Disappointed in 5V output!

I recently migrated a circuit from a breadboard to a homemade shield to plug into my Yun. I was using a large 5V supply to power my breadboard, and made the incorrect assumption that the 5V pin on my Yun would actually output 5V.

Well, after I built the shield and installed it on my Yun the results were NOT as expected! I spun my wheels trying to figure out why my circuit was working PERFECTLY before but was not working nearly as well when powered by the Yun.

As it turns out, the 5V output is more like 4.4V! That is CRAZY low for a pin labeled "5V". My shield draws a peak of 30mA. I don't know what the 5V pin is rated for but surely it should be okay with 30mA? I am powering the Yun with a micro USB cell phone charger at 5.1V. Well, now I had a decision to make: do some recalculation, reconstruction, based on the new findings, or power the Yun or shield through a different manner. I chose to deal with the 4.4V and make some modifications, but I am still not getting the performance I had previously using an actual 5V supply.

A search of this forum shows a number of other people have the same problem. So disappointing!

--Dan

It just occurred to me, I wonder if any of the pads for the POE module (which I have not installed) are common with the USB +5V input? I could tap into one of those and power my shield with a full 5.1V coming from the micro USB port.

Anybody familiar with the POE hookup? That would be an acceptable solution, I could still use the micro USB to power my Yun (clean looking) and retain full functionality of my shield using full 5V/5.1V input.

THANKS! --Dan

Click on the "Products" tab above, then on the Yun. About 1/3 of the way down the page you can download the schematic.

On page 05_Power, you can see:

  • D9 - which connects the internal VDD5V to the 5V shield pin
  • D2 - which connects 5V from the VIN shield pin to the internal VDD5V
  • D19 - which connects 5V from the USB to the internal VDD5V
  • D10 - which connects 5V from the POE module to the internal VDD5V

So, no matter how you power the Yun, you have two diode drops from the power supply to the 5V shield pin:

  • USB: D19 --> VDD5V --> D9 --> Shield
  • POE: D10 --> VDD5V --> D9 --> Shield
  • VIN: D2 --> VDD5V --> D9 --> Shield

If you must have 5V on your shield, your best bet is to bring power onto the shield first, and use that on the shield. Then, power the Yun through the VIN pin, and don't connect anything to the 5V output pin. Note that because of all of those diodes, you can still plug in a USB connector for data when needed, and it won't cause problems: neither the VIN pin, USB port, or POE pad can backfeed any other source.

ShapeShifter: If you must have 5V on your shield, your best bet is to bring power onto the shield first, and use that on the shield. Then, power the Yun through the VIN pin, and don't connect anything to the 5V output pin. Note that because of all of those diodes, you can still plug in a USB connector for data when needed, and it won't cause problems: neither the VIN pin, USB port, or POE pad can backfeed any other source.

Thanks for the detailed explanation. It is a real bummer, but I can work with it. I just wish I could power my project with the micro USB input and still get 5V to the shield.

--Dan

dankgus: I recently migrated a circuit from a breadboard to a homemade shield to plug into my Yun. I was using a large 5V supply to power my breadboard, and made the incorrect assumption that the 5V pin on my Yun would actually output 5V.

How much current does your shield draw?

None of the Arduino boards could be considered equal to a large 5v supply. If you try to draw too much current you may damage the Arduino.

...R

Yun 4.48 volt is “normal” but might cause some shield fail.

The Yun will work far better at equator than poles.

Robin2: How much current does your shield draw?

He mentioned that in the first post:

dankgus: My shield draws a peak of 30mA. I don't know what the 5V pin is rated for but surely it should be okay with 30mA?

A shield's current draw is a very valid question, as it can often cause problems. But that's not the case here. The issue is almost certainly the double diode drop.

ShapeShifter:
He mentioned that in the first post:

OOPs - so he did.

…R

ShapeShifter: So, no matter how you power the Yun, you have two diode drops from the power supply to the 5V shield pin:

  • USB: D19 --> VDD5V --> D9 --> Shield
  • POE: D10 --> VDD5V --> D9 --> Shield
  • VIN: D2 --> VDD5V --> D9 --> Shield

Looking again at the schematic, I see pin 2 of the 6 pin ICSP header is VDD5V. As a cheap workaround, it appears I could pull a little more V by using this pin to power my shield. I would skip D9 and only deal with one diode drop. Does this seem correct? Not advisable? I am going to wait until Thursday when my mouser order arrives before touching hardware again else i'd measure it.

THANKS! --Dan

dankgus:
Looking again at the schematic, I see pin 2 of the 6 pin ICSP header is VDD5V. As a cheap workaround, it appears I could pull a little more V by using this pin to power my shield.

Good catch! It sounds reasonable. Just realize that you lose the diode protection, so the usual concerns about voltages and care about backfeeding power would apply. Given your needs, and the limited amount of power you need, I wouldn’t be afraid of trying it. (Of course, if you burn up your Yun doing this, I will deny having said anything! :-X )