Electronic Ammeter and Voltmeter

Posted on Feb 7, 2014

I found that my sprite, burdened with an extra electrical load from an electric cooling fan, just barely kept the battery charged. My intended solution to this problem is an electronic voltage regulator; however, it seems obvious that, before changing the regulator, I need some way to monitor the charging. Most meters today are digital. They work by converting an analog

voltage to a digital number, displaying that number on a digital readout. Conventional analog meters are still in use, however, and probably will remain so. (To see why, just try to adjust something for a peak or null value with a digital meter!) Conventional meters are based on the principle that a coil of wire carrying a current experiences a torque when placed in a magnetic field. The torque rotates the coil, and a pointer connected to the coil indicates the current. Such meters respond fundamentally to current; we call them current-sensing devices. Voltmeters consist of an ammeter with a series resistance, so the voltage is measured, by Ohm`s law, as the current times the resistance. Moving-coil ammeters can be remarkably sensitive. They are usually designed to operate at very low full-scale currents, because it is straightforward to extend their ranges to higher currents but not lower. Back when all test meters were analog, the "gold standard" was a movement that required only 50 microamperes for full-scale deflection. To measure higher current, it is necessary to use a shunt, a resistor connected in parallel with the meter so that it bypasses most of the current. The shunt resistance is given by the relation, where Rs is the shunt resistance, Rm is the meter resistance, Im is the meter current for full-scale deflection, and Ifs is the desired full-scale value of the shunted meter. This relation illustrates the problem...

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