Negative Polarity Zener Barrier

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Are you using this Zener as overvoltage protection? Or are you using it as a shunt power supply?

See read shunt regulator

This is zener safety barrier between control system in safe area and fileld instrument located in hazard area.

OK, then your using the zener to clamp an overvoltage possibility. There also needs to be a fuse on the power source side, and a series resistor on the load side. The zener only conducts if the power source fails into overvoltage mode, or a fault in the hazard area occurs. So it is selected to prevent damage to the load in the hazard and safe areas. And in normal usage will never conduct current.

See example circuit

A telephone call to the zener barrier supplier will provide all the information needed.

If you're using a negative polarity Zener barrier then ground potential must be positive and the diode will be reverse biased, so it will work exactly the same as a positive potential barrier where ground potential is negative.

Primer for all kinds of diodes here.

It works for DC supply by limiting the maximum positive voltage level allowed to be applied to the circuit.

The Zener diode does not conduct until the applied (+) DC voltage level exceeds the Zener rating then it goes into avalanche conduction mode thereby draining/shunting all voltage greater than the Zener rating to ground instead of allowing the excess voltage to enter and circulate through the circuitry.

Example: If the Zener is 24 volt rated then it will start conducting all voltage above 24 volts to ground so that the supply is protected from any overvoltage conditions above (+) 24V.