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X-NUCLEO-LPM01A output drops from 3.3V to ~1.5V when powering Arduino UNO

Uday
Associate II

‎2025-05-12 11:46 AM - last edited on ‎2025-05-13 1:30 AM by

Hi STM Community,

I'm trying to use the X-NUCLEO-LPM01A board to measure the current consumption of an Arduino UNO during simple blinking states.

Setup Description:

  • Power output is taken from CN14 (OUT and GND pins) on the LPM01A.

  • These lines are directly connected to the Arduino UNO's 3.3V pin and GND.

  • The LPM01A is powered via USB and in normal measurement mode.

Problem:

  • Before connecting Arduino UNO, LPM01A display shows a stable 3.3V (or sometimes 3.27V).

  • After connecting the Arduino UNO, the voltage drops to ~1.5V, and the Arduino does not power on.

Working Comparison:

Previously, I tested the same setup using STM32 NUCLEO-C031C6 as the target board.

  • I removed SB2 and SB23 bridges as per the reference manual.

  • In that setup, everything worked as expected — voltage remained stable, and current measurements were valid.

My Questions:

  1. Why does the voltage drop when powering the Arduino UNO?

  2. Is it not recommended to supply power directly to the Arduino UNO's 3.3V pin using LPM01A?

Thanks in advance for your help!

 

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Andrew Neil
Super User
In response to Uday

‎2025-05-13 3:57 AM - edited ‎2025-05-13 4:00 AM

@Uday wrote:

I'm using Arduino UNO R3.

That's a 5V board - it is not designed to run at 3V

 

@Uday wrote:

I tried supplying 3.3V to the Arduino using an ESP32 board’s 3.3V output and GND, connected respectively to the Arduino's 3.3V pin and GND.

When you do that, you are pushing the 3V3 backwards through the U2 regulator!

 

AndrewNeil_0-1747133331605.png

 

As described in the regulator's datasheet, you are relying upon the intrinsic body diode - this is not intended operation, and could damage the regulator:

AndrewNeil_1-1747133709149.png

 

 

PS:

 

@Uday wrote:

  • Voltage across Arduino: 3.28V

What do you mean by that:

  • The voltage at the Arduino board's 3V3 pin, wrt GND ?
  • The VDD voltage of the ATMega MCU ?
  • other ?
A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.
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Andrew Neil
Super User
In response to Andrew Neil

‎2025-05-13 4:21 AM

@Andrew Neil wrote:
@Uday wrote:

I'm using Arduino UNO R3.

That's a 5V board - it is not designed to run at 3V

Apart from the backwards LDO, the ATMega MCU on the Uno R3 is clocked at 16MHz; this requires a 5V supply - it is out-of-spec at 3V:

https://forum.arduino.cc/t/powering-arduino-uno-with-3-3v/544396/3

 

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.
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