05-05-2011, 04:48 PM
Short-Circuit Protection inDC Low-Voltage Systems
Many a times you need topower an adjoining accessorycircuit from the powersupply used in the main module cir- Pratik Panchal cuit. Here is a circuit to derive the additionalpower supply from the main circuit.The main circuit is protected fromany damage due to short-circuit in theadditional power supplycircuit by cutting off thederived supply voltage.The derived supply voltagerestores automaticallywhen shorting is removed.An LED is used to indicatewhether short-circuit existsor not. Author’s prototypeof short-circuit protectionmodule is shown in Fig. 1.In the main power supplycircuit, 230V AC isstepped down by transformerX1 (230VAC primary to0-9V, 300mAsecondary), rectifiedby a fullwaverectifiercomprising diodesD1 throughD4, filtered bycapacitor C1and regulatedby IC 7805 togive regulated5V (O/P1).T r a n s i s -tors SK100 andBC547 are usedto derive thesecondary outputof around5V (O/P2) fromthe main 5Vsupply (O/P1).Working of the circuit is simple. Whenthe 5V DC output from regulator IC7805 is available, transistor BC547conducts through resistors R1 and R3and LED1. As a result, transistor SK100conducts and short-circuit protected 5VDC output appears across O/P2 terminals.The green LED (LED2) glows toindicate the same, while the red LED(LED1) remains off due to the presenceof the same voltage at both of its ends.When O/P2 terminals short, BC547cuts off due to grounding of its base.As a result, SK100 is also cut-off. Thusduring short-circuit, the green LED(LED2) turns off and the red LED(LED1) glows. Capacitors C2 and C3across the main 5V output (O/P1) absorbthe voltage fluctuations occurringdue to short-circuit in O/P2, ensuringdisturbance-free O/P1. The design ofthe circuit is based on the relationshipgiven below:RB = (HFE X Vs)/(1.3 X IL)where,RB = Base resistances of transistorsof SK100 and BC547HFE = 200 for SK100 and 350 forBC547Switching Voltage Vs = 5V1.3 = Safety factorIL = Collector-emitter current oftransistorsAssemble the circuit on a general-purpose PCB and enclose in asuitable cabinet. Connect O/P1 andO/P2 terminals on the front panel ofthe cabinet. Also connect the mainspower cord to feed 230V AC to thetransformer. Connect LED1 and LED2for visual indication
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