More Special-Purpose Meters


The oscilloscope was developed over 50 years ago. As a piece of test equipment, it was one of the most exciting advances in electrical testing at that time and still is. Ordinary analog and digital meters merely give numerical quantities, whereas the oscilloscope does that and in addition, gives a graphic picture of what the test signal looks like. The shapes, frequencies, amplitudes, phases, distortions, interference, and so on, all show up to be examined. One cycle or less of an AC signal, or multiple cycles, or even a stream of cycles can be examined. DC levels and combinations of DC and AC can be viewed.

The horizontal trace of the oscilloscope controls the frequency, and the vertical trace shows the amplitude; together they draw the shape on the screen. Most oscilloscopes are analog devices, relying on grid lines over the screen to show quantities related to the range settings. Very precise measurements can be interpreted with the waveforms. More sophisticated scopes have dual traces so that two waveforms can be compared. But the most sophisticated advancement is the addition of a digital readout of all the waveform data, as each is selected.



Probes are special attachments used with test equipment to increase the meter’s usefulness or accuracy. They are test leads that contain extra parts or circuits. There are high voltage probes to extend the voltage range of the meter; radio-frequency probes to measure RF signals; and oscilloscope probes which help reduce the distortion of the test signal.

The test probe has a firm body, allowing it to be gripped tightly, with a thin test prod which is placed against the test point. Usually, an alligator clip is used for a ground connection. The probe is usually connected to the test equipment through a shielded cable, especially with rf and oscilloscope probes, to prevent the leads from picking up any stray, spurious signals.

The high-voltage probe contains a very large resistor which essentially is added to the meter’s input circuit voltage divider. The high resistance of about 1000 megohms in series with meter’s I-megohm tapped voltage divider can divide the input down to about ½ooo of its value on a low scale, to bring it within the meter’s range; i.e., 40,000 volts can be reduced to about 40 volts. These probes are used with dangerous voltages.

The RF probe contains a diode detector circuit to change the radio signal to a corresponding dc voltage level. Some probes, used for small signals, can also contain preamplifier circuits.

The oscilloscope probe contains an RC (resistor-capacitor) circuit, in which the capacitor is adjustable so that the impedance of the probe at the prod end can be matched through the cable to the oscilloscope input. This reduces the distortion of the test signal.


Stray Voltage Meters

Ordinarily, all parts of a circuit which are grounded, or connected to objects that are supposed to be at ground potential, are usually kept at zero volts. No voltages should exist between any two grounded points. Unfortunately, grounds located at different places tend to build up slight resistances between them. Current flow in the area between grounds can produce small voltage drops, raising one ground potential slightly above the other. These are called stray voltages. Metallic objects, such as appliances, metal buildings, fences, etc., could cause shocks because of the stray voltages. This is a particularly annoying problem on dairy farms, where these small shocks to cows can lower milk production. There are stray voltage meters designed specifically for testing stray voltage buildup of around 3 volts. The use of the ground resistance megger can be utilized to locate these imperfect grounds, and ground-fault circuit interrupters can protect against larger ground potential buildups at their locations.



Meter Sensitivity and Accuracy

Every meter coil has a certain amount of dc resistance. The amount of resistance depends upon the number of turns on the coil and the size of the wire used to wind the coil. The strength of the magnetic field about a coil increases as the number of turns of the coil increases. Therefore, if more windings are placed on a meter coil, a small current can create a magnetic field strong enough to cause the coil to deflect full scale.

The amount of current necessary to cause the meter pointer to deflect full scale is the meter sensitivity; it is an important characteristic of any meter. Typical current meter sensitivities vary from about 5 microamperes (0.000005 amperes) to about 10 milliamperes (O.QIO ampere). Some common values are 5, 50, and 100 microamperes (µA); and 1, and 10 milliamperes (mA).

The sensitivity of a meter movement is the maximum current that the movement can measure. Any current greater than this value will very likely damage the meter. Too much current might cause the pointer to rotate past full-scale deflection and bend about the right retaining pin. Or, too much current might cause the coil to burn out. A heavy current overload sometimes causes both types of damage.


Meter Accuracy

The accuracy of a meter is specified as the percentage of error at full-scale deflection. For example, if the accuracy of a 100-mA meter is specified as ± 2 percent, not only might the meter be off by ± 2 mA at a 100-mA reading, but it might be off by as much as ± 2 mA for any reading below full-scale deflection. Therefore, the accuracy of a meter becomes progressively poorer as the pointer moves farther and farther from full-scale deflection towards zero.

For example, at a meter reading of 50 mA, the meter, since it could still be off by ± 2 mA, is only accurate to ± 4 percent. At a meter reading of IO mA, the meter could still be off by ± 2 mA, which is a true reading accuracy of ± 20 percent. This, of course, is all based on a full-scale reading of I 00 mA. By using certain meter range circuits though, this situation can be improved.


Which Meters for DC and AC?

All the basic meter movements discussed so far can be used to measure DC. However, the moving-coil meter movement is used most often because it is more sensitive and more accurate. Although concentric-vane and radial-vane moving-iron meter movements can measure both ac and dc, they are generally used to measure low-frequency ac. Even in AC applications, the moving-coil meter movement is used much more than the other types of movements, but, for it to measure AC, the AC must first be converted to DC and then applied to the meter movement.

