Meters for Beginners
November 1964 Radio-Electronics

November 1964 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

This 1964 Radio-Electronics magazine article details the operation of common electrical meters - voltmeters, milliammeters, and ohmmeters - all based on Ohm's law (I = E/R). The core component is the d'Arsonval movement, a DC-sensitive mechanism that can measure AC when paired with rectifiers. Voltmeters use multiplier resistors for different ranges, while ohmmeters employ an internal battery, producing a nonlinear scale. AC measurements rely on rectifiers to determine RMS voltage (0.707 of peak sine wave), though this method only works for pure sine waves. The article also explains practical circuits, including protection features like fuses, and discusses voltmeter sensitivity (ohms/volt), emphasizing that higher input resistance minimizes measurement errors by reducing circuit loading. Full-wave rectification improves sensitivity compared to half-wave setups.

Meters for Beginners - How Do They Work?

By Robert G. Middleton

The most widely used electric measuring instruments are voltmeters, milliammeters and ohmmeters. They measure the quantities stated in Ohm's law: I = E/R.

Most voltmeters are hooked up to measure either ac or dc voltages. Milliammeters for ac are less common, simply because voltage is measured much more often than current in practical work. On the other hand, resistance is measured almost as often as voltage. Hence, we would expect to find ac ohmmeters in wide use - and we do. Generally found in separate units called capacitor testers, they are calibrated not in ac ohms, but in microfarads.

How Basic Meters Work

The heart of a meter is its movement (Fig. 1). Basically a current-indicating device, it responds to dc only. To measure dc voltage, a multiplier resistor must be connected in series with the meter (Fig. 2). Now, if a rectifier is connected in series with the movement (Fig. 3), we get an ac voltmeter.

A vom has several current ranges, and the current-indicating circuit is elaborated as in Fig. 4. This is a milliammeter configuration with three ranges. It responds to dc only. A simple change of test-lead connections changes the configuration into a dc ammeter (Fig. 5). The ammeter function is not wired into the switch circuit, because the switch contacts are small and will not carry heavy currents satisfactorily.

A practical dc voltmeter must also have several ranges (Fig. 6). Note here that the voltmeter circuit has comparatively high input resistance, compared with the milliammeter circuit - this distinction is typical of nearly all voltage and current instruments. There are occasional exceptions. For example, in checking some of the low dc voltages in transistor circuits, a 0.25-volt full-scale voltage range can be useful. In such case, you can use the 50-μa range of a vom as a voltmeter, with full-scale deflection at 0.25 volt.

An ohmmeter is obtained by switching an internal battery into the measuring circuit in (Fig. 7). Note how the circuitry is arranged basically to indicate the current that flows from the internal battery through the resistor under test. However, the scale used on this function is calibrated in ohms. This is a simple and convenient circuit arrangement, although it results in a nonlinear ohms scale cramped at the high end (Fig. 8). When the resistor under test has the same value as the total input resistance of the ohmmeter, the pointer deflects to one-half of full scale (Fig. 9).

A practical ac voltmeter must also have several ranges. Hence, a multiplier is used, illustrated in Fig. 10. Two rectifiers are used in this circuit, although only one of them supplies current to the meter movement. The series rectifier supplies the rectified output in Fig. 11. Why, then, is the shunt rectifier used at all? It is because the series rectifier is not 100% perfect. It conducts some slight current in the reverse direction, although its back resistance is high.

To obtain the effect of a higher back resistance, the shunt rectifier is used. It provides a low-resistance shunt path around the meter to prevent any reverse current from flowing through the movement. Calibrating resistors R and R1 in Fig. 10 are set at the factory. The scale of the ac voltmeter reads in rms volts. An rms (root-mean-square) ac voltage is one that has the same heating effect as an identical value of dc voltage. The heater in a vacuum tube will get just as hot whether it is supplied with 6.3 volts dc or with 6.3 rms volts ac. It so happens that an rms voltage is 0.707 of the peak voltage in a sine wave (Fig. 12).

