TEST & MEASUREMENT HANDBOOK
Basic test instrumentation and its role in measurements
DAVID HERRES | CONTRIBUTING EDITOR
It can be helpful to understand the differences between common test gear used for bench-top development tasks.
HERE’S A QUICK REVIEW OF some basic instrumentation common to most engineering work benches.
AMMETER
The ammeter is the basis for many other electrical measuring instruments. Whether you are measuring volts or ohms, essentially inside the instrument you are measuring current. Measurement of current in a circuit is somewhat problematic because all the electrical energy to be measured must pass through the meter, so there is the inconvenience of cutting open and later re-terminating the circuit. Another problem is that conventional ammeters, as incorporated in the ubiquitous multimeter, cannot dissipate heat that is greater than just a few amps. The clamp-on ammeter is a work-around. It solves both problems by measuring the magnetic field that surrounds any currentcarrying conductor. The instrument is calibrated to read amps. The user closes the jaws around an insulated currentcarrying conductor. It doesn’t matter whether the conductor is centered within the jaws, and it may pass through at an angle. For low-amp measurements, the conductor may be coiled, multiple turns passing through the jaws in the same direction, and then the total reading divided by the number of turns. A hand-held clamp-on ammeter (trade name Amprobe) can be rated as high as 600 A, making it useful for large three-phase motor Typical clamp-on work. Specialized Hall-effect ammeters. instruments can read dc amps.
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VOLTMETER Unlike the ammeter, which is a series instrument, the voltmeter is placed in parallel across a component, conductor, circuit or power source. The full current does not pass through the instrument, only a small fraction of it. The exact amount depends upon the voltage being measured and the impedance of the voltmeter. The input impedance rating of the instrument is all-important and determines how accurately a given circuit can be measured. A low-impedance meter places a heavy load on the circuit under investigation. Used beyond its rating, or with a high-impedance circuit, the large voltage drop can damage the circuit. A high-impedance voltmeter is (relatively) invisible to the circuit under investigation. Nevertheless, it should not be used at voltages exceeding its rating. CAT ratings, which vary with precisely-defined electrical environments, must be observed. These ratings are generally printed adjacent to the inputs. A low-impedance instrument, such as the solenoid voltmeter (tradename Wiggy) is useful in checking for presence or absence of voltage and the approximate level (120 or 240 V) in residential, commercial and industrial branch
A solenoid voltmeter (tradename Wiggy) useful in checking for presence or absence of voltage and the approximate level (120 or 240 V) in residential, commercial and industrial branch circuits and load centers.
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