If the voltage to be measured is more than 300 volts, proceed as follows:
1. Shut off circuit power.
2. Discharge all filter capacitors with a shorting probe.
3. Temporarily ground the point to be measured.
4. Connect (clip on) the proper test lead to the high-voltage point.
5. Move away from the voltmeter.
6. Turn on circuit power and read the voltmeter.
7. Turn off circuit power.
8. Discharge all capacitors before disconnecting the meter.
Q-14. When taking a voltage measurement, which lead of the voltmeter should you connect to the circuit
Current measurements are not often taken in the course of preventive maintenance or testing. This is
because the ammeter (or other current-measuring instrument) must become an actual part of the
equipment being tested. The circuit must be opened (desoldered) to connect the ammeter in series with
the circuit being tested. Usually, you can take a voltage measurement and use this factor to calculate the
circuit current by applying Ohms law.
Q-15. Is an ammeter connected in series or in parallel with the circuit under test?
Resistance measurements are a valuable aid to you in locating defective circuits and components
during corrective maintenance. Maintenance handbooks for the equipment can often be used to help you
take these measurements. These handbooks often contain resistance charts that are referenced to
accessible test points within the equipment. Without these charts, taking resistance measurements in a
complex circuit is a slow process. The process is slow because one side of the circuit component must
often be desoldered to get a true resistance measurement. However, resistance tolerances vary so widely
that approximate resistance readings are adequate for most jobs.
Once the most accessible test point is found, an ohmmeter is usually used to take the resistance
measurement. Because of the degree of accuracy needed when an ohmmeter is used, proper calibration
and understanding of the meter scales is a must. (Topic 2 of this module will discuss these requirements
in detail.) When using an ohmmeter, you must observe the following precautions:
The circuit being tested must be completely de-energized.
Any meters or transistors which can be damaged by the ohmmeter current must be removed
before any measurement is made.
Q-16. What must be done to a circuit before you can use an ohmmeter for testing?