I have an Arduino Mega 2560 with a GPRS seeedstudio shield and an USB Host CircuitsAtHome shield which work fine when powered with USB or with an adjustable voltage regulator set at 8V.

When I set this voltage regulator at 7V (minimum voltage from the spec to get the better efficiency) I am still reading 5V and 3.3V on the pins of the Arduino and all the leds of the Arduino and the GPRS shield seems fine but the USB Host CircuitsAtHome shield seems to not work properly. I cannot debug it using the console because if I plug the USB cable, everything works fine...

Why 7V fail while 8V is ok?

The adjustable voltage regulator can provide up to 5A which is plenty.


The Voltage-Regulator has a dropout-voltage of 1,2V, but in line to the regulator is a diode, which adds another minimum 0,7V (dont know the exact part number, so i guessed the voltage of a Si-diode at ~1A). So you have to supply at least 5 + 1,2 + 0,7 = 6,9V. I presume the circuit doesnt draw a constant current all the time, so from time to time the regulator cant supply the 5V because you sink under the necessary dropout-voltage of the rehulator/diode-combination.

-> Use 8V or bypass the Diode D1 (with a solder-bridge) and use 7V if its a must and you are sure the input-polarity is always correct.

Arduino-Mega-2560-schematic: http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-sch.pdf

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You need to be clear about how you are hooking this up. Are you feeding 7V into the Vin on the Arduino? If so, then the voltage regulator on the Arduino is attempting to regulate 7V down to 5V. As Echo says, 7V isn't quite enough for the built-in voltage regulator.

If your voltage regulator outputs clean, noise and ripple free regulated voltage, you could set it to 5.0 volts and feed it directly into the Arduino's 5V line, bypassing the regulator on the Arduino completely. If you do this you need to make absolutely sure that you give it exactly +5V though, as you will likely destroy the board if you go much over 5V or wire it backwards.

If it's a switching regulator then you might need to add a choke to the output of the regulator to get rid of high frequency noise on its output.

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  • I am currently feeding the board using the 2.1mm jack plug of the board. Will it be safe to power the board at 5V (directly to the 5V pin of the board) with this converter : seeedstudio.com/depot/… . Is it outputs clean, noise and ripple free regulated voltage? Thanks! – benjisail Aug 21 '14 at 7:51

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