Igor's Blog

Some time during the week I noticed my digital photo frame stopped working. It uses a Raspberry Pi model A to run the jPhotoFrame software and sometimes the screen fails to turn back on after going to sleep overnight. This time the problem was different however. I noticed that the red power light on the Raspberry Pi was constantly flashing.

This is what I was seeing...



According to the R-Pi Troubleshooting page...
A blinking red power LED indicates problems with the power supply. On model A and B, it is hard-wired to the 3.3V power supply rail. If it is blinking, as one user has reported[1] it means the 5V power supply is dropping out. Use a different power supply.


Well the fix was easy then. My power supply was connected to a USB hub which then powered the RPi and the USB monitor, WiFi dongle, etc. Luckily I had an identical power supply as a spare. Plugging the other power supply let the photo frame start and boot normally. So problem fixed, but I wanted to know why it actually happened in the first place.



I checked the voltage output on both power supplies and it was pretty much the same and stable, so that wasn't it.

IMG_0830.jpg IMG_0831.jpg


I didn't want to do any rewiring so didn't measure the current output, but this is what I suspected was going wrong. The power supply is a model WT0502000-MID which is rated at 5V 2A output.

I did a rough calculation of power requirements: 1A for the USB monitor (5W @ 5V according to specs), 150-300mA for the RPi (depending on load), 100mA for the WiFi dongle, 50mA for the USB storage. That's around 1.9A and is quite generous with actual values probably quite a lot less. Still this was close to the maximum output for the power supply. Still no problems, until I realised that I also had my RGB LED clock connected to the same USB hub as well.

The clock LEDs alone add around 6x 20mA of current draw, for a total of around 120mA, and this pushes the total current draw over the 2A maximum. Running it all like this for several months is what I believe caused the original power supply to stop working at it's peak output.

Lesson learned, lets see if the reduced current draw without the clock plugged in helps.

-i

,
About — I'm an enthusiastic software engineer and consultant interested in many fields including J2EE, programming, electronics, 3D printing, video games, wood working and gardening.
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The views expressed in this blog are my own and not those of my employer.
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