How to find the magnitude of a 0-10VAC signal

AC - What frequencty ??

Will you be using the DC output for another circuit or just measuring with an ADC ??

Google for RMS converter.

donald

Look up precision rectifier.. It's an op-amp that will generate a clean DC full wave of an AC signal with out effecting the AC source to much..

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5"

If you know that only the amplitude changes (it's an unchanging waveform, ideally a sine wave), of reasonable frequency, that it comes from a low impedance source and that the DC output feeds a high impedance load, it's simple. You can use a diode and resistor in series with an adjustable shunt resistor. Adjust the resistors until the output DC voltage is numerically equal to the input AC amplitude, as measured by a multimeter. If any of the assumptions above aren't true, you'll need to follow the other poster's suggestion to use an RMS-to-DC converter.

--
John

I answered too quickly. If the amplitude is 10V, that's the peak level, and you'll need to follow the diode and resistors I mentioned with some DC gain to get 10V out.

--
John

"Michael"

** It is NOT a simple problem.

That is a dumb assumption based on emotional thinking.

** How about a " precision rectifier " ??

Look it up on Google.

This will produce either half or full wave rectification, with high accuracy, over the range from 0-10 volts.

Then all you need to add is a low pass filter to find the average DC value.

If half wave, then that average is 1/pi times the AC peak.

If full wave, that average is 2/pi times.

..... Phil

"Phil Allison"

** NOTE:

This calculation is good only for sine wave signals.

For arbitrary signals, peak detection is the ONLY method that can work.

You real problem is that YOU have failed to clearly define the measurement you want to make.

....... Phil

"donald"

** The OP was after the "amplitude " - not the rms value.

..... Phil

.

Phil - I'm fully aware of what a precision rectifier is. I'm also fully aware that that does not solve the problem. Low pass filtering the signal will give the circuit either lousy response time to changing amplitudes or lousy accuracy. Both possibilities fail to solve the problem. Of course I didn't specify the response time in my post - but I'm looking for a circuit that will change output as soon as the amplitude of the incoming sine wave changes.

-Michael

On a sunny day (Tue, 1 Jan 2008 17:17:26 -0800 (PST)) it happened Michael wrote in :

No, yes, no, your peak setector (in fact any AM detector) has a time constant, if the signal is low, then that time constant will cause the detector output to slowly go low too. So you have to ask yourself the question: 'What is the minimum speed the AC signal changes in amplitude?' Make the time constant in your detector slower then that.

So for example if you AC is modulated with 1Hz, say 1Hz pulses, then your time constant should be longer then 1 second to hold the peak, else you detector output will simply follow the modulation. Or, maybe that is what you want?

Uin--- a Diode k -------------- Uout | | R C t = R x C | | /// ///

Ideally I was hoping for something that'd change output on the peak of the incoming sine wave, and hold that value till the next peak. Unless I'm missing something, any sort of discrete solution will not be able to do this. Now I'm thinking maybe a full wave precision rectifier (so that the output gets updated twice per period) driving the input of a sample and hold circuit with a sample signal driven by a zero crossing detector looking at the derivative of the input. But that's alot of components. Argh.

-Michael

Sure - I know what that is - but how would I use that to give me a DC value? That'll just give me the absolute value of the incoming signal.

-Michael

Can you wait for the zero crossing after the peak? That makes it much easier. The idea is like this:

U1A U1B D1 !\\ SW2 !\\ IN ---+------>!----+------+---! >---/o-----+------! >-------OUT ! ! ! !/ ! !/ ! C1=== 0 C2=== ! ! SW1/ ! ! GND ! GND ! GND ! ---!\\U1C? C3 To SW2 ! >-----!!---+-----!\\U1D? ! C4 GND ---!/ ! ! >-----+--||--+-----!\\ \\ 1V-!/ ! ! >--To SW1 / \\ -1V-!/ \\ / ! \\ GND ! GND

U1 A and B are simple voltage followers. U1C, U1D, C3 and C4 conspire to cause SW2 to close briefly just after the signal goes below ground. When SW2 opens SW1 closes discharging C1 for the next cycle.

D1 can include an op-amp to dial out the drop. You need a fast op-amp that doesn't mind large dif voltages.

------!+\\ ! >--->!------+------ -!-/ ! ! ! ----------------

Some comparitors will also work for this. The comparitors output looks ugly on a scope because there is a burst of oscillations while the diode conducts but it gets the job done.

...snip...

This is what I love about the Internet. Who needs Brittany Spears when we have people like Phil?

ael

o

k,

=A0 =A0 =A0 =A0 =A0 U1B

=A0 =A0 =A0 =A0 =A0 !\\

=A0 ! =A0 =A0 =A0!/

=A0 C2=3D=3D=3D

=A0 =A0!

=A0 GND

=A0 =A0 ! =A0 =A0 ! >--To SW1

=A0 =A0 =A0 =A0\\ -1V-!/

=A0 =A0 =A0 =A0/

=A0 =A0 =A0 =A0\\

=A0 =A0 =A0 !

=A0 =A0 =A0 =A0GND

That seems like a reasonable enough way to do it. I don't have any hard requirements - I just got to thinking about the problem. The ideal solution would change immediately on the peak of the sine wave, but changing 90 degrees after that would probably be adequate for most situations.

-Michael

On a sunny day (Wed, 2 Jan 2008 05:25:10 -0800 (PST)) it happened Michael wrote in :

If you know the frequency, or to be more precise, where the maximum is, then you can sample for example the positive value at that monent in a capacitor.

\\

-------- ------------- | C | ///

Ah, it's assumed to be a sine wave, eh?

What frequency? Could you do it digitally? Maybe you just need an ADC input and a micro.

I could imagine getting to less than a single cycle response with more fancy-ness-- phase locking to it or something along those lines, either with analog or digital means.

Best regards, Spehro Pefhany

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o

lue.

Hi Sphero - this isn't for a specific purpose. It's just a theoretical question that I was trying to figure out the standard way to handle. I was envisioning a sine wave with 0-10V amplitude with 0V DC offset and a constant frequency. I was aiming for an all analog way to do it as, well, analog's cooler. Digital just feels like cheating :)

-Michael

I don't think there is a 'standard' way to handle it, because your problem description strikes me as somewhat of a rare requirement. For example, the front end in pretty much every cheap DVM uses a precision rectifier and LPF (AC-coupled, and with either a full-wave or half-wave rectifier). You can make the When I designed a line of AC panel meters, I used a precision rectifier made with a CMOS op-amp- the op-amp offset was translated into common-mode voltage on the differential inputs so it cancelled out. For visual indication and line frequencies, the response time is just peachy. And, of course, higher end meters might have RMS as an option.

Oh, suppose you had peak detectors and reset them at the zero crossings? It's a bit hard to get excited about a circuit design that is for a hypothetical application.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

value.

See...

particularly page 4.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
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