I'm using Raspberry pi2 for my mini projects. But i found different behaviour of raspberry pi powering.

  1. Red led constantly glows and green LED flashes, and rpi is working fine.
  2. Red led does not glow at all and green LED flashes, but in this behaviour also, my raspberry pi is working fine.

But whats is the difference? someone help me to understand this.

Thanks for reply!!

  • Same kernel in both cases? Dec 18, 2015 at 5:15
  • No kernel differences, i had seen in both modes
    – Assazzin
    Dec 18, 2015 at 7:11

3 Answers 3


The Pi (at least later models) has a voltage sensing circuit. If the red light is not on the supply voltage is too low. NOTE even if the voltage is below minimum for the sensor (4.63±0.07V) it may run, but cannot be guaranteed.

  1. Everything is fine (power wise)
  2. Potential power issue ahead

In scenario 2 everything may look fine right now, but there's a reasonable chance that things go wrong when you plug in extra devices, like a wifi adapter or 'more dangerous' an external drive which takes the power from the USB bus.

There is a variation to scenario 2 and that's when you use linux-image-3.18.0-trunk-rpi2 (or linux-image-3.18.0-trunk-rpi for the Pi 1) as kernel and have DeviceTree enabled. See https://github.com/debian-pi/raspbian-ua-netinst/issues/207 and (upstream report) https://bugs.launchpad.net/raspbian/+bug/1506663
In that case there may be nothing wrong but the red power led is still not on. This applies to the Pi 1B+ and Pi 2B, but not the Pi 1B.


When the PWR LED (Red light) is not on, it means that the Raspberry Pi is being undervolted. The RPI requires up to 5.25V, but only when overclocked. For example, if you give your RPi only 4V the PWR light would not stay lit.

  • 4
    NEVER repeat NEVER put more than 5.25V on the Pi. NOTE The Pi SOC does NOT even run on 5V, it has on-board regulators to supply the voltages it needs (principally 3.3V). This risks damaging other chips.
    – Milliways
    Dec 18, 2015 at 5:22
  • it does NOT need 5.25V - with or without any overclocked, you aren't doing it any favors by increasing the voltage to the upper threshold limit - in fact, it would work better at the proper 5v level (regulator more efficient) - attempting to compensate for a poor power supply by raising its voltage in anticipation of a voltage drop under load is a bad design and should be avoided Dec 18, 2015 at 15:59

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