Relay fuse: Simple battery chargers and power supplies are not normally provided with a current limiter. In many cases, however, it would be advantageous if the unit were proof against short-circuits. An electro-mechanical fuse which serves that function and which can be added to the unit is shown in the diagram. There are two variations, one for power supplies (a) and the other for battery chargers (b). The circuit will be .described on the basis of (a).
When power is switched on, the relay gets a short energizing pulse of current via C1. Since the relay contacts then change over, the relay remains energized. When a short-circuit occurs at the output terminals of the power supply, the relay is deenergized and the connection between input and output is broken. The relay is re-energized by a new Simple battery charger and power supplies are not normally provided with a current limiter. In many cases, however, it would be advantageous if the unit were proof against short-circuits. An electromechanical fuse which serves that function and which can be added to the unit is shown in the diagram. There are two variations, one for power supplies (a) and the other for battery chargers (b). The circuit will be .described on the basis of (a). When power is switched on, the relay gets a short energizing pulse of current via C1. Since the relay contacts then change over, the relay remains energized. When a short-circuit occurs at the output terminals of the power supply, the relay is deenergized and the connection between input and output is broken. The relay is re-energized by a new pulse of current via C2 after the short-circuit has been removed and Si is pressed briefly. Capacitor C2 also prevents an over-load if S1 were pressed while the short-circuit persists. The capacitor is discharged via R1 when S1 is opened. Diode D1 (biased via R2) shows when the circuit is off.
The diagram for battery chargers differs in one respect from that described: on power-on, the relay is not energized via a capacitor, but by the battery on charge via D4. In case the battery is so flat that it can no longer supply sufficient current to actuate the relay, the relay can be energized, via C3-R4, by pressing S2 briefly.
The value of bias resistors R2 and R3 depend on the LED used and the supply voltage. The relay voltage must, of course, also be in accord with the supply voltage.
