How Do Amp Meters Work?
By Don Patton
Amp meters, or more simply "ammeters,” are instruments that measure electrical current. There are two types of ammeters, those using a traditional analog needle design, and newer solid-state meters with digital readouts. Meters of each type inherently respond to direct current, or DC, only, but with additional circuitry can also measure alternating current, or AC.
At the heart of an analog meter is a galvanometer, a sensitive instrument containing a movable wire coil connected to the input terminals. The coil produces a magnetic field from the electrical current running through it, and this magnetic field reacts with a second field, usually from a permanent magnet in the meter. A spring counteracts the torque created by the interaction of the two magnetic fields. As the current increases, the spring stretches in proportion to the input current, and a needle attached to the coil indicates the magnitude of the electrical current.
A galvanometer is very sensitive, with a usable range of microamperes up to a few milliamperes. In order to read larger current values, an analog ammeter uses a combination of series and parallel resistors to expand the range of the galvanometer.
Digital meters use solid-state electronics to measure current flow. Their designs consist of a digital voltmeter connected across a calibrated current shunt. The shunt has a very small resistance, and passes the vast majority of the current while the meter reads the small voltage across it. The voltage is proportional to the current running through the shunt, and the voltmeter uses an analog-to-digital converter and other digital circuitry to translate the voltage to a digital code that the meter can display as the current value.
The direction of the coil deflection in a galvanometer depends on the polarity of the applied current, so AC currents that continually reverse direction won’t register. In order for a current meter to read AC, it must have a rectifier bridge circuit to convert the input current to DC. With this addition, the deflection always happens in the same direction, regardless of the applied current polarity. Digital meter readings also depend on the current direction, so they, too, use rectifiers to read AC current.
Ammeters usually connect directly into the current paths they measure, but some meters have clamp-on probes that allow you to take measurements without breaking into the circuit you're measuring. A clamp-on meter is not as accurate as one wired into a circuit, but it can be useful for one-time measurements or where the current or voltage levels are extreme. Some clamp-on probes use magnetic coupling in the measurement circuit, so they work only for AC currents, while others use Hall Effect sensors that respond to DC currents.
- Georgia State University Department of Physics and Astronomy: Ammeter Design
- University of Florida Agricultural and Biological Engineering: Analog Electrical Devices and Measurements
- Georgia State University Department of Physics and Astronomy: Galvanometer
- AEMC Instruments: Selection Guide to Clamp-On Current Probes
Don Patton began writing after retiring from an engineering career in 2006. He holds a Bachelor of Science in electrical engineering from the University of California at Berkeley and continued with graduate study in software engineering.