can anybody help me design a simple ac voltmeter using half wave rectifier??

 

hi,

Can you say why, you prefer a half wave 'ac' voltmeter.?

What range of frequencies and voltages.?

Starting link:
http://scitec.uwichill.edu.bb/cmp/on...20Supplies.htm

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Eric
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Bill's: www.blueroomelectronics.com/

 

What use is a half wave AC voltmeter?

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Put the AC into a diode and then through a resistor into a DC milliammeter. Draw the scale to read AC volts.

Look up the DC value of a half-wave AC waveform; it's about 50 V for 120 VRMS in but there is a PI in the formula somewhere.
50 µA full scale meter needs 50 V/50 µA = 1 MΩ resistor.
The diode PRV should be > 200 V; use 400.

Your meter will respond to the average [not the RMS] value of the AC waveform but you can put RMS on the meter face anyway. A lot of people do.

 

What about the 0.6V drop? How do you account for that?

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Can I ask why the 0.6V drop happen?? Just want to know for extra knowledge.

 

The 0.6V drop would be across your half-wave rectifier.

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For voltages below a 100V or so you could use a schottky rectifier to reduce the drop to 0.3V or less.

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hi,
If you use an OPA with the diode, that will become a 'precision rectifier' that will work close to 0Vac input. No diode voltage drop.

Google for Precision Rectifier

In your PM you say upto 50Vac input, you will have to attenuate to about 5Vac before the 'precision rectifier' OPA circuit.

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Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

 

It depends on how accurate this needs to be, for example if the full scale deflection is 120V, 0.6V only gives an error of 0.5% and 0.3V just 0.25%.

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120 vac rms is ~170 v peak; IMHO the 0.6 v is subtracted from this, then the DC value of the half sine pulse is calc. from this.

The sensitivity of this meter is 20,000 ohms/volt, right?

 

That's true, the error will be less than that, for a silicon diode it'll be just 0.353% and 0.176% for a Schottky.

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In the old days they just started the scale from the equivalent of 0.7V. No one except Clarke Kent noticed.

That reminds me of a comment made by the late Bob Monkhouse. He asked, why do people think I'm from Kent. When asked why he thought that he replied "when I walk past people I keep hearing them whispering Kent"!

Mike.