I have several RS485 devices. All of them are connected together and power by a 12V power supply. Each device has its own 5V voltage regulator. I want to use diodes for each of them to protect aginst power supply reverse polarity connection. The problem is if I place the diode on the 12V input terminal, about 125mA will flow through GND (that is wrongly connected to 12V) and A or B (that is connected to other devices and has +-5V); And if I put the diode on GND, the GND level of the devices will not be equal because of the voltage drop across the diode.


simulate this circuit – Schematic created using CircuitLab

So which is the best option?

  1. One diode on the 12V input terminal for each device.
  2. One diode on the GND input terminal for each device.
  3. One diode on the 12V and another diode on the GND input terminal for each device.
  4. Another solution.
  • 1
    \$\begingroup\$ I don't quite follow what you're doing here. A reverse polarity protection diode connected in series will pass 0 current if you connect the input the wrong way. Where is the 125 mA coming from? \$\endgroup\$
    – pipe
    Nov 12, 2020 at 18:54
  • \$\begingroup\$ @pipe: As I mentioned current flow from the power supply 12V that is wrongly connected to the GND terminal (diode is on the device 12V terminal) to A or B line of RS485 that is -5V or +5V. \$\endgroup\$
    – Vahid
    Nov 12, 2020 at 19:41
  • 1
    \$\begingroup\$ If you connect 12V to GND while other devices are connected correctly, you short out the supply and nothing you do can fix that. If you connect 12V to GND with nothing else connected, nothing will happen. If you connect 12V to GND and supply GND to input 12V, the series diode will block it. Nothing will draw 125 mA. You probably need to use the schematic editor to show exactly what you mean by "connected together". \$\endgroup\$
    – pipe
    Nov 12, 2020 at 19:44
  • \$\begingroup\$ @pipe: I added the circuit diagram. I tested it, there is a 125mA current flow. \$\endgroup\$
    – Vahid
    Nov 12, 2020 at 20:18
  • 1
    \$\begingroup\$ You can also check this topic for alternative solutions electronics.stackexchange.com/questions/164306/… \$\endgroup\$ Nov 12, 2020 at 20:39


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