3/7/2023 0 Comments Optical isolator relay![]() 6.6.5 Solid State Relay Safety Features SSR Safety Features More information on SSRs can be found by searching for Solid State Relays on manufacturers websites such as Infineon Technologies or at semiconductor suppliers such as RS Componentsįig. SSRs are also manufactured in a range of output voltages and current ratings, with a range of package types ranging from small surface mount components through complex multi pin chips and large heavy current examples for rack mounting in electrical control cabinets. Other chips are available that act as double pole, Normally Closed (NC), Normally Open (NO), and Changeover relays with a wide variety of extra facilities. A number of similar SSRs are available to meet different AC and DC output voltage and current requirements, a typical example is the PVT412 SSR from International Rectifier (now part of Infineon Technologies) manufactured in several versions as a 6 pin DIL package and capable of replacing a single pole mechanical relay to switch AC or DC voltages up to 400V (peak) with currents up to 140mA AC or 210mA DC. Because a single photodiode will only produce a very low voltage, the diodes in the photovoltaic unit are arranged in a series/parallel array to produce sufficient voltage to turn on the MOSFETS.įigure 6.6.4 represents a basic example of a MOSFET SSR, showing how the outputs can be arranged to allow the SSR to switch either AC or DC loads. ![]() The (infra red) light from the LED falls on the Photovoltaic unit that comprises a number of photodiodes. ![]() A current of about 20mA through the LED is sufficient to activate the MOSFETs that take the place of mechanical relay contacts. For this reason adequate heat sinks are required to prevent overheating.Ī typical circuit of a basic MOSFET SSR is shown in Figure 6.6.3. ![]() Even so, some power will be dissipated by the semiconductor switch when in either 'on' or 'off' state with either AC or DC currents. the MOSFETs are turned off) the SSR has a nearly infinite resistance across its output terminals, and an almost zero resistance when 'closed' (i.e. One of the most important features of SSRs is for the optocoupling to provide complete electrical isolation between its low power input circuit and its high power output circuit. Also special very fast acting fuses or circuit breakers are needed to prevent damage to the semiconductors due to current overloads. For example, adequate heat sinks for the solid-state devices used. In addition to these criteria, the circuit around the control device must also provide safeguards against dangerous situations. The control device chosen must also ensure electrical isolation between the input and output circuits. Also high values of surge current (much higher than the normal 'running current') that occur for example when loads such as motors or incandescent lamps are switched on, can require that the control device must be rated to handle surge currents up to 40 or 50 times higher than normal ´running´ current. The control device must be able to handle the high voltages, including very high voltage spikes that may occur in either AC or DC output circuits due to back emf from inductive loads and voltage spikes that may be randomly present on the mains (line) power supply. Many high power circuits today are controlled by low voltage, low current circuits such as microprocessors, using opto electronic devices such as Opto−Triacs, Opto−Thyristors and Solid State Relays to isolate the low and high power circuits. Fortunately there are readily available solutions to this problem. Physical isolation (meaning that there is no electrical connection at all between the input and output) is what is needed. When faults occur (and they are more likely to do so in high power circuits) the results can be catastrophic, not only to the circuit components but also to the users of such equipment. Relying on a layer of silicon oxide, a few atoms thick to provide the required insulation is not really an option in such conditions. Devices that are used in the control of high voltage/high power equipment need to have good electrical insulation between their high voltage output and low voltage input.
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