The multimeter had its roots in a surprising observation by Hans Christian Ørsted during a lecture in 1820. He saw that a compass needle deflected from its alignment with the earth’s magnetic north pole when a metal conductor carrying electric current was moved close by.
This action suggested that an analog meter could be built incorporating a needle that would indicate on a calibrated dial the amount of electrical current flowing in a circuit.
The amount of electrical current conveyed in a conductor, originally called Intensity (I), is precisely denominated by the number of electrons passing a given point in one second: 6.25 × 1018. This is a high number, and it corresponds also to an electrical charge of one coulomb.
Nineteenth-century electrical researchers built large numbers of galvanometers, incorporating small wire coils that rotated inside permanent magnet fields. An attached pointer moved across a calibrated scale with a spring that moved the pointer to the zero position when there was no magnetic flux producing a field to deflect it.
This instrument could be converted to a voltmeter by simply inserting it into the circuit in series with the coil resistors to make a voltage divider, then recalibrating the scale. A small internal voltage, typically 3.0 Vdc, can be placed in series so an unknown resistance can be ascertained, the scale now calibrated in ohms. This is the basis for an ohmmeter.
Today galvanometers are currently reconfigured with additional functions as multimeters. While the hand-held multimeter is a superb diagnostic instrument, the bench-type multimeter will continue to dominate the lab bench due to its high precision and many features.
The Fluke 8808A bench-type multimeter is an exceptional example of a smart meter in a small, durable enclosure. Its modest cost puts it within reach of the working technician while its high precision and many available features make it suitable for advanced lab work.
An important and valuable feature of the Fluke multimeter is its dual-display capability. This parallels certain aspects of MDO and MSO oscilloscopes, where time domain and/or frequency domain signals can be viewed simultaneously in real time. The applications are similar in that insight can be gained regarding the way in which electronic equipment is functioning during product development and also for troubleshooting when repairs become necessary.
In the past, to make two measurements on a common signal, separate meters were required. A single Fluke 8808A performs these operations simultaneously and presents the results in the dual display.
To demonstrate, first power up the meter. Then, with the probe leads inserted into the usual multimeter ports and the probe tips inserted into a live 120-V receptacle, we begin by checking the single display readings. As expected, the ac volt reading is about 121 V with some decimal point fluctuation. The frequency is a relatively stable 60 Hz when the instrument has warmed up.
The readout, as in other Fluke instruments, is bright and clear regardless of ambient light, and the fonts are very sensible for excellent readability.
To see ac volts and frequency simultaneously in the dual display, the procedure is first to access the reading that is to be in the larger primary display at the bottom of the readout. Then press and release Shift followed by the button that corresponds to the reading that is to be in the secondary display. (It is important not to keep Shift pressed while activating the secondary display, or it won’t appear in the readout.) To exit the dual display mode, simply press either of the two function buttons.
The value of the dual-display mode goes way beyond the convenience of not having to switch back and forth to view the two parameters. A number of measurement combinations can be shown in the dual display, but some combinations are not compatible. For example, dc volts cannot be displayed with frequency or with ohms. AC volts can be (and often is) displayed with frequency but not with ohms. Current and volt measurements, ac or dc, are frequently taken together.
The applications are numerous, limited only by the creativity of the investigator. For example, Volts dc and Volts ac can be displayed in combination to ascertain ripple in the output of a power supply and also to troubleshoot amplifier circuits.
Volts dc and current dc can be displayed together to evaluate power supply regulation and to check voltage in a circuit under various loading conditions. Volts dc and current ac in combination will reveal the status of dc-to-ac and ac-to-dc converters. Volts ac and current ac in combination can be used to perform line and load evaluation and to determine transformer and other types of magnetic saturation.
Volts ac and frequency in the dual display, a useful combination, has many applications including determination of ac power quality and small signal analysis, measuring frequency response of an amplifier, adjusting an ac motor controller, adjusting a small electrical generator for correct power output, checking noise in telecom equipment, and setting network frequency compensation.
Current dc and current ac readings taken together can be used in evaluating a switching power supply for ripple and current draw, checking current loss in protective fuse resistors in power supplies and measuring line noise and ripple.
An important feature of the Fluke multimeter pertains to resistance measurements. The conventional method for measuring ohms is to set the meter in the ohms mode with the auto-ranging function on, and to place the probes across the portion of the circuit under investigation. A certain degree of fluctuation or drift in the reading is inevitable because of probe tip contact resistance at terminals or leads.
Low resistance readings may be problematic in terms of getting a stable and precise reading. To mitigate this difficulty, the Fluke bench-type multimeter offers an alternate method used at the option of the operator. This is the four-wire resistance measurement, essentially deploying two pairs of probes in parallel to measure a resistance.
It may be awkward to hold all four probes and touch each pair of tips at a small circuit node. The Fluke 8808A offers a more convenient option using a single pair of probes with a four-wire adapter that plugs into the meter. When either of these methods is employed, it will be noticed that the reading goes to a higher resolution and is more stable. This is particularly apparent when touching the probe tips together to see something approaching a zero-ohm reading.
With the pressing of the button identified by sound and diode icons, the instrument can check continuity and test diodes. This button toggles between the two modes. The continuity test is often used because the operator can quickly test circuit board and electrical equipment and numerous points without having to look at the readout. An audible tone beeps when the input is below 20 Ω.
Any ohmmeter can test diodes. The voltage at the probes used for measuring resistance, in most meters about 3.0 V, will either forward-bias or reverse-bias a diode by moving the charge carriers toward or away from the semiconductor junction. Accordingly, a good diode will measure low resistance when it is forward biased and high resistance when it is reverse biased.
A multimeter that has diode checking capability performs a more sophisticated diode test by measuring the actual voltage drop across the diode when it is forward biased. Taking advantage of this capability, a bipolar junction transistor (BJT) can also be tested. This should not be construed as a complete dynamic test of the device, but if the diode function or lack thereof among the three leads are known, most failed transistors will be revealed.
One might expect that if a transistor has a metal case, it will invariably be connected to the base. One might also expect that the middle of three pins or leads will be the base, but this is not always the case. If there is no data sheet available, diode tests on the three pins or leads can reveal the identity of the leads and whether the device is npn or pnp:
In a BJT, diode activity is between base and collector and also between base and emitter, but not between collector and emitter. In an npn transistor, the anodes of both virtual diodes are connected to the base. In the pnp, these anodes are connected to the collector and emitter.