Fuse Saver

Posted on Feb 5, 2014

This circuit will be particularly useful to those hobbyists who use a breadboard` to try out ideas and who also use a simple home-made` DC power supply consisting of a transformer, rectifier, smoothing capacitor and protective fuse, that is, one without over current protection! In this circuit, the detecting element is resistor R6. Under normal co

Fuse Saver
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nditions, its voltage drop is not high enough to switch on transistor T1. The value of R6 can be altered to give a different cut-off current, as determined by Ohm`s Law, if required. When a short circuit occurs in the load, the voltage rises rapidly and T1 starts to conduct. This draws in the relay, switching its contacts, which cuts off power to the external circuit, and instead powers the relay coil directly, latching it in this second state. The circuit remains in this state until the primary power supply is switched off. Capacitors C1 and C2 hold enough charge (via D3, D4 and D6, which prevent the charge from being lost to the rest of the circuit, whichever state it is in) to keep T1 switched on and power the relay while it switches over, and R2 and R4 provide slow discharge paths. LEDs D1 (red) and D5 (green) indicate what state the circuit is in. Inductor L1 slows the inrush of current when the circuit is switched on, which would otherwise cut off the circuit immediately. D2 and D7 provide the usual back-emf protection across the coils. In use, the input of the circuit is connected to the main transformer-rectifier-capacitor-fuse power supply via K1, and the output is connected to the (experimental) load via K2. Note that the input voltage must be a floating supply if Vout is grounded via the load, as Vin and Vout must not be connected together. Some consideration needs to be given to a number of components. First,...

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