Only 3.6 Vdc on 5Vdc out

Questions and Discussion concerning the Bare Bones Board
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Tobis84
Posts: 2
Joined: Mon Aug 25, 2008 2:05 pm

Only 3.6 Vdc on 5Vdc out

Post by Tobis84 » Wed Aug 27, 2008 7:05 am

Hi
I just got my BBB up and running quite well, but in my current project I'm using one of the 5Vdc outputs on the board to drive some external circuit.
Unfortunatly I discovered that my board only supplies 3.6Vdc out instead of the printed 5V.
Does anyone have a solution to that?

EDIT: another problem is the ever present 'not in sync' issue.
is this sync-thing about the resonator missing a resistor? In the BB_Arduino_InstrRevC.pdf it says that the resonator is bypassed by a 1M ohm resistor. But my ver. D2 BBB hasn't got this. Don't know if it is necessary.

EDIT_2: I have solved the problem by bypassing the voltage regulator on the BBB to provide a constant 5VDC to the board. This I think is a layout error because the FTDI cable provides a stable 5V directly and doesn't need any regulation. The voltageregulator creates a voltagedrop itself and then the 5V is not 5V on the output.
I use th 7805 voltageregulator to be able to provide enough amps to my external circuit. This is what makes the 1.4V voltagedrop.

paul
Site Admin
Posts: 734
Joined: Mon May 12, 2008 4:19 pm

Re: Only 3.6 Vdc on 5Vdc out

Post by paul » Wed Aug 27, 2008 11:46 pm

tobias84,

Glad you got it figured out. The board was designed with the USB cable running through the regulator to protect the USB line. The 300 mA regulator only shaves off a few mV so it's not a big deal. If you use a larger regulator I'd recommend the LM2940 which I'm going to sell on my website soon. To use the 7805 just use the external jack with a 9v wall adapter. You can also do what you did too, if using USB, which is just to bypass the USB positive pin directly to the 5V bus on the board.

Yet another way to do this would be to power the power rail of a breadboard from the regulated 5V and leave the PRJ shunt on the 5V setting. That will route the power rail voltage to the 5V rail (regulator output) on the BBB. I have some schematics of some of these setups that I'll get up on the site soon.

Paul

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