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Topic: Analog Input with 4.6V (Read 996 times) previous topic - next topic

javiguardiola

Hi:

I'm having some problems with analog input. I'm using an Arduino Mega2560 and I had working a temperature controller with an LM35 sensor. The analog input A0 was working properly, but now without nothing connected to the arduino board, I have 4.6V in A0, and the temperature controller is reading this value instead the value offered by the sensor.

Any idea?, I used a 19V power source for the board and peltier module. The A1 pin has 3.6V and the others seems to be ok you can mesure 0.xV.

pluggy

Floating Input.  A pin without a connection or bias will read all over the place, this is perfectly normal.

WalterM

#2
Dec 10, 2010, 02:38 pm Last Edit: Dec 10, 2010, 02:40 pm by 3WalterM4 Reason: 1
How have you supplied the LM35?
Also with 19V or with the 5V comming out of the Arduino? If the first case is true, the input A0 could be damaged.

By the way, 19V for the Arduino should be too high. Max. 12V is recommended

javiguardiola

I've supplied with +5V coming from arduino board. I know that the recommened suply is 12V, but it supposed that arduino can manage until 20V.

I obtain the same 4.6V if a connect the LM35. BTW A0 has 4.6V, A1 has 3.0V and A2,3,4,5... and so on have 0V in "floating connection"

Thank you so much for your quickly answer, Any more ideas? I think that the ping A0 and A1 is damaged, but I want to know what can be the problem.

Thanks

MarkT

Quote
I've supplied with +5V coming from arduino board. I know that the recommened suply is 12V, but it supposed that arduino can manage until 20V.



Well it might manage, but with any extra load from modules powered from the 5V regulator on the Arduino it could push the regulator power consumption over the edge - normally voltage regulators shutdown if they overheat, so you will risk reliability.

Quote

I obtain the same 4.6V if a connect the LM35. BTW A0 has 4.6V, A1 has 3.0V and A2,3,4,5... and so on have 0V in "floating connection"


If floating inputs are reading a steady 0, they are either pulled-down to ground or damaged - a CMOS input that's floating _really will float_!

One last possibility - you aren't accidentally causing the analog pins to be OUTPUTs or enabling their internal pull-ups?

You mentioned 19V earlier - that will instantly fry any inputs on a 5V logic chip, so you really really need to be careful wiring things up.  Another reason for not running Vin at such a high voltage.
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