Reverse Biasing a Voltage Regulator

[Jdo300]

Hello All,

Simple question here. I have a microcontroller development board that plugs into the wall and has a garden variety LM7805 type 5V regulator on the board for powering the microcontroller and other peripherals. But also on this board is a header that allows the 5V from the board to power external circuits. My question is, what happens if I connect the 5V pin on the board to an external 5V power supply without the wall plug connected to the board?

In other words, I have a situation where I am supplying 5V to the 5V rail while there is no input voltage on the input side of the 5V regulator. Does it care if there is no input voltage or will this cause damage to the regulator?

Thanks,
Jason O

[vk6kro]

There is often a diode placed across the regulator to avoid this happening in normal use.
It could happen if there was a charged capacitor on the 5 Volt line and you turned off the input power.

So, yes, this is risky. The plug should probably disconnect the output of the regulator from the 5 Volt line if the 5 volts was going to come from elsewhere.

[rbelli1]

Plus you are powering all of the internal circuitry of the 7805 with your alternate supply. If it a battery it will drain faster.

There are some regulators that are designed to be operated like you are describing. Micrel MIC5235 for example.

BoB