Posted on Oct 15, 2012

The circuit in Fig. 56-lla lights the signal lamp upon detecting a line current consumption of more than 5 mA, and handles currents of several amperes with appropriate diodes fitted in the D1 and D2 positions. Transistor T1 is switched on when the drop across Dl/D2 exceeds a certain level. Diodes from the well-known 1N400x series can be used for currents of up to 1 A, while 1N540x types are rated for up to 3A. Fuse F1 should suit the particuiar application.

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The circuit in Fig. 56-llb is a current-triggered alarm. Rectifier bridge D4 through D7 can only provide the coil voltage for Rel when the current through D1/D2 exceeds a certain level, because then series capacitor C1 passes the alternating main current. Capacitor C1 needs to suit the sensitivity of the relay coil. This is readily effected by connecting capacitors in parallel until the coil voltage is high enough for the relay to operate reliably.

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