“Solid state relays can be divided into AC type (AC-SSR) and DC type (DC-SSR) according to the load type. They are respectively used as load switches on AC or DC power sources and cannot be mixed. Changhui Instrument only introduces the commonly used AC solid state relays. The shape and principle of the AC solid state relay are shown in Figure 1. It can be seen from the figure that it is a four-terminal active device with two input control terminals and two output controlled terminals. It has both amplifying driving effect and isolation effect. It uses opto-isolator to electrically isolate the input and output.By adding a DC or pulse signal to the input, the output can be converted from an off state to an on state (no signal
Solid state relays can be divided into AC type (AC-SSR) and DC type (DC-SSR) according to the load type. They are respectively used as load switches on AC or DC power sources and cannot be mixed. Changhui Instrument only introduces the commonly used AC solid state relays. The shape and principle of the AC solid state relay are shown in Figure 1. It can be seen from the figure that it is a four-terminal active device with two input control terminals and two output controlled terminals. It has both amplifying driving effect and isolation effect. It uses opto-isolator to electrically isolate the input and output. When a DC or pulse signal is applied to the input, the output can be converted from an off state to an on state (blocking state when there is no signal), thereby controlling a large load.
Solid State Relay Classification
AC solid state relays are divided into voltage zero-crossing conduction type (referred to as zero-crossing type) and random conduction type (referred to as random type) according to the switching mode; according to the output switching element, there are triac output type (common type) and one-way type SCR anti-parallel type (enhanced type); according to the installation method, there are pin-in type used on the printed circuit board (natural cooling, no need for a radiator) and the installation type fixed on the metal base plate (cooled by a radiator); In addition, the input terminal has a wide range input (DC3-32V) constant current source type and series resistance current limiting type.
①The difference between zero-crossing type and random type solid state relay
When a valid control signal is applied to the input terminal, the output terminal of the random solid state relay is turned on immediately (with a speed of microseconds), while the zero-crossing solid-state relay will not turn on the conduction until the load voltage zero-crossing region (about ±15V). Pass. When the input terminal cancels the control signal, both the zero-crossing and random-type solid state relays are turned off when the current is less than the holding current. Although the zero-crossing solid state relay may cause a delay of half a cycle, it reduces the impact on the load and the generated radio frequency interference, making it an ideal switching device, which is widely used in “single pole single throw” switching occasions. Random solid state relay is characterized by fast response, it can control the phase-shift trigger pulse to easily change the voltage of the AC grid, and thus be used in resistive loads such as ground temperature regulation, light dimming and some inductive loads.
②The difference between the ordinary type of triac output and the enhanced type of unidirectional thyristor anti-parallel output
In the case of inductive load, when the SSR is turned off from the on-state, due to the inconsistency of the current and voltage phases, a large voltage rise rate dv/dt (commutation dv/dt) will be generated at both ends of the triac , such a value exceeds the commutation dv/dt index of the triac (typical value is 10V/μs), it will lead to delayed shutdown or even failure. The one-way thyristor is in a unipolar working state, and is only affected by the static voltage rise rate dv/dt (typical value is 100V/μs). The enhanced SSR ratio composed of two anti-parallel thyristors is composed of one The commutation dv/dt of the ordinary SSR composed of only triacs has been greatly improved, so the enhanced SSR should be selected in the case of inductive or capacitive loads.
Use of solid state relays
Solid state relays have been widely used in computer peripheral interface devices, electric furnace heating and constant temperature systems, numerical control machinery, remote control systems, industrial automation devices; signal lights, flashers, lighting stage lighting control systems; instrumentation, medical equipment, copiers, automatic washing machines; automatic Fire protection, security systems, as well as the switch of power capacitors for power factor compensation of the power grid, etc. In addition, they are widely used in chemical, coal mines and other occasions that require explosion-proof, moisture-proof and corrosion-proof.
Advantages and disadvantages of solid state relays
Solid-state relays work reliably, have a long life, no noise, no sparks, no electromagnetic interference, fast switching speed, strong anti-interference ability, and small size, shock resistance, vibration resistance, explosion-proof, moisture-proof, corrosion-proof, and can be compatible with TTL, DTL, Compatible with logic circuits such as HTL, it can directly drive large current loads with tiny control signals. The main shortcomings are that there is an on-state voltage drop (requires corresponding heat dissipation measures), an off-state leakage current, the AC and DC cannot be used universally, the number of contact groups is small, and the indicators such as overcurrent, overvoltage, voltage rise rate, and current rise rate are poor.
