1-14
Ohmmeter applications include resistance measurements; continuity checks; and inductor, capacitor,
and transformer checks. A transformer, for example, may be tested by checking whether there is an open
or short, low-insulation resistance to ground, or improper continuity between transformer windings. A
capacitor may be tested to determine whether it is open or shorted. Ensure that capacitors are properly
discharged before you test them; otherwise, damage to the multimeter may occur. When an ohmmeter is
placed in series with a capacitor, the changing current will cause a meter deflection that is proportional to
the capacitance. The deflection obtained is compared with the deflection from a similar capacitor of
known value. This deflection may be small or large, depending on the type and size of the capacitor and
the voltage of the battery within the meter. An external series battery will increase the sensitivity of the
instrument.
Q-10.
How do you compensate for the resistance of the test leads of a meter?
DIGITAL MULTIMETER
The two major advantages of using a digital multimeter are its ease of operation and accuracy. Most
digital multimeters can be ordered with an optional battery pack, which makes them just as portable as an
analog multimeter. Another advantage is that their LED or LCD readouts are much easier to read than the
scale on an analog multimeter. Digital multimeters also are ideally suited for measuring sensitive devices
that might otherwise be damaged by the excessive current associated with analog multimeters —
maximum current flow through the component being tested is typically limited to less than 1 milliamp.
When measuring small values of resistances, remember to consider the resistance of your test leads. Most
digital multimeters cannot be zeroed in the way analog multimeters can. With digital multimeters, you
have to short the leads, read the lead resistance displayed, and then subtract the reading from subsequent
component measurements that you make.
Q-11.
Why are digital multimeters well suited for testing sensitive devices?
RCL BRIDGES
The 250DE+1325 is a typical resistance, capacitance, inductance (rcl) bridge. Like the vtvm, the rcl
bridge has several disadvantages. It requires ac power and a lengthy warm-up period, and its accuracy is
limited to 2%. The rcl bridge uses a tuning indicator electron tube, commonly referred to as the bridge's
"eye," and an internal arrangement of resistors that form a Wheatstone bridge. As discussed in NEETS,
module 16, the rcl bridge can be a time-consuming method of performing resistance measurements.
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.
