It is a seldom-known fact that the Fluke 893 ac-dc differential voltmeter can be used for measuring
extremely high resistances from 10 megohms to 106 megohms with a typical accuracy of 5%. This
measurement method, however, requires some basic calculations on your part. The obvious advantage of
the differential voltmeter is its capability of measuring extremely high resistances. Consult the Fluke 893
technical manual for initial switch settings and a more detailed explanation of its operation.
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 F, 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 safedischarge 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
EEVblog #643 – Mailbag
Mailbag Monday Spoilers: Dual voltage source selection circuit Casio FX-7000G...