An ammeter is an instrument which measures current flow in units such as amperes, milliamperes (mA) or microamperes (µA). Digital DC ammeters can be extremely accurate. For example, DC digital ammeters by Laurel Electronics are rated for 99.99% full scale accuracy. They offer four full-scale current ranges from 2.0000 mA with 0.1 µA resolution to 5.000 A with 1 mA resolution, plus five voltage ranges from the 200.00 mV to 600.0V.
The voltage input ranges available with a digital ammeter are for use with external current shunts. Since high DC currents in excess of 5 amperes would burn out most ammeters, such currents are normally determined by measuring the voltage drop across an external low-resistance, precision resistor in series with the circuit. Such resistors are named “current shunts,” since only a tiny fraction of the current to be measured passes through the meter while the rest of the current is “shunted” through the external resistor. According to Ohm’s law V = I x R, so the voltage drop V across the shunt resistance R will be directly proportional to current I. By knowing the resistance R, current I can be determined from the voltage drop. For example, a 1 milliohm shunt having a current of 50 Amps flowing through it will produce a voltage of 0.001 ohms ×50 A= 50 millivolts. Typical voltages across a shunt at maximum rated current are 50 or 100 millivolts.
Laurel’s digital meters are scalable in software so that the current in amperes through the shunt can be displayed based on the millivolts measured across any type of shunt. Laurel’s meters go one step further by having a user-selectable built-in 10 milliohm current shunt and providing a special 5.000A scale for the meter. Calibration factors for this 5A scale, as well as all other DC current scales down to 2 mA, are stored in EEPROM on the signal conditioner board in the meter. This allows signal conditioner boards to be changed with no need to recalibrate the meter.