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

[Uday]

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!

[TDK]

It should work fine. What pins are you using? Should be using pins 1 and 3 of CN14. Perhaps show a picture of your setup.

[Uday]

[Uday]

Thank you for the response.

Yes, as specified in the user manual, I’m using pin 1 (GND) and pin 3 (VOUT) of CN14 on the X-NUCLEO-LPM01A board.

[Andrew Neil]

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@Uday wrote:

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

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Surely, that pin is an output - not an input?

What happens if you try to apply 3.3V to that pin from a standard power supply, and measure it with a standard meter - does that work?

PS:

Also, the Arduino UNO is a 5V system - it's not designed to run on 3.3V.

Is it an Arduino UNO R3? or R4? or what??

R3 Schematic: https://content.arduino.cc/assets/UNO-TH_Rev3e_sch.pdf

R4 Schematic: https://docs.arduino.cc/resources/schematics/ABX00080-schematics.pdf

[Andrew Neil]

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@Andrew Neil wrote:

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Surely, that pin is an output - not an input?

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@Uday  Indeed it is - as the User Manual shows:

https://docs.arduino.cc/resources/datasheets/A000066-datasheet.pdf#page=8

If you just apply power to the 3V3 pin, you are just driving the output of the LDO - so there is no power to the MCU or the rest of the system!

PS:

Similarly for the R4:

https://docs.arduino.cc/resources/datasheets/ABX00080-datasheet.pdf#page=12

[Andrew Neil]

@Uday I don't think that ST have anything which will supply 5V - you'll have to look elsewhere.

eg,

https://www.microchip.com/en-us/development-tool/atpowerdebugger

https://www.nordicsemi.com/Products/Development-hardware/Power-Profiler-Kit-2

No doubt others, too.

@STTwo-32 This has always struck me as a major omission on ST's part - that's why I have the other two, mentioned above!

[Uday]

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@Andrew Neil wrote:

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What happens if you try to apply 3.3V to that pin from a standard power supply, and measure it with a standard meter - does that work?

Thanks for your response!

I'm using Arduino UNO R3.

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. The Arduino powered up successfully — the onboard LED was blinking as expected.

I also measured the voltage and current using a multimeter:

• Voltage across Arduino: 3.28V

• Current drawn by Arduino: 49.65mA (measured in series between ESP32 and Arduino)

[Uday]

Yes, you're right — the X-NUCLEO-LPM01A only supplies a maximum of 3.3V output. Unfortunately, I cannot use any other board since I’m required to use this one specifically for my project.

[Andrew Neil]

Who "requires" you to do that?

Clearly, it's not suitable for what you're doing - so something will have to change!