If a Zener diode fails in an open state, what happens to the output voltage?

When a Zener diode fails in an open state, it no longer conducts current. In its normal operation, the Zener diode is used to provide voltage regulation by allowing current to flow in reverse when a specific breakdown voltage is reached. When it fails open, it effectively creates a break in the circuit through which current cannot flow.

As a result, the load connected to the output side of the diode will no longer receive the regulated voltage it relied upon. Instead, the output voltage will begin to rise and approach the input voltage. This is because, with the Zener diode open, there is no longer any voltage drop across it, which would have otherwise maintained the regulated output. Therefore, the output will tend to fluctuate toward the input voltage, as it loses the reference voltage to hold it down.

This behavior illustrates an important aspect of Zener diode operation: it is designed to maintain a stable output voltage under certain conditions, and when it fails, that regulation is lost, leading to potentially dangerous voltage levels for connected components.