Direct-current monitor

EE 60 July/August 1986 135 Many direct-current monitor circuits use a resistor in series with the current-carrying wires, and actuate some indicator by the ensuing volt- age drop across that resistor. The drop causes a reduction in the avail- able load voltage, which at relatively high currents can be appreciable. In the present circuit, use is made of a reed relay, around which the current- carrying wire is wound a number of times. The consequent losses are minimal. This method has a bonus in that a switch contact is immediately available for a number of appli- cations. One possible application is that of a low loss lamp monitor. As long as the lamp (here represented by RI) is on, the LED lights. The number of turns depends on the relay used and the load current. As a guide, most reed relays operate at 50 ampere-turns, so that in the case of, say, a car headlight (60 W at 12 V gives a current of 5 A) about 10 turns are required. The more complex circuit diagram shows an electroni...