Thermocouple meters can be used to measure both AC and DC. In electrical and radio work, however, it is used almost exclusively to measure radio- frequency (RF) currents. The frequencies of these currents range from a few kilohertz to thousands of megahertz and can only be measured by a thermocouple meter because it operates on the heat produced by the current and is insensitive to frequency. Other meter movements are inaccurate for high-frequency measurements.



Moving-Coil and Moving-Iron Meter Scales

Scales for Moving-Coil Meters

Moving-coil meter movements have a linear scale; that is, a scale in which the space between numbers is equal. The distance that the pointer deflects across the scale is directly proportional to the amount of current flowing through the meter coil.

When the full rated current of a moving-coil meter movement flows through the coil, the pointer deflects full scale; when one-half the rated meter current flows through the coil the pointer will move one-half the distance across the scale, and so on. The reason for this is that the magnetic flux produced by the coil increases in direct proportion to the current, so the interaction of the fields also increases proportionally to give a linear reading. This is not true for moving-iron type meters.


Scales for Moving-Iron Meters

As stated previously, the scale for a moving-coil meter is linear. If the amount of current through the meter doubles, the distance the pointer deflects doubles; if the current through the meter triples, the distance the pointer deflects triples. This relationship does not hold for moving-iron meter movements, however. Instead, the deflection increases with the square of the current. If the current through the meter doubles, the strength of the magnetic field about each vane doubles. Therefore, the repulsion of each vane becomes twice as great. Since the repulsion of each vane is now twice as great, the combined repulsion of the two vanes becomes four times as great. If the current is tripled, the repulsion of each vane becomes three times as great, and the combined repulsion of the two vanes becomes nine times as great. Thus, the deflection varies as the square of the current, rather than in a linear manner.

Since deflection is nonlinear, the scale of a moving-iron meter must be nonlinear. The numbers at the low end of the scale are crowded, and they are farther and farther apart toward the high end of the scale where deflection is greater.


Edgewise Scales

The standard stock meter movement design uses the broad round or rectangular face that is higher and wider than it is deep. The large face area of the stock meter permits room for the scales to be highly legible and provides room for multiscales on its face.

For equipment where panel space is at a premium, the edgewise reading meter can be used. These meters run deep, but take up only a small rectangular area for the scale. The edgewise meters can be designed for either horizontal or vertical use and are often made available so that they can be stacked. With such an arrangement, a few edgewise meters will take up the same panel space as one stock meter.

Stock meters are more accurate than edgewise meters because their pointers arc in a plane that is parallel to the scale face. With an edgewise meter, the arc of the pointer causes the scale face to curve to the same sweeping arc as the pointer. The curved scale increases parallax error possibilities from one end of the scale to the other.



Resistance Testing: Part 1

There are several different types of resistance tests. Resistance tests differ from voltage and current tests because they are rarely performed on a dynamic basis, that is, while the equipment is operating. Resistance tests are usually performed with the power off and usually with the component disconnected to make sure that there are no short circuits to cause misleading readings.

Resistance testing of a component that has pure resistance is straightforward. The component resistance reading should be the same as it would be in both ac and dc circuits. But many components used in ac circuits have both a pure resistance and an impedance, which affect circuit operation. Any component containing coils or capacitances has an ac impedance. Ohmmeters measure only the DC or pure resistance of a component. There are, however, AC component testers, to measure impedance.


Wire Continuity Tests

Many electrical troubles are caused by breaks, or opens, in wire continuity, as well as increased resistance in a connection. Continuity testing is merely a test to show that there are no complete breaks in a wire, whereas resistance testing generally measures for a more specific reading in ohms. Since a reading is not always necessary in a continuity test, many continuity testers sound an audible to notify the user when continuity exists, while an analog or digital meter would show zero or very low resistance.

Testing the continuity of a wire requires connections to both ends of that wire. With a short wire, this is not difficult, but with a long wire, where one end might not be accessible, that end might have to be tied to another return wire so that the continuity of both wires can be tested in series.


Wire Resistance Tests

Continuity tests are general indicator tests that are useful in most cases. But, in some cases, particularly where a stranded cable is used, a wire can have continuity but too high a resistance if one or two or more strands are broken. Also, a bad connection will allow continuity, but a high resistance reading. Resistance tests are made the same as continuity tests, except ohm readings are obtained on a meter. For long wire lengths, the normal resistance of the length of wire should be known.


Infrared Tests

Because of some long-line testing, and the fact that wiring is often buried or hidden from view, many ordinary continuity and resistance tests are challenging. There are several infrared scanners available which allow the user to test for high resistance effects while the equipment is operating. The effect of high resistance in a circuit carrying current is that the resistance dissipates power and generates heat. The infrared testers are also called thermal testers. The tests are made by first aiming the tester at a known normal temperature target to get an ambient reference reading; and then aiming the tester at various suspected targets in a circuit, usually connections. An audible tone on some units may guide the user to a suspected target, while visual displays, such as lights, show the severity of the heat. Other testers can give the actual temperature readings.