A digital ammeter is a device that is inserted in a circuit to measure electrical current in amperes (A). Some models are powered by electricity, but most are battery operated and can be used for measurements done outdoors. Like an analog ammeter, a digital ammeter should have a very low resistance so that the characteristic of the circuit under test will not be changed when the digital ammeter is inserted.
Ammeters generally have very low resistances, such that they seem like a short circuit. For instance, when the power switch is turned off, a very high resistance actually exists between the terminals of the switch. By connecting an ammeter in place of the off switch, it is able to re-establish the continuity of current. A very small resistance of the ammeter makes current measurement possible.
Ammeter resistance is typically less than 0.1 ohm, which is almost a short circuit. An example of an ammeter element is a shunt resistance 0.001 ohm in resistance. The digital portion of the digital ammeter relies on the Ohm’s law equation that relates the voltage across a 0.001-ohm resistance and the current through it. A 1-volt (V) drop on a 0.001-ohm resistance corresponds to 1,000 A. Ohm’s law states that voltage is the product of the resistance in ohms and the current in amperes.
The voltage reading on a 0.001-ohm shunt resistance is in proportion to the actual current through the digital ammeter. For example, 0 V means 0 A, and 0.001 V means 1 A. The digital reading circuit of the sample digital ammeter is calibrated to internally measure voltage but displays a current panel that is able to indicate 0 to 1 A. It is assumed that the shunt may safely dissipate 1 watt (W).
If a current ammeter were to dissipate 1 W, then it may be practical that this ammeter be used in applications where the original power dissipated is at least a hundred times the ammeter rated full-power dissipation. For example, a 110 V power outlet is supplying 1 A current to a 110 V light bulb with a resistance of 1 ohm. If a digital ammeter is inserted, the total resistance that will be displayed is 1.001 ohm. The digital circuit of the digital ammeter will read a current very close to 1 A or 0.99999A, due to the slight increase in total resistance seen by the 110 V supply when the ammeter was inserted.