Functions and use of SSRs
SSR solid-state relays have three basic functions. The first one is galvanic separation, i.e. separation of electrical potentials. There are projects where it is necessary to isolate the control circuit from the working circuit in order to protect the control planes from possible malfunctions and failures, as well as interference in the area of interference currents. This is only possible with a tightness of at least 3 mm between the transmitter and receiver. The semiconductor relay should withstand a voltage of at least 2.5 kV. The second function of the relay is to process signals that appear between separated circuits. It is designed for this transoptor, one of the basic components of which the relay is built. It is responsible for precise matching of different voltage potentials of signals that occur in both operating and control circuits.
The last function of the SSR solid-state relay is switching gain. If the current and voltage are higher than the photoelectric transistor rating, an extra circuit is used to amplify the switching. During switching, the base current is activated in the conveyor by means of a phototransistor and an LED. The second semiconductor takes control of the base current.
Performance of SSRs
The construction of the SSR semiconductor relays is based on a transoptor. It consists of at least one transmitter and a receiver. The task is to separate the control circuit from the working one. The transoptor is an electronic element, without mechanical parts, which are susceptible to rapid wear. In the control circuit a light signal is triggered by means of an LED, which initiates switching operations. The reflected light intensity is measured with a phototransistor. The function of the transmitter is usually performed by the LED, which emits infrared light.
As the optoelectronic elements are arranged opposite each other, it is possible to receive the light directly, for which the receivers are responsible. If the transmitter and the receiver are arranged in one plane, the light beam is transmitted by reflection, i.e. according to the light-guide principle. However, it is necessary to know that the optocoupler allows for one-way flow of signals from input to output circuit.
The optocoupler phototransistor can only be loaded with low current and voltage values. If they are higher, it is necessary to use additional semiconductor element. Its function is to switch the corresponding rated voltages and currents. A bipolar transistor is used for this purpose, which works very well at low load currents ≤0.5A. For high load currents, a unipolar transistor with an insulated MOSFET gateway is used, generating low leakage currents <10 μA. A triac whose functioning is similar to that of diodes can also be used, so it is a good option for AC currents.