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Topic: Wrong analogRead values (Read 2112 times) previous topic - next topic

gmbroh

Feb 13, 2013, 11:04 pm Last Edit: Feb 14, 2013, 08:41 am by gmbroh Reason: 1
I am trying to measure the voltage of an 5volt AC rectified voltage at 65Hz. Using the UNO R3 Below is the current code.




Code: [Select]
const int numReadings = 25;
int ac_input[numReadings];
const int analogInPin = A0;//Pin for input for AC voltage     
float ac_output = 0;   
int index = 0;

void setup()

{
  Serial.begin(9600);
  pinMode(A0, INPUT);
  for (int thisReading = 0; thisReading < numReadings; thisReading ++)
    ac_input[thisReading] = 0;
}

void loop()

{
  ac_input[index]= analogRead(analogInPin);
  ac_output = ((ac_input[index]/1023.0)*5.0);                             
  Serial.println(ac_output); 
  delay(2);
  index = index + 1;

  if (index >=numReadings)
    index =0;

}




The problem is that the output on the serial monitor only shows the peak values of the voltage. That is from 4-5v instead of zero.

5
4.98
5
5
4.95
4.62
4.27
4.26
5
4.93
4.7
4.58
5
4.79
5
5
4.35
4.53
5
5
5
4.91
4.84
4.52
5
5
4.67
4.54
5
4.69
5
5
4.43
4.32
5
4.92
5
5


I have tried changing the board I'm using aswell as the input pin.

Has anyone experienced anything similar ?

AWOL

Have a look at this thread
http://arduino.cc/forum/index.php/topic,144195.msg1084872.html#msg1084872

Please use code tags when posting code.

Code: [Select]
for (int thisReading = 0; thisReading < numReadings; thisReading ++)
    ac_input[thisReading] = 0;
they're already zero; this is pointless.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

Nick Gammon

What are you expecting exactly?  A 65 Hz signal would have a period of 15 mS, and you have a 2 mS delay between readings. What is your circuit?
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

PeterH


I am trying to measure the voltage of an 5volt AC rectified voltage at 65Hz


How have you connected the Arduino to the source?

Where does the rectified signal come from - is it possible that it includes some smoothing?
I only provide help via the forum - please do not contact me for private consultancy.

michinyon

The phrase "5 volt rectified AC voltage"  is gibberish.   

If you have rectified it,  it isn't an AC voltage.  The electron flow is no longer alternating in direction.

You will expect to see a DC voltage with a voltage ripple.  Which is exactly what you are getting.
Depending on the type of rectifier,  you will see 65 Hz or 130 Hz ripple.    I can't tell from your
sampling rate, exactly which you are getting.

Your DC voltage has a peak of 5 volts and a minimum of about 4.5 volts.  That is exactly what you
are getting.

PeterH

#5
Feb 14, 2013, 03:23 am Last Edit: Feb 14, 2013, 03:25 am by PeterH Reason: 1

The phrase "5 volt rectified AC voltage"  is gibberish.  


To me that seems like a reasonable way to describe a DC signal which has been obtained by rectifying an AC signal.


Your DC voltage has a peak of 5 volts and a minimum of about 4.5 volts.


Do you mean that this is what would be expected from a rectified sin wave? If so, how did you predict the minimum value?
I only provide help via the forum - please do not contact me for private consultancy.

Nick Gammon


I am trying to measure the voltage of an 5volt AC rectified voltage at 65Hz. Using the UNO R3 Below is the current code.


Is that full-wave or half-wave rectification?
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

Nick Gammon

Read this before posting a programming question

Please edit your post, select the code, and put it between [code] ... [/code] tags.

You can do that by hitting the # button above the posting area.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

retrolefty



I am trying to measure the voltage of an 5volt AC rectified voltage at 65Hz. Using the UNO R3 Below is the current code.


Is that full-wave or half-wave rectification?


And with or without a filter capacitor?

Lefty

Nick Gammon

See reply #2:

Quote

What is your circuit?
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

gmbroh


What are you expecting exactly?  A 65 Hz signal would have a period of 15 mS, and you have a 2 mS delay between readings. What is your circuit?


gmbroh


The phrase "5 volt rectified AC voltage"  is gibberish.   

If you have rectified it,  it isn't an AC voltage.  The electron flow is no longer alternating in direction.

You will expect to see a DC voltage with a voltage ripple.  Which is exactly what you are getting.
Depending on the type of rectifier,  you will see 65 Hz or 130 Hz ripple.    I can't tell from your
sampling rate, exactly which you are getting.

Your DC voltage has a peak of 5 volts and a minimum of about 4.5 volts.  That is exactly what you
are getting.


The input to the arduino is 0 to 5volts ac  I dont understand why you think its dc. I have attached a print screen of the input to the arduino

gmbroh




I am trying to measure the voltage of an 5volt AC rectified voltage at 65Hz. Using the UNO R3 Below is the current code.


Is that full-wave or half-wave rectification?


And with or without a filter capacitor?

Lefty


It is full wave rectification without a filter capacitor.

Grumpy_Mike

Quote
It is full wave rectification without a filter capacitor.

Without any load resistor the sample and hold capacitor on the arduino input will act as a smoothing capacitor.
Put a 10K load ( resistor) between the analogue input and ground.

AWOL

Quote
The input to the arduino is 0 to 5volts ac  I dont understand why you think its dc.

Because your scope trace clearly shows everything above 0V.
Alternating current implies a current reversal.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

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