Question T7D10
From subelement T7 - T7D
What reading indicates that an ohmmeter is connected across a large, discharged capacitor?
Why is this correct?
When an ohmmeter measures a discharged capacitor, it applies a small voltage that charges the capacitor. Initially, the uncharged capacitor acts like a short circuit, showing low resistance. As the capacitor charges from the ohmmeter's test voltage, it increasingly opposes current flow, causing the resistance reading to climb steadily upward over time until reaching a very high or infinite reading.
Memory tip
Remember the charging pattern: discharged capacitor = low initial resistance that climbs. This 'resistance ramping up' behavior is unique to capacitors and helps distinguish them from other components during troubleshooting. Resistors show steady readings, while short circuits stay low and open circuits start high.
Learn more
This behavior occurs because ohmmeters use a small test voltage to calculate resistance via Ohm's law. As the capacitor charges, less current flows for the same applied voltage, creating the appearance of increasing resistance. In amateur radio circuits, this principle helps identify filter capacitors in power supplies or coupling capacitors in RF stages during troubleshooting.
Think about it
Why do you think a fully charged capacitor would eventually show an infinite or very high resistance reading on the ohmmeter, and what would happen if you reversed the test leads?