More Meters for Beginners
December 1964 Radio-Electronics

December 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.

Prior to the advent of meters with a super high input impedance (>1 MΩ) featuring field effect transistors (FETs), accurate use of a voltmeter or ohmmeter required knowledge of the meter's internal resistance and the resistance of the device under test (DUT). As can be seen in Figure 2, various range settings on the meter resulted in different internal resistances. The internal resistance of the meter in series with the resistance of the DUT form a voltage divider where the indicated voltage is less than that which would be indicated if the meter's internal resistance was much higher. Author Robert Middleton presents examples of possible errors when measuring voltage, current, and resistance. Many modern digital multimeters (DMMs) have an input impedance of at least 10 MΩ, so very few scenarios would require minding the meter characteristics. With modern DMMs there is also no need to learn how to interpret the multiple needle scale markings, or to take care to avoid parallax error as a result of not looking at the needle from directly perpendicular to the scale markings, or keeping track of the reading units (mV, V, mA, A, Ω, kΩ, MΩ, etc.) Life as a meter reader is certainly much simpler these days.

More Meters for Beginners

By Robert C. Middleton

Most of the meters used in TV servicing have test leads. The resistance of the test leads is important on the R x 1 range of an ohmmeter. Fig. 1 shows the basic principle of ohmmeter action. An internal battery supplies current to a multiplier resistance R. This current flows through the meter movement, through the test leads, and through R_o, resistance to be measured. This diagram, which is reduced to the essentials, makes it obvious that the test leads will introduce an error in measurement if their resistance is abnormally high. Thus, if the leads become frayed internally, for example, R_o will appear to have a falsely high value.

This error might seem unimportant in a practical ohmmeter which includes a zero-adjust control (Fig. 2). The zero-adjust control is useful in setting the pointer to reference zero on the scale, when the battery voltage falls off from its "fresh" value. Hence, you might conclude that the zero-adjust control compensates for test lead resistance. To illustrate the error of, such a conclusion, let us take a practical example.

A peaking coil has a resistance of 5.5 ohms. The ohmmeter reads correctly when the test leads are in good condition. A frayed lead caused the ohmmeter to indicate that the coil's resistance was 4.6 ohms even though the ohmmeter was correctly zero-set. The abnormal lead resistance subtracts from the true resistance value.

Battery Condition

Fig. 2 will show that the same basic error crops up when the ohmmeter battery approaches the end of its useful life.

Ohmmeters, dB Scales, Current Probes

Fig. 3 shows the reason. A battery has internal resistance. When the battery is fresh, its internal resistance is low. When the battery weakens, its internal resistance increases although its emf (electromotive force) remains practically the same. In other words, a weak battery will seem to be good if tested with an ordinary voltmeter, Under load, its voltage measures below normal.

Such a battery appears "good" on a voltmeter test, and "weak" under load because a substantial portion of its emf is dropped across the abnormally high internal resistance. In an ohmmeter, the battery's internal resistance is added to the test-lead resistance. Hence, a weak battery has the same effect as defective test leads. It is easy to measure the internal resistance of a battery (Fig. 4).

First, measure the battery voltage on open circuit. Then connect a rheostat (R) across the the battery as in Fig. 4, and set the rheostat to the point where the voltmeter indicates one-half of the open-circuit voltage. The resistance of the rheostat is then equal to the battery's internal resistance. For example, a typical "good" size-D flashlight cell might have 0.4 ohm internal resistance.

This test works as shown in Fig. 5. When load resistor R has a value equal to the internal resistance R_int, a voltage divider is set up which applies one-half of the battery's emf to the voltmeter. Note that this is also the basic principle used in ordinary battery testers, which indicate good or bad on the basis of a battery's terminal voltage under normal load.

