We found this circuit in a 1977 technical documentation. Its purpose is to protect instruments against overloads. The zener is 20 V and the transistor can be a BD135. The indicator LED can be any color.

We found this circuit in a 1977 technical documentation. Its purpose is to protect instruments against overloads. The zener is 20 V and the transistor can be a BD135. The indicator LED can be any color.