Power Thru USB Yeilds 5v on rail, but Only 3.5V using Ext 5v Power

I’ve read a handful of threads on powering FEZ devices using USB and the external power supply, but none seem to answer my question. Before I noted the Panda II external power supply specs that dictate input voltage should be between 6 and 9 volts on the Ext power feed connector, I bought a few 5v power supply wall warts rated at varying milliamps (650, 2A, 3A)

What is the the semi-nerdy reason why the external power 5v won’t work, other than "it’s not designed that way dummy"
I assume if I wanted I could use the USB connector voltage pins to use my 5v wall warts? I suppose I could wire the wart directly to the 5v rail as it seems to work: http://www.tinyclr.com/forum/2/6444 [/ol]

Voltage Table

Source        Voltage On Rail
USB            4.45v
9v Ext         4.72v
5v Ext         3.45v

The best way to understand this is by looking at schematixs.

Generally speaking, the raw power input needs to be at least 1.2 volts over the regulator voltage. 6.5 would be ideal, but hard to find. 7.5 works well and is available. Anything beyond 7.5 wastes power and heats the Panda on board regulator. Jenson manufactures a multi voltage wart that will allow you to determine that for yourself.

If you overrun the onboard regulator it heats and affects the analog readings.

Randomr, thank you. That makes sense.

You’re welcome. Diode forward voltage must be overcome before a regulator can function and some of the newer ones can operate as low as .7 V but virtually all can operate at 1.2 v plus. Anything beyond that must be dissipated by the regulator and the on board regulator for the panda dissipates through the circuit board. If the circuit board heats then the values of the onboard ADC are affected as there is no built in compensation for the temperature rise.

Having said that, the Panda is still very useful as a DAQ board. It is a very inexpensive platform that includes a built in SD card reader for long term acquisition, and as long as it’s limitations are known a very useful device. I hope that GHI doesn’t abandon the platform in favor of the simplistic SPIDER format.