reading voltage.

Hi mates,

I have a simple question but i can solve it.

I have a output signal that it comes from AC signal i converted to DC but it looks like this (left image).

When I measure the signal using arduino i get values of 900 ~= 4.5 v and the oscilloscope shows me that the top reaches 2v, how it is possible? What am i missing? because it's impossible that the oscilloscope gives me a values of 2v and the arduino a value of 4.5

edit: the schematic

can you post your schematic-s ?

You can never be sure until you measure it with a multimeter :slight_smile:

sure!

The AC side of your schematic is screwy. And you're showing the meter on the AC side. Or,is that a motor?

Is your 'scope connected connected to the same points as the Arduino, including using the same ground?

I don't think that you can make a floating current shunt that way. At the very least, you should use a transformer to provide DC isolation.

And that must be a lot of current to overcome the two diode drops with a 0.22 ohm resistor!

With 100 Ohm load resistor and an unknown AC input impedance, anything can be displayed!

Drop the circuit into SPICE and plot it out...several free ones online. One:
http://www.falstad.com/circuit/

Remember, scope shows peak and probes should be on DC if viewing from the uC point of view. "M" is AC-centric... Is this an RMS meter?

Use an old battery if you think your scope vs Arduino are wrong... I have never seen a uC be more than 1 mV different from a calibrated scope. And always check that the scope is on DC vs AC... It is a small thing and easy to miss.

  • Connections to the Arduino are missing (unless its A2 & GND) and the AC return signal is missing. In any case, could you post a more detailed diagram?
  • Is the purpose of this circuit to measure the current load through an AC motor?
  • You show a ground connection at the motor ... is that the return line of your AC or GND at the Arduino?
  • Is the AC power source isolated?
  • An on-line schematic editor you could try.

yes, the goal is measure the load through the AC motor, i have a tester to link Voutput from my schematic to load of the motor.
by the way, i hust have seen a ACS712 sensor that is designed for that.
I will buy one for testing. I think i would be better option.

asboxi:
i just have seen a ACS712 sensor that is designed for that.

Now you're talking. It is designed for the job.

Your original circuit was wrong in just so many ways!

The output of a full bridge rectifier is a differential signal - and superimposed on some
other voltage here due to the load. You need to work out what the maximum/minimum
voltage can be w.r.t. the real ground, and then measure differentially using a circuit
that can tolerate the voltages involved (typically an opamp in differential configuration
or a specialist high-side current measuring chip)

KeithRB:
I don't think that you can make a floating current shunt that way. At the very least, you should use a transformer to provide DC isolation.

This.

There's no way in hell that can work unless your Arduino and everything else is running off batteries (including your 'scope!)

PS: Remember that the crocodile clip on your oscilloscope probe is connected directly to mains ground. If you connect that up to the point marked "GND" in your drawing then almost anything could happen.