I. Solid State Relay Principle
SSRs can be divided into two main categories according to their application: AC type and DC type. They are used as load switches on AC or DC power supplies, respectively, and cannot be used interchangeably. The following uses the AC type SSR as an example to illustrate its working principle. The following diagram is its working principle block diagram. Components ① to ④ in the diagram constitute the main body of the AC SSR. Overall, the SSR has only two input terminals (A and B) and two output terminals (C and D), making it a four-terminal device.
During operation, by applying certain control signals to A and B, the "on" and "off" states between C and D can be controlled, realizing the "switch" function. The coupling circuit provides an input/output channel for the control signals input to A and B, but electrically disconnects the (electrical) connection between the input and output terminals of the SSR to prevent the output from affecting the input. The component used in the coupling circuit is an "optocoupler," which is sensitive, has a high response speed, and a high insulation (withstand voltage) level between the input and output terminals. Since the load at the input terminal is a light-emitting diode, it is easy to match the input signal level of the SSR. In use, it can be directly connected to the computer output interface, i.e., controlled by the logic levels of "1" and "0". The trigger circuit generates a trigger signal that meets the requirements to drive the switching circuit ④ to work. However, since the switching circuit will generate radio frequency interference and pollute the power grid with high-order harmonics or spikes without special control circuitry, a "zero-crossing control circuit" is specially designed for this purpose. The term "zero crossing" refers to the fact that when a control signal is applied and the AC voltage crosses zero, the SSR is in the on-state; and when the control signal is disconnected, the SSR waits until the boundary point (zero potential) between the positive and negative half-cycles of the AC current before becoming off-state. This design prevents interference from higher harmonics and pollution of the power grid. The absorption circuit is designed to prevent spikes and surges (voltages) from the power supply from impacting and interfering with the switching device, the bidirectional thyristor (SCR), (even causing malfunctions). It is generally constructed using an RC series absorption circuit or a nonlinear resistor (varistor).
II. Application Examples of Solid State Relays
(I) Voltage Regulation Application
SSR and TSR voltage regulating modules can achieve linearly adjustable output voltage by using an external analog signal to trigger the module. For example, a PLC or temperature controller can output an analog signal: 1-5V, 4-20mA trigger system. Domestic single-phase and three-phase thyristor trigger boards, in conjunction with thyristors, can also be used with external analog signals to adjust the trigger board. The trigger board then triggers the module to achieve a linearly adjustable output voltage, controlling the thyristor conduction angle to achieve voltage regulation.
( II) Exchange and Adjustment of Qi
"AC power adjustment" is a common method used in Z-type SSRs and can also achieve PID regulation. This involves controlling the number of half-waves of the AC sinusoidal current within a fixed period to achieve power adjustment. Analog circuits often use voltage comparators to compare the sawtooth voltage of a fixed period with the error voltage from the preceding stage, outputting a square wave for adjustment, as shown in Figure 3. Alternatively, a timing algorithm can be used on a computer to generate square wave pulses with adjustable duty cycles. For example, the SR22, FD20, and E5 series intelligent temperature control products from Japanese companies SHIMADEW and OMRON, when used with Z-type SSRs, achieve adaptive "automatic flip-flop" control, where a computer generates a disturbance and calculates the optimal PID control parameters.
(III) Three-phase current
The HS series SSR products can be directly used for the control of three-phase motors. The simplest method is to use two SSRs for motor on/off control, four SSRs for motor commutation control, and leave the third phase uncontrolled.
When commutating a motor, it's crucial to ensure that, due to the motor's inertia, commutation should only occur after the motor has come to a complete stop. This prevents situations similar to motor stalling, which can lead to significant voltage and current surges. In the control circuit design, it's essential to avoid the possibility of simultaneous conduction of commutating SSRs at any given time. The power-on/off sequence should follow the order of applying power to the control circuit first and then disconnecting it, followed by applying power to the motor first and then disconnecting it. Simply using inverters to connect commutating SSRs is not advisable, as this can cause a phase-to-phase short circuit if one SSR remains on while the other is conducting. Furthermore, fuses, phase loss relays, and temperature relays in the motor control system are also essential protective devices to ensure the system's normal operation.