Because ac voltmeters like the one diagrammed in Fig. 10 are calibrated to read rms voltages of sine waves, these instruments will not read the rms voltage of a square wave or a sawtooth or any complex wave. Thus, this type of ac voltmeter must be restricted to measuring sine-wave voltages. This is no great handicap; other types of instruments (especially the oscilloscope) are available for measuring the voltages of complex waves found in TV receivers.

How about measuring alternating current? The simplest, if not the most convenient, method is to insert a precision resistor in series with the line, and to measure the alternating voltage across the resistor (Fig. 13). Then, the current can be calculated by Ohm's law. For example, if you measure 2 volts across the 5-ohm resistor, the current flow is evidently 0.4 ampere. A somewhat more professional method of measuring alternating current is to use a current transformer ahead of the voltmeter (Fig. 14).

Complete VOM Circuit

The complete circuit for a standard vom is shown in Fig. 15. The fuse in the common lead is a protective device. Since the ohmmeter has a low input resistance on the R x 1 range, the 11.5-ohm resistor could be burned out if the test leads were accidentally connected into a "live" circuit. The fuse will blow in such case, protecting the ohmmeter circuit.

Note also in Fig. 15 that the two instrument rectifiers are connected in a bridge circuit with two 5,000-ohm resistors. In this configuration, full-wave rectification takes place (Fig. 16). This does not change the ac voltage indication, because the meter scale is calibrated to read rms volts, as it is in the case of the half-wave configuration. It is interesting to observe the meter current in each case (Fig. 17). When a half-wave instrument rectifier is used in a vom, the movement responds to 0.318 of the rectified peak voltage, and the scale is calibrated to indicate 0.707 of peak. On the other hand, when a full-wave instrument rectifier is used in a vom, the movement responds to 0.636 of the rectified peak, and the scale is calibrated as before to indicate 0.707 of peak. Basically, therefore, the full-wave arrangement is twice as sensitive as the half-wave configuration.

Observe the shaded areas in Fig. 17.

These emphasize the equal areas in the rectified current sine wave, which determine the average value of the rectified wave. This is 0.318 of peak for a half-rectified sine wave, and 0.636 of peak for a full-rectified sine wave.

Voltmeter Sensitivity

All voltmeters have a rated sensitivity which is specified as ohms per volt. What does this mean? This rating refers to the input resistance of the voltmeter. To find the ohms-per-volt sensitivity of a vom, divide the full-scale indication on any range into the input resistance on that range. Thus, if your meter has an input resistance of 50,000 ohms on its 2.5-volt range, its sensitivity is 20,000 ohms per volt. This same meter will have an input resistance of 200,000 ohms on its 10-volt range. In other words, a vom has the same sensitivity on all ranges, but the input resistance is low on the low ranges, and high on the high ranges. A 20,000-ohms-per-volt meter has 100 megohms of input resistance on its 5,000-volt range.

It must not be supposed that a vom will have the same ohms-per-volt rating on its ac voltage range. Thus, a meter which has 20,000-ohms-per-volt sensitivity as a dc-meter, commonly has 1,000- or 5,000-ohms-per-volt sensitivity on its ac-voltage function. Meters with half-wave rectifiers may have an ac sensitivity of 1,000 ohms per volt, while meters with full-wave rectifiers generally have one of 5,000 ohms per volt. Why the difference? Because instrument rectifiers are contact rectifiers. To control the characteristics of contact rectifiers satisfactorily, lower-impedance circuits must be used than on the dc function. Hence, vom's using contact rectifiers will necessarily have lower input resistance on their ac-voltage function.

The sensitivity of a meter matters to the technician, because it tells him how much the meter will load a circuit, or, it indicates that circuits exceeding a certain impedance cannot be tested accurately. In practical work, we prefer to keep the input resistance of the vom at least 10 times higher than the impedance of the circuit under test. Thus, in measuring agc voltages with a vom, it is usually desirable to use as high a range as possible that will still provide a readable indication. In this way, the measurement error due to loading is minimized.