how to build a circuit for ac input voltmeter?

Question

how to build a circuit for ac input voltmeter?
Is Digital Voltmeter for this website:
www.electronics-lab.com/projects/test/014

Answer 1

In front of the '+' and '-' input of the schematic diagram, put in a bridge rectifier and a filter capacitor.

The diodes must be rated with a reverse voltage much greater than the largest AC voltage you want to measure. Keep in mind that the 120 VAC from a wall outlet is really about 170 volts peak, so your diodes should have a PIV rating of at least 200 volts. Something like a 1N4005 or 1N4006 will work.

The filter capacitor may be very small (much less than 1 uF), because the current draw from the 7107 IC is in the micro-amp regime. That voltage divider string of resistors will probably draw more current than the IC, but it is still very low so, you don't need much filter capacitance.

For AC sine waves, you will get an AC peak-voltage measurement. If you want an RMS approximation of AC sine waves (doesn't work for other waveforms), then you have to put in a resistor voltage divider with a ratio of 0.707 after the filter capacitor. 300k and 700k Ohms in series is 1 MegOhm, tapped on the 700k resistor gives approximately 0.7 voltage division. The more accurate your resistors are, the more accurate the RMS approximation. You can make a 300k resistor out of separate resistors (270k + 27k + 2.7k + 270 + 27 + 2.7) Or get a 330k resistor and trim it with MegOhm valued resistors in parallel, until you get 300.0k . You can do the same to make a 724 k resistor.

Since the diode will drop about 0.6 volts, this circuit will not work for very small AC voltages, and the lower you try to measure (6 V AC for example), the higher the error of the diodes will be. You can compensate for this by changing the RMS approximation voltage divider ratio, but then things get more complicated for wiring in different ratios for different scale settings.

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Answer 2

one factor you will possibly be able to desire to compliment first is, do you prefer/prefer a "genuine rms" length, or will the meter only be used to degree organic, single-frequency sinusoidal indicators? A circuit for genuine-rms measurements may well be extra complicated.

Answer 3

One thing you need to decide first is, do you want/need a "true rms" measurement, or will the meter only be used to measure pure, single-frequency sinusoidal signals?

A circuit for true-rms measurements would be more complicated.

Answer 4

Need more info, plus web site no good....