Special-Purpose Meters

Frequency Meters

Most components in a power system are designed to run on a specific ac frequency or limited range of frequencies. The frequency meter is important to test these frequencies. The frequency meter works very much like the digital voltmeter. A digital meter can generate a pulse train to be counted. Since the test frequency already has waveforms that can be counted, the clock and decade divider circuits only have to supply the gate pulse. The test signal is amplified, then converted to square wave pulses, generally by a Schmill trigger stage. The pulses are gated, then further shaped, and then counted to determine their frequency for display.

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Digital Amplifiers

Electronic amplifiers are used with analog meters to increase their ranges, to make them more sensitive, to improve their accuracy, and to minimize their effects on the circuit under test. But, any analog meter movement can be used effectively without amplifiers; they need only multiplier resistors and/or shunt resistors to work on different ranges.

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Electromagnetic Meter Construction: Part 2


In both the moving-coil and moving-iron meter movements, the current being measured flows through the coil. Except for this similarity, the coils in each type of movement are different. When current flows through the coil of the moving-coil meter movement, a magnetic field is produced that causes the coil to rotate. For the coil to rotate easily, it must be as light as possible. To make the coil light, it is wound on an aluminum frame. Furthermore, the coil is made from very fine wire, and when compared with the coil in the other meter movements, it contains very few turns to keep it as light as possible.

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