Fun with power supplies

So I’ve got the Panda II and the connect shield (running latest firmwear), really an amazing combo for the amount of things you can do with them!

Unfortunately, there are several traps I’ve run into along the way of getting my project working. Posting to maybe help someone else in the same situation and humbly suggest some possible improvement.

First, I have a sensor attached that needs exactly 5v, which I think would be a common problem. Any less than 5v and it starts acting funny. I’ve found that the 5v rail is typically 4.5-4.7 which is not enough for this sensor. The voltage never gets to 5v even if I’m powering from 9v. I even connected a 5.3v charger I had through usb (queue yelling at me for doing this) just to see if I could fudge it and man did the board not like that, the connect shield lit up like a chirstmas tree (ALL the lights turned on), fun easter egg or melting something?

Ok throw out powering through USB, will use the Vin to power the sensor as it can tolerate up to 12v. Found a spare 9v switching power supply 9v. Hey everything is working… for a little bit, then system unresponsive… Oh I’ve created a microcontroller bbq, both the connect shield and the parent board get extremely hot! OK my fault, searched the forums and it turns out in the connect shield only likes 6-7.5v.

So I’ve got a bench supply running 7.5 and the wiznet chip still gets a bit hot nearing 50 C but hasn’t locked up so far (and I see 50c is well within specs). Time to find one online.

TL;DR: 5vUSB = my porridge is too cold. 9v power plug = my porridge is too hot (literally). 7.5v is just… bearable

So for noobs like me it would be really nice if this was possibly stated as a requirement on the purchase page (I know it’s on the bottom of page 2 in the docs but this is supposed to be Freakin EZ :)) or if it could be fixed possibly in the panda III’s regulator so I could just use an easy to find 9v (or even better 12v) source? And why is my 5v rail never 5v? (at least according to my RSR multimeter)

Feel free to correct where I’m wrong and thanks for reading!

Welcome to the community and thank you for feedback.

The panda states the voltage next to the power connector 6V to 9V

We will see what we can do to make this clearer in docs.

Thank you for your excellent products!

This is exactly the problem, it says 6-9v but really it’s 6-7.5v if you’re using the Connect Shield. A confusing thing for a newbie :slight_smile:

Also 5v is less than 5v on my board for some reason.

Consider using a powered USB hub. Sound like your USB voltage is low.

The higher the voltage you put into the power jack the more heat that has to be disipated by the 5V regulator. Nine volts requires around (4 X current) watts to be disipated as heat.

6-7 volts is the optimal voltage to put into the power jack.

Consider using a powered USB hub. Sound like you USB voltage is low.

Thanks for the response Mike. Even when I connect with 9v I still only get 4.66. Where as with the same power supply, my arduino has a nice 5.02v. Is there some difference in the regulators I don’t know about? Problem with my board? Just the way things go?

If put 9V into the 5V regulator and do not get 5V out, then either the regulator is bad, which I doubt, or you are drawing too much current, and it is going into shutdown. What current does you sensor require? A high current draw could explain the heat.

What voltage do you see when you measure the 5V rail with nothing attached. No shield etc? Just a naked board. :-[

No shield, nothing attached with a 9v 2amp switched power supply: 4.72v on 5v pin

The sketch is reading a digital pin every 10th of a second and toggling the on board LED accordingly, really shouldnt be drawing too much power.

So looks like its the board? Do I contact the seller or GHI?

[quote]Do I contact the seller or GHI?

I suggest you wait for someone from GHI to discuss your problem.

This is expected. There is protection diode on 5V line so you will see 5V minus diode drop.

Also, USB cable almost never provide 5V so that is not reliable source for 5V

So you’re telling me 5v pin will NEVER be 5v. Even with a 9v power supply?

That’ll teach me to read specs and expect them to be what they say :slight_smile:

Edit: I think you assume I’ve only used USB, this is not the case. The 5v regulator wouldn’t have a protection diode after it, would it? Sorry just trying to understand why I can’t have 5v on my board ever.

If you check the schematic for the Panda II you will see that diode D2 is on the output of the 5V regulator. I assume this is to protect the requlator when an external 5V power supply is being used.

If you really need 5V, and will never use an external 5V power supply, you could short D2 and then you should get the full 5V. You can check the voltage on the regulator side of D2.

Do I hear Gus screaming? :stuck_out_tongue:

**** SMOKE ALERT ****
If you remove the diode you will not be able to USB power.

Never take hardware advise from a software person. :-[

Yes 5V pin will ALWAYS be a bit short of 5V!

The 5V pin is provided ot power shields and common sensors, not to be a precision power supply.

Thank you both for clearing this up for me, I’m a software person too Mike, hence the hardware confusions.

I’m also coming from arduino where the 5v was always 5v (when using external power), and apparently that 0.3-0.4v difference is too much, oh well learn something new every day, and I love the built in extra features with the FEZ.


And we are always here ready to help :slight_smile: