I have a PI 4, WHEN using a 3.5A USB power measured 5.1v supply, the red LED is constantly on. When using a power supply which was rescued from a DVD player, checked supply 5.1v I get a flashing red LED (once every 2/3 seconds). Is there any difference as to how the voltage is connected ie USB-C connector or via GPIO pins 2 and 6 ? what does a red flashing LED every 2/3 seconds mean? All measurements taken from GPIO pin 2 an 6 using a volt meter. I have done several projects and using either way to power the unit up it does not seem to bother the PI 4 Projects here ANy feedback welcome

  • The flashing LED means your power supply isn't providing enough current even though it appears to provide the right voltage.
    – Dougie
    Commented Aug 23, 2020 at 13:43
  • As everyone has already pointed out the fact you've neglected to consider the electrical current being supplied, here's something that might help.
    – Seamus
    Commented Aug 23, 2020 at 16:52
  • @Dougie NO the LED indicates low voltage - which may have many causes. I have a 10A supply which happily supplies 10A - it just has poor regulation
    – Milliways
    Commented Aug 23, 2020 at 23:00

2 Answers 2


A red flashing LED is an insufficient power supply.


  • Well I measured via a volt meter and the voltage is 5.14 so better than the USB-C power supply which was 5.10, must be something else.
    – FXV300
    Commented Aug 23, 2020 at 12:41
  • 1
    Voltage is one thing - then there's current (amps) - you can't start a car with 8 x AAA batteries - even if they do add up to 12 volts Commented Aug 23, 2020 at 13:01
  • @FXV300 use an oscilloscope to monitor the power supply voltage ... it will probably reveal brief voltage drops that a voltmeter is unable to detect
    – jsotola
    Commented Aug 23, 2020 at 16:53
  • @JaromandaX that comparison is not quite accurate , the output of those AAA cells would not stay at 12 V ... if the output of the battery pack stayed at 12 V then the vehicle would start
    – jsotola
    Commented Aug 23, 2020 at 16:59
  • @jsotola - I'm sure with a multimeter with fast enough response time, the OP would see his 5.14V isn't stable either Commented Aug 23, 2020 at 22:38

Despite the many posts (on this site and others) if the red LED flashes (except on reboot) the Voltage is too low i.e. BELOW 4.63±0.07V - no amount of speculation about this will change the fact.

The voltage monitor chip responds to transient voltage, and the kernel stretches this to 3 sec so the flashing is more obvious.

Multimeters are designed to measure AVERAGE Voltage and will not show show transients.

Even if the LED is lit the voltage may still be below the USB minimum (4.75V), although the Pi itself may still work.

This MAY be due to a poorly designed Power Supply which cannot supply the required current while maintaining the required voltage. (Although all Pi - including the Pi4 will not require more than 1A itself.)

No matter how good your Power Supply if you use poor quality cables to connect to the Pi you will have problems. Many (the majority?) of μUSB cables are designed to carry data, and have very thin wiring. This makes the cables thin, light and inexpensive but they are unsuitable for power.

As an Electrical Engineer I have designed many computer systems and power supplies. Expecting a 5V supply to power a device which may require 3A over a 1m cable without ridiculously large cables is impossible (just look at the size of multiple wires on an ATX PS to supply 5V over a much shorter distance), so the official Pi supplies are designed to provide 5.1V and have decently sized cables to minimise the inevitable voltage drop.

See Raspberry Pi Power Limitations for further detail.

  • Many thanks to all for the comments. I have now solved the problem by implementing a voltage regulator. This can be tweaked from 12V down...so have adjusted to 5.4V and am monitoring the voltage and current using a voltage and ampere readout, I have not seen the current ever rgo over 0.7Amp so hopefully this will not blow up my PI4. Take a look at my video if you have the time youtu.be/_C6qLpJ5XR0
    – FXV300
    Commented Aug 24, 2020 at 10:36
  • @FXV300 setting voltage above 5.25V will exceed USB maximum. There is also a quite low maximum for the MxL7704 PMIC (I don't recall off hand) which risks damaging the Pi. This is NOT a reliable solution.
    – Milliways
    Commented Aug 24, 2020 at 10:44
  • MxL7704 PMIC spec sheet - Input voltage range: 4.0V to 5.5V but I guess I should find a different solution.
    – FXV300
    Commented Aug 24, 2020 at 12:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.