# Zener diode for overvoltage protection and earthing

I am designing a small circuit where the goal is to measure the current. The CT measures the current, which passes through a shunt resistor. The ADC convert the signal so it can be further processed.

I am thinking about using jack port for the input of the CT. However, someone might think it is a power supply input and might plug in a 230 V cable.

So I want to protect my circuit against high voltage. Apparently, placing a zener diode is the simplest solution.

However it looks like my circuit is wrong. The zener diode will be connected to itself. Can someone tell me what I am doing wrong?

First, that's not the symbol for a Zener diode. That may be the fault of your layout software.

For a Zener to protect the IC it would go between the pin and ground, with the cathode (line end) to the pin, something like this:

simulate this circuit – Schematic created using CircuitLab

The positive voltage on the pin will be limited to the Zener voltage $$\V_Z\$$ (more or less) and a negative voltage on the pin will be limited to the Zener's forward voltage drop (approx. 0.7 V) with respect to ground.

The current will need to be limited, otherwise a voltage higher than $$\V_Z\$$ will cause excess current in the Zener, likely burning it out. This can be done by using a series resistor after the shunt like this:

simulate this circuit

If you are worried about someone plugging 230 VAC into the jack you should use a jack that doesn't accept a 230 VAC plug, using something that can be mistaken for an AC receptacle for anything else is just asking for trouble.

• Thanks for your answer. The problem with using another resistor is that the shunt resistor becomes useless. Is there an alternative to using a resistor ? Sep 12 at 13:45
• @cobdmg How does the shunt resistor become useless? Current goes through the shunt dropping a voltage across it, you read that voltage with the ADC. The series resistor between the voltage being read and the ADC input isn't going to make much difference. Maybe the diagrams are unclear, I'll edit them. Sep 12 at 16:25
• I had something else in mind, it is more clearer now, thanks Sep 13 at 11:50

Just cut the connection upward from "1" on the Zener to break the loop. You also need to make sure that both resistor and diode are good for the resulting current from applying 230V if that is your concern.