Help me building an AC voltmeter

[joy4u]

plz help me build an AC voltmeter with a AVR or PIC and a 16 X 2 display

I want the the display to show two voltage readings at the same time one on the above line of the display and another on the lower line of the display

I want to measure line voltage that is about 200 - 250 volt AC

 

[schmitt trigger]

if you rectify and average (filter) a sinewave into pure DC, the relationship between that DC voltage and AC RMS voltage is 1.11

 

[crutschow]

You can rectify and average the voltage as stated by schmitt trigger, but when measuring mains voltage, you must be careful of the shock hazard. Do not connect the measurement circuitry directly to the mains. The best way to to use isolation transformers and measure their output voltage. You could use a small 6 or 12V transformer for this purpose, but you would probably need to use a precision rectifier (Google) to minimize the effect of the diode drops on your measurement. After the circuit is built, you will need to calibrate this output versus the mains voltage using a standard digital multimeter.

 

[schmitt trigger]

Carl added more meat to my original response.........
Another thing I must add is that in order to make precise measurements, a stable voltage reference (again, Google) must be employed, such that the A/D conversions are meaningful.

 

[MrAl]

Hello there,


The simplest way is to use a half wave rectifier (one diode for rectification and one for shunting) and largish cap and divider resistors. Using a 1 megohm resistor in series with a 10k ohm for example gives you 3.5 volts for the uC ADC input at 250vac when you use a large enough cap. You can measure a few known voltages and calibrate in software if you like, or adjust resistor values to get more exact results.
As others have pointed out, the circuit will no longer be isolated from the line when you do it this way however so precautions have to be taken to make sure the user can not touch anything at all in the circuit that is electrically part of the circuit. This isnt always a problem, and sometimes the circuit is run from an offline power supply anyway so this is an option for some products.
If you need isolation then yes a transformer isnt a bad idea, and you can sometimes use the same transformer that powers your circuit to measure the line voltage with.

If you dont isolate then the pots you use (if any) should have plastic shafts. Other controls must be plastic too not metal.

 

[schmitt trigger]

All the recommendations provided so far are the front end of the meter. For the backend, that's another story.
You mentioned that you wish to use an AVR or PIC; thus you must be familiar with those devices and their development environments. Whether you choose to code in assembler or a high level language, again that depends on what you are most comfortable with.
Personally I find assembly language a chore, but others swim thru it easily.
The LCD may be either serial or parallel, and again which option you choose depends on your coding abilities.

 

[joy4u]

Thank a lot for ur replies....

Can anyoe please design a circuit please ....

With the avr and the 16 X 2 lcd....
ohhh yeah.... i need the source code too...

 

[joy4u]

dual channed pic voltmeter..

I searched a bit and found this voltmeter... here


**broken link removed**

**broken link removed**


But the problem with this voltmeter is that it can measure only from 0 - 70 volt...


Is there any way that i will use a step down transformer to drop down the voltage to say 12 volt and then measure that but the readings will come in hundreds...

that is i mean to say that if the transformer output is 12v then input is 220v

 

[Nigel Goodwin]

You just need to alter the existing attenuator to bring the voltage down - there's also PIC ADC code in my tutorials, using a precision voltage reference, opamp buffering (to give a decent input impedance), and an attenuator (which again would need scaling for higher voltages.