Definition
The voltage measured across a semiconductor device, such as a diode, when it is conducting current in its normal (forward-biased) direction. For a typical silicon diode this drop is approximately 0.7 volts; for a germanium diode it is approximately 0.3 volts. The remainder of the supply voltage appears across the rest of the circuit.
Plain English
When current flows through a diode the way it is meant to, the diode itself uses up a small, fixed amount of the voltage. That small amount is the forward voltage drop.
Context Anchor
Seen in aircraft electrical system troubleshooting, especially when checking diodes, charging circuits, and electronic components.
Derivation
Forward refers to the direction in which the diode is designed to pass current. Voltage drop comes from the idea that voltage is 'used up' or 'lost' across a component as current flows through it, similar to a drop in pressure across a restriction.
Why Pilots Care
When troubleshooting avionics or electrical components, a diode that shows the wrong forward voltage drop (too high, too low, or zero) is a quick indicator of a failed part. Knowing the expected drop helps technicians and pilot-owners interpret simple meter readings correctly.
Intuition Check
Do not read “drop” as the circuit simply losing power for no reason. Here it means a measured voltage decrease across a specific electronic part while current is flowing through it.
Example Sentence 1
The technician measured the forward voltage drop across the diode and read 0.68 volts, confirming it was conducting normally.
Example Sentence 2
Designers must allow for forward voltage drop when sizing the aircraft battery and charging circuit.