The electrical symbol for a solid state relay is: SSR. its graphic symbol can be represented by adding a bidirectional thyristor graphic load in a box.
The input circuit of a solid state relay is to provide a loop for the input control signal to become the trigger signal source of the solid state relay. Solid state relay input circuit is mostly DC input, individual AC input. DC input circuits are further divided into resistive inputs and constant current inputs. The input control current of the resistive input circuit varies linearly and positively with the input voltage.
Constant-current input circuits, the input voltage reaches a certain value, the current no longer with the rise in voltage and significantly increased, this relay can be applied to a fairly wide range of input voltage.
The drive circuit of the solid state relay can include three parts: isolation coupling circuit, function circuit and trigger circuit. Isolated coupling circuit, the current use of photocoupler and high-frequency transformer two circuit forms.
Commonly used photocouplers are light - transistor, light - bi-directional silicon controlled, light - diode array (light - voltaic) and so on. High-frequency transformer coupling, is in a certain input voltage, the formation of about 10MHz self-excited oscillation, through the transformer core will be high-frequency signals to the transformer secondary.
Functional circuits may include various functional circuits such as detecting and rectifying, over-zeroing, accelerating, protecting and displaying. The function of the trigger circuit is to provide a trigger signal to the output device.
Expanded Information
Selecting the type specification of SSR is mainly to select the appropriate rated current solid state relay (SSR) except for special instructions, rectifier, controllable and other power modules as well.
The current ratings of SSRs are selected according to different types of loads. Resistive, inductive, and capacitive loads have high instantaneous currents when they are first started. Even purely resistive, due to a positive temperature coefficient, the resistance value is smaller when cold, thus having a larger starting current. The current when the furnace is first turned on is 1.3-1.4 times higher than when it is stabilized.
Incandescent lamps turn on the current for the steady state 10 times. Some metal halide lamps not only stay on for up to 10 minutes, but have up to 100 times the steady state pulse current.
Asynchronous motor starting current for the rated value of 5-7 times, DC motor starting current is even greater. Not only that, inductive loads also have a high reverse potential. This is an indeterminate value, varying with L and di/dt. It is usually 1-2 times the supply voltage, so that it is superimposed with the supply voltage. There are up to three times the supply voltage.
Capacitive loads are more dangerous, because when starting, the capacitor (load) is equivalent to a short circuit because the voltage across the capacitor can not be changed abruptly. This kind of load in the selection of more special attention.
Baidu Encyclopedia-Solid State Relay