By Carl Babcoke, CET
Following are brief descriptions of basic VOMs, VTVMs and DMMs, with a list of advantages and disadvantages of each type.
Volt-ohm-milliammeter--VOMs include an analog meter with a pointer that is deflected by cur rent alone (although some voltage is required to overcome the de meter resistance). This power to deflect the meter is taken from the dc or ac voltages it is measuring.
Various currents are tested by placing proper values of shunt resistors in parallel with the meter.
DC voltage is tested by reading the amount of current that flows through a series resistor of known value. The amount of undesired loading on the circuit tested is determined by the meter-current rating for full-scale readings. A 1mA meter, for example, must bleed 1mA of current from the tested circuit in order to have a full-scale reading. (That is a 1000-ohms-per-volt rating.) A 50/4A meter only bleeds 5014A from the voltage source it is testing. This is called a 20,000-ohms-per-volt (or 20k/V) sensitivity, and is the most popular rating for portable VOMs.
Ac voltages are rectified by diodes into dc voltages before they are measured by the dc voltmeter action. Ac voltage ranges usually have lower sensitivity than dc.
Resistances are measured by monitoring the current drawn from an internal battery through an internal reference resistor that is in series with the external unknown resistance.
Vacuum-tube voltmeters--VTVMs and newer transistorized multimeter or FET-meter have either tube or solid-state amplifiers with cur rents that operate the analog meter according to the amplitude of the input signals. The amount of meter current has no effect on input resistance. These amplifiers operate from internal batteries or power sources. Ac voltages are changed to dc by tube or solid-state diodes before they are applied to the dc input voltage divider (the same divider is used for all ranges).
For resistance tests, the voltage across the unknown resistance is measured by the dc voltage circuit.
The current to be tested produces a voltage drop across a precision resistor, and this voltage is measured by the dc-voltage function.
(VOMs measure by current; VTVMs and DMMs by voltage.) VTVM advantages--Most VTVMs have dc-voltage input resistances of 10M 0 or higher and all dc ranges have this same high resistance. A 0.5V range, for example, loads the circuit (that is being tested) by only 10 M-o . On the average, VTVMs have more ranges and can test lower voltages and resistances than VOMs. Ac voltages can be calibrated for sine-wave RMS and peak-to-peak. Bandwidth for ac readings is wide, although the stray capacitance adds loading to signals of several MHz. Resistance measurements usually can be made to 1,000 M-o. Forward-biased diodes can also be tested.
VTVM disadvantages-Loss of power-supply voltage eliminates all functions. Input resistance for ac voltages is about 1 M-o . Some zero-setting and resistance-calibration drifts occur, thus correction adjustments may be necessary. To minimize drift (which affects the meter's accuracy), some technicians run VTVMs constantly. The polarity switch setting must be changed each time the input dc signal is negative. Accuracy of measurements is average, and may be degraded by parallax.
Digital multimeters--A typical DMM uses an analog-to-digital (A/D) converter. A capacitor is charged by the input dc voltage while the A/D circuit measures the time required for the capacitor charge to reach a designated voltage. The digital signal representing the time is processed by a binary coded decimal (BCD) integrated circuit and a display driver before it lights the required number of 7-segment decimal digits in the readout. The usual measurement functions are: dc volts; ac volts; dc current; ac current; and resistance.
Advantages of DMMs--Many digital multimeters have 10 times better accuracy of readings than VOMs or VTVMs. Accuracies of 0.25% to 0.1% for dc voltage are common, even in portable units. The readings are given in decimal numbers, and no conversion factors are needed.
Input resistance for dc voltage measurements is 10M or higher.
Parallax viewing errors are impossible with these readouts. Except for function and range selection, most new DMMs have no external adjustments. They offer automatic-polarity indication, automatic decimal placement and automatic zeroing. A few models feature automatic ranging (the DMM selects the best range for each measurement). Current drain is so low that many DMMs can be operated from internal batteries. Specific models feature a visual or audible indication of continuity or overvoltage, high- and low-power ohms, and special diode tests.
Disadvantages of DMMs--One common objection to DMMs is delay in obtaining a stable reading.
Digital counting systems have a time of uncertainty at the beginning or end of each count. Dc voltages having ripple, pulses or other disturbances usually force a DMM to display wandering readings that are centered around the correct average voltage. Several years ago, several DMMs gave wildly incorrect readings when called on to measure half-wave unfiltered dc voltage. The integration evidently was not sufficient for this rare condition. Present production units do not have the problem. The ac frequency response of some DMMs is flat only to about 400Hz, with a rapid decrease above that point. This bandwidth is sufficient for line-voltage measurements, but is unsatisfactory for audio work. If wide bandwidth is needed, a few units have good response to 50kHz. Most DMMs cannot test resistances of more than 2M O. Ohmmeter current (even with a high-power range) often is not sufficient to show useful differences between normal forward-biased silicon junctions. Some older DMMs have been known to show wild variations of readings when near the horizontal-sweep section of color sets or in a strong RF field.
Many newer models have internal This DMM is portable and battery operated. (Courtesy of B&K-Precision) shielding to eliminate interference.
Most DMMs cannot follow peaks and nulls or other fast-changing signals.
For most uses, the advantages of DMMs overcome all minor short comings. However, the best multi-meter depends on the measurements being made and the accuracy that's needed.
----------- A portable VOM. (Courtesy of Mura); Typical of VTVMs and FET-meters
is this Volt Ohmyst. (Courtesy of VIZ); This DMM is portable and battery operated.
(Courtesy of B&K-Precision)
Also see: Unique features of DMMs