standards are multifarious. The solid state relays are usually classified according to the following criteria.
1. Load Power Supply Type:
The Solid-State Relay can be divided into DC solid state relays (DC-SSR) and AC solid-state relays (AC-SSR) according to the type of load power supply. The DC type solid state relays use the power semiconductor transistors as the switching element (such as BJT, MOSFET, IGBT) to control the ON/OFF state of the DC load power supply, and the AC type solid state relays use the thyristors (such as Triac, SCR) as the switching element to control the ON/OFF state of the AC load power supply
1.1 DC-SSR:
Based on the input form, DC type SSR can be divided into the Resistive Input Type DC solid state relays and the Constant Current Input Type DC solid state relays.
1.2 AC-SSR:
The AC type SSR can be classified according to the following standards.
1.2.1 Control Trigger Mode:
According to the control trigger mode (the timing of turn-on and turn-off), the AC SSR can be divided into zero-crossing Type AC solid state relays, Random Conduction Type AC solid state relays, and Peak Conduction Type AC solid state relays.
 1) Zero-crossing AC solid state relays (Figure 4.2), are also known as Zero Crossing Trigger AC solid state relays, Zero Switching AC solid state relays, Zero Voltage AC solid state relays, or Synchronous AC solid state relays. For the zero-crossing SSR relays, their switching state of the output circuit are synchronized with the output signal, that is, "synchronous" to the power supply.

§6 What Is The Working Principle Of Solid-State Relays?
Because of the different application environments, solid-state relays have slightly different internal components, but the working principle is similar. The internal equivalent circuit diagram of ordinary solid-state relays is shown in the figure below (Figure 6.1). The principle of solid-state relays can be simply described as: for the NO-SSR, when the appropriate control signal is applied to the Input Terminal (IN) of the solid-state relay, the Output Terminal (OUT) will be switched from the off state to the on state; if the control signal is cancelled, the Output Terminal (OUT) will be restored to the off-state.In this process, the solid state relays realize non-contact control of switch states of the load power supply which is connected to output terminals. It should be noted that the input terminal can only be connected to the control signal, and the load should only be connected to the output circuit.

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§7 What Is The Application Of The Solid-State Relays?

Lighting Control System:

Remote Control System:

Computer Numerical Control Machinery:

Heating/Cooling Equipment:

Medical Devices:

Electric Vehicles:

Chemical and Mining Industry:

Computer Control System:

§8 How To Select The Solid-State Relays?
The following are options to consider when selecting the appropriate solid-state relays based on the actual requirements:
 1) Load Voltage- AC or DC
 2) Load Current - Maximum Current and Minimum Current
 3) Load Type - resistive, inductive or capacitive
 4) Input Control Signal - AC or DC
 5) Mounting Method - PCB, Panel or DIN rail mounting
 6) Ambient Temperature - for calculating the Derating Factor and the size of the heat sink
 7) International Certification – Underwriter Laboratories (UL), Canadian Standards Association (CSA), British Approvals Board of Telecommunications (BABT), Verband Deutscher Elektrotechniker (VDE), Technischen Uberwachungs Vereine (TUV), Conformite Europeene (CE) or other.

§9 Attentions When Using Or Installing Solid-State Relays
1) The actual application conditions of the product must fully comply with the requirements of the parameters and characteristic curves of solid-state relays.

2) SSR should not be used in applications with a large number of low or high harmonic components (for example, multiple sets of loads at the output of the inverter need to be switched separately). If the solid state relay is used in inverter as an electronic switch, because of the higher harmonics, the solid state relays may not be able to switch reliably, and the RC circuit inside the SSR relays will be exploded due to overheating.

§10 Attentions When Testing Solid-State Relays
1) First the relationship between the output current and the shell temperature (ambient temperature) should be known before testing to avoid permanent damage to the SSR caused by overload, because the rated output current will drop when the case temperature rises or with no radiator.

2) When testing the turn-on and turn-off voltage of DC-SSR, the input voltage cannot remain in the state between on and off for too long, otherwise the power consumption of the output terminal rises sharply and burns out the output switching components.

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