AC solid state relay basic performance test circuit
Under normal circumstances, the multimeter cannot judge the quality of the SSR. The correct method is to use the test circuit in Figure 2: when the input current is zero, the voltage measured by the voltmeter is the grid voltage, and the lamp does not light (the lamp power must be above 25W); When the input current reaches a certain value, the light is on, and the voltage measured by the voltmeter is the SSR conduction voltage drop (below 3V). (For first-time users, please pay attention: the leakage current is caused by the RC circuit inside the SSR, so it cannot be equivalent to the ordinary contact type relay and contactor, please refer to the following precautions).
Selection of solid state relays
The model names of solid state relays have not been unified, and the manufacturer’s instructions should be referred to when selecting models. The following conditions should be considered when selecting parameters.
①Rated input voltage
Refers to the allowable voltage rms value of the steady-state resistive load that can be withstood under given conditions. When used for inductive loads, the selected rated input voltage must be greater than twice the power supply voltage value, and the breakdown voltage of the selected product should be higher than twice the peak value of the load power supply voltage. When the AC load is 220V resistive load, SSR with 220V voltage level can be used; when the AC load is 220V inductive load or 380V resistive load, 380V voltage level SSR can be used; when the AC load is 380V inductive load, 480V voltage class SSR can be selected; 480V voltage class SSR can be selected for the forward and reverse rotation of the motor; 660~800V voltage class SSR can be selected for single-phase or three-phase motor loads with frequent starting.
②Rated output current and surge current
The rated output current refers to the rms value of the current that can be endured under given conditions (ambient temperature, rated voltage, power factor, with or without radiator, etc.). Solid state relays are highly sensitive to temperature. When the operating temperature exceeds the nominal value, it must be derated or used with an external heat sink. Usually, the manufacturer provides a thermal derating curve.
Inrush current is the non-repetitive peak current that is allowed under given conditions (ambient temperature, rated voltage, rated current and duration, etc.) without causing sexual damage. For resistive loads, the SSR can be used at full or derated by 10%. The inrush current of the electric heater and contactor at the moment of switching on can reach 3 times of the steady-state current, and the SSR should be derated by 20%~30%.
Use and maintenance of solid state relays
When the solid state relay is wired, do not reverse the input and output terminals. Pay attention to the following matters when using it.
①Influence of inrush current on SSR All loads have inrush current during operation. For example, the electric heating element is a pure resistive load with a positive temperature coefficient. The resistance is small at low temperature, and the current is large when the initial power is turned on. If the heat inside the SSR cannot be dissipated in time, the thyristor may be damaged, and a certain current margin should be reserved when selecting the SSR. When using resistance or electric heating load, the current level of SSR should be 2-2.5 times greater than the rated current of the load.
②Influence of ambient temperature The load capacity of solid-state relays is greatly affected by ambient temperature and its own temperature rise. Good heat dissipation conditions must be used in use. SSRs with rated working current above 10A should be equipped with radiators, and SSRs above 100A should be equipped with heat sinks. Equipped with radiator and fan for forced cooling. When installing, pay attention to the good contact between the bottom of the SSR and the heat sink, and apply thermal grease. For SSRs operating at 40-80°C for a long time, the current level should be derated.
③Overcurrent and overvoltage protection measures Overcurrent and load short circuit will cause damage to the SSR. A fast fuse and an air switch should be set in the control loop for protection. An RC absorption circuit and a varistor can be connected in parallel at the SSR output end for protection.
④Input loop signal
If the input voltage or input current exceeds the specification, a voltage divider resistor can be connected in series at the input end or a shunt resistor can be connected in parallel at the input end to make the input signal meet the requirements of SSR. The input signal line should be kept away from the source of electromagnetic interference to prevent malfunction.
The driving signal of solid-state relays usually comes from thermostats or phase-shift triggers. If you want to use solid-state relays proficiently, you should go to the product pages of thermostats or phase-shift triggers to learn more about these two supporting products and wiring of solid-state relays. Knowledge!
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