SCR Protection Circuits

Posted on Feb 6, 2014

SCRs are sensitive to high voltage, over-current, and any form of transients. For satisfactory and reliable operation they are required to be protected against such abnormal operating conditions. Because of complex and expensive protection, usually some margin is provided in the equipment by selecting devices with ratings higher (3 or 4 times hi

SCR Protection Circuits
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gher) than those required for normal operation. But it is always not economi ­cal to use devices of higher ratings, hence their protection is imperative. High forward voltage protection is inherent in SCRs. The SCR will breakdown and start conducting before the peak forward voltage is attained so that the high voltage is transferred to another part of the circuit (usually the load). The turn-on of SCR causes a large current to flow and poses a problem of over-current pro ­tection. Over-current protection can be provided by connecting a circuit breaker and a fuse in series with the SCR, as usually done for the protection of any circuit. However, there are some reservations to their use. A semiconductor device is capable of taking overloads for a limited period, so the fuse used should have high breaking capacity and rapid interruption of current. There must be a similarity of SCR and fuse I2t rating without developing high voltage transients which endanger those SCRs in the off or infinite impedance condition. These are contradictory requirements necessitating voltage protection when fast-acting fuses are employed. Fuses when used, their arc voltages are kept below 1. 5 times the peak circuit voltage. For small power applications it is pointless to employ high speed fuses for circuit protection because it may cost more than the SCR. Current magnitude detection can be employed and is used in many applications. When an...

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