DIY RMS Voltmeter?

[Stocker]

Does anyone have any idea what sort of a filter to use to measure AC (true RMS! ) on a DC voltmeter?

 

[Bill Fitzpatrick]

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.

 

[johnnyx]

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.🙂

 

[Stocker]

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?

 

[jackinnj]

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.

 

[pinkmouse]

Guys, stick to the topic please...

 

[peranders]

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/productDetail.do?navId=H0,C3,P1285
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1086,P1701
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1086,P2505
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1086,P7526
http://www.linear.com/press/pressNewsDisplay.do?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?

 

[Stocker]

Thanks everybody, that's a bunch of good starting points.

(everybody except Bill, of course).