transform-less AC voltmeter

[unknown77799]

I was assigned to make a voltmeter using a breadboard and an Atmega328 that measure both AC and DC currents. I am struggling with the AC part of the voltmeter as I am required to do it without a transformer. Everything else works fine, anyone please recommend what to do I am so confused. Thanx a lot

 

[Nigel Goodwin]

Why would you imagine you need a transformer?.

An AC meter is just a DC one with a rectifier in front of it.

 

[unknown77799]

So am I supposed to make another DC meter circuit with a rectifier in front of it or is it possible to add the rectifier in the same circuit? sorry if the question seems dumb, I am only a mere student and have little knowledge about electronics.

 

[KeepItSimpleStupid]

I'll urge you to read this: **broken link removed**

Most multimeters are average responding and RMS reading and only work correctly for a sine wave input.

A square wave input won't give you the right answer. By design, it's not supposed too.

 

[Les Jones]

Is your restriction that you need to power you meter from the AC supply or is the restriction on just scaling the voltage down to a level to feed into the ADC on the ATMEGA 328 ?

Les.

 

[unknown77799]

I'll urge you to read this: **broken link removed**

Most multimeters are average responding and RMS reading and only work correctly for a sine wave input.

A square wave input won't give you the right answer. By design, it's not supposed too.

I have been able to add a low pass filter that allows me to get a sign wave. And thank you I will read it now.

 

[unknown77799]

Is your restriction that you need to power you meter from the AC supply or is the restriction on just scaling the voltage down to a level to feed into the ADC on the ATMEGA 328 ?

Les.

I think it is just scaling down but here is the task regarding AC signal sensing attached. Again sorry for the hassle but this is my first ever electronics project and I have a poor background of electronics.

 

[KeepItSimpleStupid]

You basically assume you have sine wave input (e.g. 50- 60 Hz), precision rectify, and multiplay by a fudge factor that would read RMS. The average is a mathematical average.

e.g. Vac = k*Average(abs(V(t))

K, you can find with mathematics. https://www.wolframalpha.com/input/?i=ave+(abs(sin(t)))+t=1+to+2*PI

So k is about 1/0.68 the average value. Hope I did the math right.

If you put a numbers in I know derived from 120 VAC, it works out.

 

[Nigel Goodwin]

I think it is just scaling down but here is the task regarding AC signal sensing attached. Again sorry for the hassle but this is my first ever electronics project and I have a poor background of electronics.

The PDF seems to have little or no relation to your original question?, and actually explains what you need to do - simple attenuator, and add a DC offset.

 

[ronsimpson]

After a quick look at the .pdf; it looks like they don't want a "rectifier". They just want the computer to read voltages in the +/-10 volt range. The computer can not read that so a circuit is made to convert -10 to 0, 0 to 2.5 and +10 to +5. (remember the computer can only see 0 to 5V)

There are several ways to do this.
C1 strips off any DC from the signal to measure.
R2, R3 causes the ADC input to want to go to 2.5V or 1/2 of the supply.
R1 & (R2//R3) makes a voltage divider. (reduces the voltage down to what the computer can read)

From the .pdf your posted: The -10 to +10V signal gets changed to 0 to +5V for the computer to read.

 

[KeepItSimpleStupid]

I agree. making a multimeter and the assignment don't really agree. Anyway, I mentioned how making an AC voltmeter might work.

Do what's asked.

 

[ronsimpson]

Do what's asked.

Yes.... I think in this case the "rectification" and RMS/averaging is done in software. (school project so not done at all)