1-9
CURRENT MEASUREMENTS
Unless an ammeter is already an integral part of the circuit under test, current measurements are
rarely taken. In the case of a high-resistance circuit, it will contain such a small amount of current that it
cannot be measured accurately with ordinary field test equipment. In lower resistance circuits, current
measurements can be taken only if the ammeter is placed in series with the circuit under test. These
measurements require that a circuit connection be unsoldered or otherwise opened to insert the meter in
series with the circuit. An easier method you may use to obtain a current measurement is to take a voltage
measurement across a known resistance and calculate the current with Ohm’s law. The accuracy of
current measurements depends on the internal resistance of the meter as compared with the resistance of
the external circuit. If the total circuit current is decreased by increasing the load, then the percentage of
error will decrease. Therefore, greater accuracy is obtained if the meter resistance is considerably less
than the load resistance. A method of obtaining greater accuracy of current measurement is to decrease
the total internal meter resistance with respect to load resistance. This is accomplished by connecting two
ammeters in parallel with each other and in series with the circuit in which the current is being measured.
Additional ammeters may be connected in parallel in the same manner for increased accuracy. This
method also increases the range of measurements that can be taken. The arithmetical sum of the
indications of all the parallel meters represents the total current flow in the circuit. You should note that
this is not a common test method and that your test equipment may be damaged if connected incorrectly.
MULTIMETER METHOD
As previously mentioned, current measurements are usually taken by breaking the current path of the
circuit under test and electrically inserting a meter in series. This is normally accomplished by
disconnecting a wire from a terminal or unsoldering one end of a component and electrically inserting the
meter in series using the meter leads. This method is both time consuming and usually requires the use of
a soldering iron, which can damage components. Most analog multimeters cannot be used for measuring
ac current and are only accurate to within 2% on dc ranges.
Q-8.
What are the advantages of connecting ammeters in parallel when performing current
measurements?
DIGITAL MULTIMETER METHOD
Unlike the analog multimeter, the digital multimeter will measure ac current as well as dc current.
Again, current measurements are taken by breaking the current path and inserting the meter in series.
Regardless of whether you’re using an analog multimeter or digital multimeter, this procedure for
measuring current is time consuming. However, there is a major advantage to be gained by using the
digital multimeter — its high degree of accuracy. The Fluke 8000A digital multimeter, for example, is
accurate to within 0.3% when measuring dc current and 1% when measuring ac current. These
accuracies are representative of most medium-priced digital multimeters.
CURRENT TRACERS
For the purpose of discussion, we have selected the Hewlett-Packard 547A, shown in figure 1-7, as a
representative current tracer. A current tracer will not actually measure current; it is designed to indicate
the presence of current and the relative magnitude of one source of current as compared to another. The
Hewlett-Packard 547A is a hand-held probe that enables you to precisely localize low-impedance faults in
a circuit. The probe senses the magnetic field generated by a pulsing current and lights an indicator lamp
near the current tracer tip. The brightness of the indicator lamp is proportional to the magnitude of the
current. The sensitivity of the indicator lamp can be adjusted with a thumb-wheel potentiometer located
on the probe. Figure 1-8 depicts a typical logic circuit application for a current tracer. Current tracers are
