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Old 31st December 2004, 09:46 PM   #1
Stocker is offline Stocker  United States
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Default DIY RMS Voltmeter?

Does anyone have any idea what sort of a filter to use to measure AC (true RMS! ) on a DC voltmeter?
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Old 31st December 2004, 10:08 PM   #2
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Aren't you supposed to be reading a book on evolutionary biology or something?

Depends on what a.c. you're trying to measure. Are you trying to measure audio signals? If so, you would be better off buying an audio a.c. voltmeter on ebay. There are always some nice analog HP units available.
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Old 31st December 2004, 10:39 PM   #3
johnnyx is offline johnnyx  United Kingdom
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There isn't a filter that can make a dc meter read true RMS. dc meters are average reading, they are scaled by 1.11 (the form factor) so that a sine wave of say 1v RMS reads 1v. If you don't have a sine wave, then it's not RMS. Analog Devices make chips that do the conversion, involving log amps and filters.

Look here

Hope this helps.
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Old 1st January 2005, 12:10 AM   #4
Stocker is offline Stocker  United States
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Thanks Bill but I'm already up to speed on evolution: it's bunk, but that's ANOTHER thread.

The purpose of the question was to see if I could get some help turning my DIY DC Voltmeter into an AC voltmeter as well. It would probably be easier to do an average AC meter it sounds like. Is this as simple as a bridge rectifier or simple discrete circuit, or do I hafta hafta have a special chip?
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Old 1st January 2005, 12:56 AM   #5
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Quote:
Originally posted by Stocker
Thanks Bill but I'm already up to speed on evolution: it's bunk, but that's ANOTHER thread.
Let us all know when you get your PhD in molecular biology.

In the interim (I know it's going to be a long wait) look at the application notes at the Analog Devices and Linear Technology websites on true RMS detectors, or read the treatises on the www.agilent.com website on calorimetric methods.
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Old 1st January 2005, 09:18 AM   #6
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Guys, stick to the topic please...
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Old 1st January 2005, 05:13 PM   #7
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Default Re: DIY RMS Voltmeter?

Quote:
Originally posted by Stocker
Does anyone have any idea what sort of a filter to use to measure AC (true RMS! ) on a DC voltmeter?
http://www.linear.com/pc/productDeta...Id=H0,C3,P1285
http://www.linear.com/pc/productDeta...54,C1086,P1701
http://www.linear.com/pc/productDeta...54,C1086,P2505
http://www.linear.com/pc/productDeta...54,C1086,P7526
http://www.linear.com/press/pressNew...?releaseID=936

LT1088
LT1966
LT1967
LT1968

Check those IC's but you may forget about the LT1088 because it's old.

May I ask why you want True RMS?
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Old 5th January 2005, 11:28 PM   #8
Stocker is offline Stocker  United States
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Thanks everybody, that's a bunch of good starting points.

(everybody except Bill, of course).
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