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Short-Circuit Protection in DC Low-Voltage Systems

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Short-circuit Protection in
Dc low-Voltage SyStemS                                                                                                      s.c. dwiv
                                                                                                                                       edi



   Pratik Panchal




M
          any a times you need to                       cuit. Here is a circuit to derive the addi-   Working of the circuit is simple. When
          power an adjoining acces-                     tional power supply from the main cir-        the 5V DC output from regulator IC
          sory circuit from the power                   cuit. The main circuit is protected from      7805 is available, transistor BC547
supply used in the main module cir-                     any damage due to short-circuit in the        conducts through resistors R1 and R3
                                                                     additional power supply          and LED1. As a result, transistor SK100
                                                                     circuit by cutting off the       conducts and short-circuit protected 5V
                                                                     derived supply voltage.          DC output appears across O/P2 termi-
                                                                     The derived supply volt-         nals. The green LED (LED2) glows to
                                                                     age restores automatically       indicate the same, while the red LED
                                                                     when shorting is removed.        (LED1) remains off due to the presence
                                                                     An LED is used to indicate       of the same voltage at both of its ends.
                                                                     whether short-circuit exists         When O/P2 terminals short, BC547
                                                                     or not. Author’s prototype       cuts off due to grounding of its base.
                                                                     of short-circuit protection      As a result, SK100 is also cut-off. Thus
                                                                     module is shown in Fig. 1.       during short-circuit, the green LED
                                                                          In the main power sup-      (LED2) turns off and the red LED
Fig. 1: Prototype of short-circuit protection in DC low-voltage
                                                                     ply circuit, 230V AC is          (LED1) glows. Capacitors C2 and C3
systems                                                              stepped down by trans-           across the main 5V output (O/P1) ab-
                                                                                  former X1 (230V     sorb the voltage fluctuations occurring
                                                                                  AC primary to       due to short-circuit in O/P2, ensuring
                                                                                  0-9V, 300mA         disturbance-free O/P1. The design of
                                                                                  secondary), rec-    the circuit is based on the relationship
                                                                                  tified by a full-   given below:
                                                                                  wave rectifier          RB = (HFE X Vs)/(1.3 X IL)
                                                                                  comprising di-          where,
                                                                                  odes D1 through         RB = Base resistances of transistors
                                                                                  D4, filtered by     of SK100 and BC547
                                                                                  capacitor C1            HFE = 200 for SK100 and 350 for
                                                                                  and regulated       BC547
                                                                                  by IC 7805 to           Switching Voltage Vs = 5V
                                                                                  give regulated          1.3 = Safety factor
                                                                                  5V (O/P1).              IL = Collector-emitter current of
                                                                                       Transis-       transistors
                                                                                  tors SK100 and          Assemble the circuit on a gen-
                                                                                  BC547 are used      eral-purpose PCB and enclose in a
                                                                                  to derive the       suitable cabinet. Connect O/P1 and
                                                                                  secondary out-      O/P2 terminals on the front panel of
                                                                                  put of around       the cabinet. Also connect the mains
                                                                                  5V (O/P2) from      power cord to feed 230V AC to the
                                                                                  the main 5V         transformer. Connect LED1 and LED2
Fig. 2: Circuit diagram of short-circuit protection                               supply (O/P1).      for visual indication. 




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