Over Voltage Protection


Im designing a shield which will connect to the Mega2560. The shield will have its own power supply (5V0 and 3V3) which will in turn supply the Mega2560.

I am looking to be able to connect a USB cable to the Mega2560 to download data but obviously, this will introduce a second power supply, ie the shield PSU and the USB 5V0 and I am concerned that the Mega2560 might start smoking, glowing and going BANG if I dont include some sort of over voltage protection on my shield but I have no idea how to do this. I have googled how to protect dual power supplies but all the info I could find included fitting diodes to both power supplies and obviously, I cant fit a diode into the USB 5V0 on the Mega2560 without redesigning the board! Can anyone point me in the right direction please?

Cheers, Stav.


Not knowing anything about your shield, is it possible to simply not connect the +5V coming from the USB?

@Rob the shield has an mcp3208 12 bit ADC on board and the shield power supply is for the sensors, the shield and the Mega2560. Under normal running, the USB won't be plugged in. The whole unit is being supplied by an external PSU connected to the shield. Occasionally, I want to plug in my laptop via USB and download the data that has been collected. It's only at this time, there will be the external PSU and the USB 5V from the laptop.

Hope that helps.


A Mega is normally powered externally through the DC socket (min. ~7.5volt) or the V-in pin (min. ~6.8volt).
Then the onboard mosfet switchover circuit will switch automatically between USB and external power.
Powering the shield and Mega with 5volt on the 5volt pin could back-feed 5volt into the laptop, and could damage the USB supply inside the laptop.
A solution could be a powered USB hub without the supply being connected to the hub.
Or make a custom USB cable with the (red) +5volt wire cut.

I’ve got a lot of projects that have their own power and also plug in USB while they’re powered. I’ve never had a serious* problem.

Any high-current devices (eg, more than 100mA) are powered separately from the project’s 12V but the 5V regulator that’s powering the Arduino is just hooked straight to the Arduino’s 5V pin. If I plug in USB then maybe it’s powering my computer or maybe the computer is powering the project.

*I have of course blown up Arduinos when the 12V wire from something else touched the wrong pin. I’ve even had the laptop freak out from 12V on its USB but unplugging the laptop battery for 5 minutes got it working again.