So I have a small problem with my Arduino Pro Mini here. May be worth noting that it is a clone, but otherwise this is still some weird functionality. I have done some experimenting so I'll just set the scene for you. I have everything that requires power connected to the + and - lines on my breadboard, that way I can avoid drawing more than about 20mA across any individual pins. I currently have my Arduino Pro Mini hooked up to the 5V power supply via the RAW and GND pins, but the supply is a regulated 5v wall supply. It actually is currently outputting 5.36V.

I have made sure all my pins I'm using for output are indeed set as output, the problem only arises when I attempt to power the device from something OTHER than USB power. I tested the output from my wall adapter connected to RAW and to a random VCC pin, both have the same effect.

When powering the device via USB (USB to Deek-Robot ASP, then to Pro Mini), if I set my digital output pin to HIGH, I get 5v. This is enough to turn on a 5v relay that then supplies power from the supply to a small water pump. However, if I use the regulated wall-power, I get about 3.7 to MAYBE 4.15v, which sadly is not enough to switch the relay to the NO position.

I have tested the voltage coming from the USB ASP as well, it is 4.95v.

EDIT: The wall-plug voltage drops from 5.36 with no load except the meter to 4.7v with the Arduino and a couple LEDs attached, if that means anything.

  • 1
    RAW pin requires at least 6 V
    – Juraj
    Mar 9, 2020 at 8:51

2 Answers 2


"Arduino Pro Mini hooked up to the 5V power supply via the RAW and GND pins, but the supply is a regulated 5v wall supply. "

That is the problem. RAW pin expects more like 6V to 6.5V for the regulator to output 5V.

If you are supplying 5V, connect to the 5V, or VCC and GND pins.

If you are connecting to USB via an FTDI Basic or equivalent, and the 5V supply at the same time, then the computer may not like having the USB 5V line back driven from an external source (mine complains about a "power surge" and shuts down the USB port). Make up a breakout cable between the FTDI and the Promini and either disconnect the 5V line, or put a diode in series so the Promini can't supply 5V back thru the FTDI and into the USB port. That way the FTDI chip is powered from USB, but the Promini is not, and all the hardware is happy.

Make sure to use series resistors with LEDs to avoid overcurrent from the Promini output pin, which will either damage the IO pin, possibly damaging the 328P, or damaging the LED from overheating from excess current. (5.36V - 2.2V)/.02A = 156 ohm. A 180 ohm, 220, 270, 330, some value in that range will do. 1K will probably be fine, LEDs these days can be quite bright even at lower current; 20mA can be blindingly/annoyingly bright.


Power supply was dropping to 3.8 volts under basic load, once it turned on some LEDs it was just too low to activate the relay switch. Crappy Chinese parts.

  • So your problem is mainly related to the weak power supply, and to your attaching it to the wrong pin (RAW instead of Vcc). However, external supply at Vcc + USB in parallel is forbidden. Additionally, there are only few 5V relais that need only 20 mA. Mar 9, 2020 at 13:50
  • The question does not mention relays that I see. However, there are many 5V, 20mA coil relays. A simple search at Digikey turns up 42 for example in SMD and thruhole (with some duplication for differents methods of delivery packaging) digikey.com/products/en/relays/signal-relays-up-to-2-amps/…
    – CrossRoads
    Mar 9, 2020 at 16:51

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