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Real-World Control Device Interfacing
Published in A. Arockia Bazil Raj, FPGA-Based Embedded System Developer's Guide, 2018
Another popular application of the Triac is as an optoisolation relay. An application circuit with the Triac providing the optoisolation is shown in Figure 9.31. These active semiconductor devices use light instead of magnetism to actuate a switch. The light comes from an LED. When control power is applied to the device's input, the light is turned on and shines across an open space. On the load side of this space, a part of the device senses the presence of the light and triggers a solid-state switch that either opens or closes the circuit under control. Often, solid-state relays are used where the circuit under control must protected from the introduction of electrical noises. Advantages of solid state relays include low electromagnetic interference (EMI), long life, no moving parts, no contact bounce, and fast response. The drawback to using a solid-state relay is that it can only accomplish single pole switching. The quantity of electrical current that flows through the contacts directly influences the characteristic of the contacts.
Computer-Based Instrumentation: Sensors for In-Line Measurements
Published in Gauri S. Mittal, Computerized Control Systems in the Food Industry, 2018
Interfacing a microprocessor system to external devices involves both hardware and software. The commonly used bus standards are RS232C, IEEE488, and CAMAC. RS232C is the classical binary serial interface, and transmission speed up to 9600 baud is used. IEEE488 is also known as “General Purpose Interface Bus (GPIB)” or ASCII bus. CAMAC is IEEE’s Standard 583, and it specifies a parallel bus with specific physical modules. Analog signals from sensors must be converted to digital signals for further processing by the microprocessor. Digital output of the microprocessor is converted to analog form to actuate the valves and switches at the process equipment. Thus the process interface brings signals into and out of the control system. In addition to analog signals from sensors, these signals include contact inputs and outputs such as valve position, pump on/off status, etc. Further signal-conditioning modules are needed to provide bridge excitation, zero balance, gain, span adjustment, and low pass filtering. For interfacing, plug-in boards are available that provide programmable gain, analog to digital, and digital to analog conversions. Solid state relays are used for applications such as contact closures, interfacing to low power state circuit, maximum speed and control of contact closure, etc. Complex control strategies can be implemented simply by changes in software, once the inputs and outputs are connected to the microprocessor.
Problems with International Standards
Published in Vladimir Gurevich, Digital Protective Relays, 2017
Again, in the “Scope” section of IEC 62314-1, it is noted that the “solid-state relays are components (not stand-alone devices) and, as such, do not perform a direct function. Therefore, no EMC requirements are included in this standard.” In my opinion, this is rather debatable. First, very often solid-state relays are used for direct switching of electric motors, solenoids, heating elements, lighting lamps, and so on, in which case all control equipment consists of this relay only, without any additional electronic control circuits, and as such it is a stand-alone electronic device incorporated into power equipment which does not contain any other low-signal electronic elements. The electromagnetic compatibility (EMC) of such power equipment is determined completely by the solid-state relay only. The consumer has to know about the EMC of the equipment (solid-state relays in this example). Second, in itself even the simplest solid-state relay contains many complex built-in electronic components—light-emitting diodes (LEDs), photo diodes or phototransistors, electronic amplifiers and triggers, synch circuits with a network voltage, elements of overvoltage protection, and so on—that characterize it more as a complex electronic device and not a simple element. Third, today in the market there are so-called intelligent solid-state relays with expanded functions, supplied by complex electronics and sometimes even a built-in microprocessor.
Light controlled (super) cascode, LC(S)C, a power device with optical turn-on and -off
Published in EPE Journal, 2020
An application could be the combination with a two or three phase diode bridge rectifier to build an optically isolated one or three phase switch, see Figure 16 left. This forms a solid-state relay for DC, single or three phase AC loads. If necessary a varistor or a RCD-snubber must be added to avoid overvoltage at turn-off. Current solid-state relays use a series connection of Si-photodiodes to excite a MOSFET designed for the full blocking voltage.