Output Voltages differ for different power supplies to the arduino.

Hi,

I generally power my Arduino UNO (rev 3) in three ways.

  1. Standard USB cable from PC.
  2. a wall adapter (similar to iphone charger) with USB interface.
  3. a wall adapter with normal co-axial connectors 5vdc 1.04 amps.

also
I use digital pin 5 as output pin and use digitalWrite on it and it has also been configured accordingly.

the problem that I have is:
==>when I power the Arduino board using methods #1 and #2, the voltage from pin 5 is around 4.67 VDC approximately,
BUT
==>where as when I power using #3 I only get approximately 3.45 VDC.

I have checked the power sources before connecting them to the board separately and all of them are providing around 5VDC (+/- 0.25)
and current range is from 0.7 amp to 1.11 amp.

is this a known issue? If not what might be causing this? any ideas?

I am planning to shift my entire project to a ATiny85, and now after this weired observation, am pretty worried, if that would also have similar issues.

Also, this is my first question and first day in this forum, so if i have posted the question in a wrong place, then please be kind enough to guide me to get the right audience.

thanks
BlueRat.

Yes, it's a known 'issue'. The regulator drops some voltage, which is why the recommendation for the barrel socket input is:

Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V

From http://arduino.cc/en/Main/ArduinoBoardUno

You can use the #3 directly in the GND and VCC pin (not Vin) so you bypass the regulator and get 5V

If I remember right there's a bit of wiggle-room in what devices consider a logic high/low. I don't think it HAS to be bang-on 5.0V, just above a certain threshold. I power my stuff using LDO 5V regulators and usually, unless it wants a lot of current, a PP3 battery (because I have hundreds of the bloody things), and it always ends up pretty close to the 5V mark :slight_smile:

Thanks for the replies.

And yes Bullet.., you are right; In digital-electronics high and low can merely differ by small unit of voltages.
and I till date have never used the LDO regulators, need to have a look at those.

I was disappointed in one of my approaches to make a digital switch, where i used a NPN transistor with 5vdc from Collector and 3.45 vdc from arduino to BASE, i had my load at the emitter, i did manage to get a good amount of current but the voltage was still not to my expectation, then i tried cascading more than one of these transistors but every time the voltage was disappointing, and my relays won't respond.

Anyways I'll try the LDO regulators now, consequently if anyone of you more experienced ppl out there, have any other ideas please enlighten me.

Thanks All, Thanks a lot.

regards,
BlueRat.

bluerat:
Anyways I'll try the LDO regulators now, consequently if anyone of you more experienced ppl out there, have any other ideas please enlighten me.

Assuming your wall adapter is already regulated, there is no need for a LDO or additional regulator.

"I was disappointed in one of my approaches to make a digital switch, where i used a NPN transistor with 5vdc from Collector and 3.45 vdc from arduino to BASE, i had my load at the emitter, i did manage to get a good amount of current but the voltage was still not to my expectation,"

How did you wire that up?
Generally an NPN is wired as an Open Collector circuit. The source voltage goes thru a pullup resistor to the collector.
When the NPN base is low, the collector is pulled up by the resistor, with the current available determined by the resistor.
When the NPN base is driven high, the collector pulls the signal low, down to about 0.7V if the emitter is grounded.

Typically PNPs (or P-channel MIOSFET), or a pullup resistor, are used to source current (and provide a high voltage) and NPNs (or N-channel MOSFET) are used to sink current (and cause a low voltage).