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I'm reading that the lightning bolt in the top right corner of my pi means that it is currently underpowered.

I won't be able to get a new cord for a few days and I'm wondering if running under powered will have any long term negative effect on the pi?

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    It depends on how many and the type of peripherals connected to the Pi. Having circuits connected to the GPIO is going to have an impact as well. The main risk is corrupting the the filesystem on the SD card, you might also see some slow down in responsiveness as well. I would suggest against using an under-powered Pi, or at the least minimizing any voltage load on it. The model of Pi would be useful to know. – Darth Vader Feb 21 '17 at 20:11
  • I saw this in HNQ and thought it was a math question... What does happen if you don't at least Π^2, though? – MarioDS Feb 22 '17 at 10:39
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The main risk I can think of is file system damage. Your Pi is reasonably likely to unexpectedly shut down in the event of insufficient power. If you (or a background process) happened to be in the middle of writing something to the SD card at that point you run a decent risk of corrupting the file system. That might put your Pi out of action until you can re-image the SD card.

If your power supply is extremely underpowered you run a reasonable risk of physically damaging it (the power supply, not the Pi). Exercise a little common sense when you're considering your options and don't try to power the Pi from a 100mA supply.

Aside from that I think you'll be OK - underpowered components usually just don't work, rather than coming to harm. If you wanted to be super cautious I suppose you could unplug everything from the GPIO pins and avoid connecting the Pi to external storage. That should minimise the risks of damaging anything electrical or corrupting drives.

  • Thanks that's exactly what happened. I was able to find another cable and am now reimaging. Luckily I didn't have anything important on the pi. – Philip Kirkbride Feb 21 '17 at 20:07
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If it is necessary to do this, you can minimize damage as follows.

STEP 1: Remove all USB peripherals you can, and power the rest externally.

Place a single, POWERED USB hub between ONE USB port on the Pi and all USB peripherals. (Powered hubs have their own bricks. Unpowered hubs will make things worse. If you need more ports, DAISY CHAIN THE HUBS, do not use two ports on the Pi.)

RATIONALE: USB peripherals initially use up to 35mA per port, but can negotiate up to 500mA per port after that initial connection. If you used two powered hubs, you could still be drawing up to 70mA (though probably not). Using a single USB port connected to a powered hub guarantees your lowest possible host power draw.

STEP 2: Lower all clock rates-- CPU clock, GPU clock, memory clock, and the SPI clock in use for SD cards. (You can find the last one in the instructions for overclocking SD cards).

Put the clock rates back to normal after you get a proper supply. (You might even find you can overclock now, provided you use proper heatsinks and cooling.)

RATIONALE: The amount of power of power used per instruction is fairly constant. If you have less power available, you need to use less energy per second-- fewer instructions per second means less power used.

This step is easy, just tedious. Tweak one setting at a time, and test thoroughly. Start by knocking 25% off all the stock speeds. If possible, wire up an ammeter and MEASURE CURRENT DRAW BEFORE AND AFTER EACH CHANGE, it is easy to do and will be very useful for you to know.

STEP 3: Disable all GPIO equipment you can live without.

This includes individual features on each device, and this is often overlooked. Consider:

  • Turning down the gain on transmitters and receivers
  • Shutting off or dimming backlight displays
  • Lowering display framerates
  • Minimizing volumes on speakers
  • Allowing longer acquisition times for GPS antennas
  • Falling back to lower speed wireless standards
  • Reducing ADC and DAC sampling rates
  • Running motors one at a time, when possible (do not pan and tilt at the same time, etc.)
  • Reduce signaling speeds (serial baud rates, etc.)
  • Reduce optical gate/gray code wheel duty cycles

RATIONALE: Your payload is your purpose, but you may be able to meet your design goals in a round-robin fashion, using one device at a time, rather than just leaving all of them on all the time. This is the hardest step, but will also be the most rewarding for future designs.

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    I'm pretty sure unnegotiated USB power is 100 mA. Not a RPi thing, that's just the USB standard. – MSalters Feb 22 '17 at 9:31
  • Thanks, I'll check the spec. That's actually pertinent to me right now, so I appreciate the note. – breakpoint Mar 29 '17 at 0:01

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