Protecting RS-232 Serial Port from Overvoltage

Updated Jan 19, 2023

Reported In


External Circuitry

Issue Details

How can I protect my RS-232 serial port from overvoltage damage?


You can build an overvoltage protection circuit using metal oxide varistors (voltage dependent resistors) or any fast semiconductor based component, such as Zener diodes.

Metal Oxide Varistors (MOVs) or Voltage Dependent Resistors (VDRs):
Varistors offer a large voltage surge-handling capability, but a delayed response time.  In the following circuit, the MOVs will shunt the current to the GND lines when the surge exceeds the MOV's rated voltage.

Zener Diodes:
Zener Diodes have a better response time over MOVs, but can only handle a small amount of current.  The resistors in the diagram below will prevent a large surge current from damaging the diode.

Additional Information

The RS-232 standard states that the maximum open-circuit voltage of a signal cannot exceed +/- 25 V.  Unfortunately, most RS-232 ports are not protected from voltage surges that exceed this specification.

The circuits detailed above are designed to clamp harmful voltage surges and protect your equipment. While every attempt has been made to ensure their accuracy they are provided As Is. They have not been tested or validated as a product, for use in a deployed application or system, or for use in hazardous environments.  You assume all risks for use of these circuits