# Voltage divider from Ac signal

Hi, i want to ask how i could connect my arduino to this supply circuit that i have already make.i want to measure my voltage with arduino. its the purpose what im try to do now beside my large target is to make my own power meter. Im not sure where to put my voltage divider. can anyone suggest me how to connect it. im confusing with + - terminal and ground.

here my supply circuit

Connect 0V to Arduino Gnd.
Put 2 resistors in series from +12 to Gnd, bring the junction to an arduino pin.
Use 12K to 5K to Gnd. Will measure ~3.5V across the 5K & Gnd. Tweak the values if desired:
Vout = Vin * R2/(R1 +R2) R2 = 5K, R1 = 12K, R1 can go as low as 6.8K

Do not connect to the -12V.

@CrossRoads if i not connect the -12V can i still have sine wave to plot?

and one more is the Vout from the circuit is in RMS value or peak-to-peak .

You can’t measure the voltage at the 7812 output - it’d be always the same +12V !
Check on a link below, I have a power quality meter (1) using a transformer, you would need to bias / divide voltage from transformer .

Yes Magician..
i have look at that link.. some use full stuff i have read..
but my problem here, how can i get sine wave from this circuit to arduino. i tried the voltage divider from +12V to 0V, still no sign of value in sine wave. here my update circuit that i try,

How you power up arduino? If grounds of rectifier and arduino connected, than you may use arduino own +5V pin, connect 2 resistor 4.7 k as it shown on this circuit:
http://coolarduino.files.wordpress.com/2013/01/pqa_ver2_w.png
than bring ether end of secondary power transformer to middle point ( I use 22 k, but you probably need little bit higher value, 33 or 47 k depends on voltage AC). 3 resistors overall.

arduino is power from usb cable. u mean like this?
make a dc offset to the signal.?

If you are looking for a sine wave then either you seem to misunderstand what the circuit function is, or you are using the words "sine wave" incorrectly. The circuit is a stabilised power supply whose output is DC. The circuit converts (rectifies) the AC output of the transformer then uses a pair of constant voltage chips to provide a constant DC voltage level of +12 and -12.

Can you please explain what it is that you are trying to measure and we will endeavour to assist.

owh..sorry for misunderstood. actually i want to get AC waveform from the supply, so i can measured the voltage, current. just like power meter. i thought from transformer i make the rectifier circuit to stabilize it and i can get the smooths signal. SO if i want to get the waveform should i put voltage divider directly to transformer or else.?

ac in to your transformer… out pops a low voltage ac sinewave… at most you’d simply filter the signal and then measure the low voltage ac side with a voltage divider…

the voltage regulators and the rectifier simply convert it to dc current, which is a step too far?

u mean by the output of rectifier only dc voltage. not ac volt? so i must get it directly from transformer not from rectifier?

Take a look at this page. It shows the output of the full wave rectifier.

http://www.osha.gov/SLTC/etools/electric_power/illustrated_glossary/substation_equipment/rectifiers.html

As others have pointed out, the rest of this circuit filters the output to steady DC.

now i get it..after doing simulations i get the result that the output is a straigh line.. here the pic from my simulation

.
so now if i want the sine waveform i have to get it output from transformer not from the rectifier right?

Correct. Refer back to the diagram and see what is on the inputs of the full wave rectifier.

http://www.osha.gov/SLTC/etools/electric_power/illustrated_glossary/substation_equipment/rectifiers.html

ok.. my new simulation.. tell me if im wrong..

if i analogread the output with arduino. what voltage did i get? is it in Vp-p, vdc or Vrms.

It’s great that you’re adding a voltage divider to make the voltage levels safe enough to be read in by an analog port!

but, I still don’t know why?.. Power is Amps * Volts (Watts) you’re reading Volts, so it’s not so much a meter which measures current, but potential voltage at any given point?

My best guess is OP want to measure AC RMS (root mean square) voltage.

Precision Micropower ?? RMS-to-DC Converter

In case OP need measure AC current

\$5.90, Digikey