Testing the Test Leads

If you suspect that the test leads are frayed internally or that contact resistance is high, it is easy to check. Remove the leads from the ohmmeter and replace them with a short jumper of heavy copper wire. Turn the zero-adjust control to bring the pointer to zero. Then remove the jumper and plug in the test leads. Short the leads together and note the meter reading. A typical value is 0.1 ohm. A substantially higher value, such as 1 ohm or more, confirms your suspicion.

The Decibel Scales

These are used chiefly in audio test work. Unlike the voltage scales, a decibel scale (Fig. 6) is nonlinear. Its usefulness is based on the fact that the perception of loudness is proportional to logarithmic units like decibels, and not to voltage. Ten decibels equal 1 bel. When the system was established, it was based on the premise that 2 bels represented a sound level twice as loud as 1 bel. As a matter of fact, many persons will judge that an increase from 1 bel to 1.8 bel, for example, doubles the loudness of a sound. This difference in individual judgments, however, does not affect the utility of db measurements now that the db has been established and its meaning fixed.

As illustrated in Fig. 7, the decibel ranges are referred to a standard load such as 600 ohms. In other words, unless the test leads are applied across a 600-ohm load, the db indication will be incorrect. What is the reason? Simply that the decibel is fundamentally a power ratio, and a vom is not a power meter. Since a vom operates as a voltmeter on its db ranges, it must be applied across a known resistance so that the scale reading will be proportional to power.

This is not to say that db measurements are meaningless when made across other than the standard load value. For example, if both the input and the output of an amplifier have the same impedance it is possible to make db input and output measurements and to subtract the input reading. The difference is the actual gain of the amplifier in db. But note carefully - both the individual measurements are incorrect (in absolute terms), although the difference between the readings is a valid, correct value.

It is not possible to use this method of measuring gain when amplifier has different input and output impedances. In such a case, rather involved correction factors must be used. They are often impractical in a busy shop.

It is essential to add the specified number of db to the scale reading when you use any range other than the first ac-voltage range, as in Fig. 7. To put it another way, if the vom is switched to its 2.5-volt range, the db scale reads directly. But if the instrument is switched to its 10-volt range, we must add 12 db to the scale reading. Note the positive and negative scale sectors above and below zero db. These won't cause confusion, if you remember simply to observe the interval between readings (Fig. 8). In other words, if the first reading is -10 db and the second reading is +10 db, the total interval is 20 db.

Current Probes

Until recently, current (ac) had to be measured in one of two ways. Voltage can be measured across a series resistor in the circuit, and the current calculated from Ohm's law. If there is no series resistor, the circuit must be broken. This of course is time-consuming, and limits the convenience of current tests. In most shops, current is measured only when absolutely necessary.

However, it is now possible to measure current as easily and quickly as voltage. In fact, it is easier to measure current with a current probe (Fig. 9) than to measure voltage, because no connection is made to the circuit. The current probe (made by Hewlett-Packard) is basically a miniature "half-transformer" enclosed in a probe housing. Clamped around a wire, the probe becomes the secondary, and the wire is equivalent to a one-turn primary. Circuit loading is extremely light, because there is only a small magnetic coupling to the wire. The current probe is shielded, so it does not respond to electrostatic fields. Only the magnetic flux surrounding the wire contributes to the probe output.

The probe is used with a vtvm. It contains a transistor amplifier and a gain control (only a maintenance adjustment). Thus, the probe can be calibrated to read current values with high accuracy. Such probes are available with uniform response from near de to 400 cycles. The probe illustrated in Fig. 9 is designed for ac measurements only, and has flat response from 60 cycles to 15 mc.

It is not practical to use a current probe with a vom, because the input resistance of the instrument changes when the range switch is turned. The probe would have to be recalibrated each time the vom range was changed. On the other hand, a vtvm has constant input resistance on all ranges.

The probe indicates current on the voltage scales of the vtvm. A typical probe calibration factor is 1 millivolt per milliampere. Thus, a current flow of 75 ma produces a probe output of 75 mv. If small currents are to be measured, this type of probe is used with an audio vtvm, which has suitable low-voltage ranges.

Posted February 26, 2024