RCL BRIDGES

Difficulty may be experienced when you attempt to measure wire-wound resistors. To obtain a sharp balance on the indicator, you can shunt the resistor with a variable capacitor and adjust the capacitor for the clearest indication. The resistance measurement will not be affected by this reactance neutralization.

MEGGERS

Meggers produce the large voltages that are required to measure resistances as high as 10,000 megohms - only high resistance values can be measured. The unknown resistance is connected between the megger terminals, and the hand generator part of the meter is cranked. Some meggers are capable of producing in excess of 500 volts, so use caution when you operate them. Typical applications for a megger are testing unterminated transmission lines and ac power cords for insulation breakdown.

DIFFERENTIAL VOLTMETERS

CAPACITOR MEASUREMENTS

Capacitance is that property of a circuit that produces an electrostatic field when two conducting bodies separated by a dielectric material have a potential applied to them. Capacitors are made by compressing an insulating material (dielectric) between two conductors (plates). The farad is the basic measurement of capacitance. It is dependent upon the area of the plates, the distance between the plates, and the type of dielectric used. Electrically, the farad is a measure of 1 coulomb of potential charged by 1 volt. A coulomb (the amount of current flow maintained at 1 ampere that passes a given point of a circuit in 1 second) is a large charge. Most capacitors are measured in millionths of a farad (microfarad), expressed as mF , or in one-millionth of a microfarad (picofarad), expressed as pF.

Capacitors incur various losses as a result of such factors as resistance in the conductors (plates) or leads, current leakage, and dielectric absorption, all of which affect the power factor of the capacitor. Theoretically, the power factor of an ideal capacitor should be zero; however, the losses listed above cause the power factors of practical capacitors to range from near 0 to a possible 100%. The average power factor for good capacitors, excluding electrolytics, is 2% to 3%. Current leakage, which is an inverse function of frequency, is important only at the lower frequencies and becomes negligible at higher frequencies. Dielectric absorption (sometimes referred to as dielectric viscosity) results in losses that produce heat. The effect of this type of loss is the same as resistance in series with the capacitor.

You have probably learned the hard way that some capacitors can retain a charge long after the voltage has been removed. The electrical charge retained by capacitors in de-energized electronic circuits is, in many cases, sufficient to cause a lethal shock. Be sure you and those working with you consider this hazard before performing any type of maintenance on any electrical or electronic circuit and before making connections to a seemingly dead circuit. Use extreme caution prior to working on or near de-energized circuits that employ large capacitors. Be safe - discharge and ground all high-voltage capacitors and exposed high-voltage terminal leads by using only an authorized shorting probe, as shown in figure 1-11. Repeat discharge operations several times to make sure that all high-voltage terminations are completely discharged. It is of the utmost importance that you use only an authorized safety shorting probe to discharge the circuits before performing any work on them. An authorized general-purpose safety shorting probe for naval service application may be requisitioned using the current stock number listed in the ELECTRONICS INSTALLATION AND MAINTENANCE BOOK (EIBM), General NAVSEA 0967-LP-000-0100, Section 3, Safety Equipment. Certain electronic equipment are provided with built-in, special-purpose safety shorting probes. These probes are not considered general purpose. Use them only with the equipment for which they are provided and only in a manner specified by the technical manuals for the equipment. It is considered to be poor practice to remove them for use elsewhere.

Figure 1-11. - Safety shorting probe.

When using the general-purpose safety shorting probe, always be sure first to connect the grounding clip to a good ground connection (if necessary, scrape the paint off the grounding metal to make a good contact - paint can be replaced, lives can't). Then, while holding the safety shorting probe by the handle behind the protective shield, touch the end of the metal rod to the points to be discharged. Touch each point several times to ensure that the circuit is completely discharged. Be extremely careful that you do not touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot" terminal. Don't develop a nonchalant or routine attitude about these procedures. It pays to be safe; use the safety shorting probe with care.

Large capacitors, dormant in storage, can also develop a large static charge. This charge is caused by environmental conditions such as a close proximity to an rf field. An easy way to avoid this condition is to short the stored capacitor's terminals with a piece of wire before putting it in storage. Remember to remove the wire before installing the capacitor. If you receive a large capacitor that is not shorted, short the terminals together. Remember, CHARGED CAPACITORS CAN KILL.

Q.12 Charged capacitors can kill. True or false?

BRIDGE-TYPE MEASUREMENTS

Capacitor tests involving quality and value must be made in the course of everyday troubleshooting. You must make the important decision of whether to reject or continue to use a certain capacitor after it has been tested. Capacitance measurements are usually accomplished by either a bridge-type or a reactance-type capacitance meter. The bridge-type capacitance meter is much more accurate than the reactance-type meter. You may want to review rcl bridges in chapter 1 of NEETS, module 16, before reading further. Capacitance tolerances vary more widely than resistance tolerances and are dependent upon the type of capacitor, the capacitance value, and the voltage rating. The results of capacitance tests must be evaluated to determine whether a particular capacitor will fulfill the requirements of the circuit in which it is used.

The power factor of a capacitor is important because it is an indication of the various losses attributable to the dielectric, such as current leakage and dielectric absorption. Current leakage is of considerable importance, especially in electrolytic capacitors.

Figure 1-12 is a simplified schematic of a capacitance bridge. As you can see, a capacitance bridge is very similar in construction to a resistance bridge with the exception of the standard capacitor (C_S) and the unknown capacitor (C_X). Because current varies inversely with resistance and directly with capacitance, an inverse proportion exists between the four arms of the bridge. The following expression shows the inverse proportion between resistors A and B and capacitors C_S and C_X:

Figure 1-12. - Simplified capacitance bridge.

It is actually the capacitive reactance, rather than the capacitance, that is balanced in this circuit.

In addition to its reactive properties, the capacitor under test always exhibits some loss. This loss may have the characteristics of either a shunt or series resistance, or it may be a combination of both. Regardless of its true nature, the loss can always be represented as a simple series resistance, which is shown in figure 1-12 as R_X. This loss is balanced by the calibrated resistor R_S. Rather than calibrate this control in terms of resistance, it is convenient to calibrate it in terms of the dissipation factor (the ratio of the energy dissipated to the energy stored in a capacitor). The R_S control then provides the means for completing the capacitance balance, and its dial reading indicates a loss figure for the capacitor under test.

Q.13 Which is more accurate, the bridge- or reactance-type meter?