# How to determine the need for a blocking diode

I am powering an Arduino using a DC/DC converter to the Vin pin. The supply voltage is 32V and the output is 12V with a 500mA limit. The manufacturer recommends using a blocking diode "if current can flow backwards into the output".

How can I determine if such a condition exists? I know the value of the diode should be between 25-50V and 0.625-1.25A.

I'm not powering any motors or other devices that, when shut down could drive current backward. I AM powering a DMX output to LEDs (separately powered) and an RS485 network with 42 transceivers (150 ft. of cable). Each node on this network will have another Arduino attached with an identical power supply.

• Is there any other source of energy in your system besides the DC/DC converter? For example, do you also have batteries connected to Vin? Jan 8, 2014 at 22:25

For current to flow backwards, against the voltage generated by the DC/DC converter, you need some other energy source. If you have no such energy sources, then it can't happen. Possible sources:

• motors
• batteries
• connectors that someone might accidentally connect to one of the above

If you have any connectors at all, it's good practice to assume someone will connect them to something bad at least once. Putting diodes on them to clamp the maximum voltages seen here to the supply rails is pretty common, but then if these diodes start shunting excess voltage into the supply rails, where does it go?

simulate this circuit – Schematic created using CircuitLab

It's even common for CMOS ICs to have diodes like this internally on all their inputs and outputs for ESD protection. The microcontroller in your Arduino certainly has them. Are any of those pins accessible from external connectors that might be abused?

When excess voltage is encountered on these, it's dumped into the supply rails. If that current has nowhere to go, the supply voltage increases and things go boom.

The robust solution is some over-voltage protection, usually in the form of a crowbar circuit. Adding a diode to isolate the DC/DC converter from everything else will just protect the converter, leaving the overvoltage fault to destroy everything else.

• Capacitive components can also cause current to flow back into the power source when the power is turned off. Some regulators don't deal with that well and should be protected if capacitive loads are present. Jan 9, 2014 at 10:07