True RMS Multimeter heads up

[Tim A]

Those of you looking, Sears sells a very nice True RMS manual ranging meter for $65, auto-ranging for $69.

[DJPhatman]

Thank you for the heads up, Tim! Is it listed in an advertisement? Would you mind posting a Sears product number? Thanks!

[Rick Auricchio]

It could be this one: http://www.sears.com/shc/s/p_10153_12605_03481077000P

If the waveform is distorted or has a DC offset, then one needs a true-RMS meter. But assuming we're pumping fairly good sine waves into the speaker---with neither a DC offset nor heavy distortion---almost any meter should give an acceptable RMS reading.

[SoundInMotionDJ]

I got mine from the Shack. $30. Mostly automagic.

http://www.radioshack.com/product/index ... Id=2104114

--Stan Graves

[gdougherty]

I got mine from the Shack. $30. Mostly automagic.

http://www.radioshack.com/product/index ... Id=2104114

--Stan Graves

That one's not RMS, this one would be.
http://www.radioshack.com/product/index ... Id=2104750

Not as nice a value as Tim's.

[Tim A]

Thank you for the heads up, Tim! Is it listed in an advertisement? Would you mind posting a Sears product number? Thanks!

The one Rick posted a link to.



Rick, we went through this a few months ago. The consensus was that unless it's RMS, the reading could be off by quite a bit. Maybe Bill will chime in here.

[SoundInMotionDJ]

Rick, we went through this a few months ago. The consensus was that unless it's RMS, the reading could be off by quite a bit. Maybe Bill will chime in here.

This has me a little paranoid.

Some reading revealed that my little automagic version will give me RMS - but only for "pure" AC, and it will do that by multiplying the peaks by 0.707 (the square root of 2) - and that is a decent approximation. The meter will break down if the AC signal is not a "perfect" sine wave, or if the AC signal has a DC offset. I do not think that is an issue for setting the limiter on an amp with a pure tone...but I lack the test equipment to confirm that.

This article says that a meter that claimed to be "True RMS" was not:
http://www.enginova.com/true_rms_volts.htm

Here is an article that explains what True RMS is...
http://www.globaltestsupply.com/article-090806.cfm

I may invest $65 at Sears on the way home and do a little side by side comparison.

--Stan Graves

[Bill Fitzmaurice]

The consensus was that unless it's RMS, the reading could be off by quite a bit. Maybe Bill will chime in here.

If anything the reading will be higher non-RMS, giving a higher margin of protection.

[Rick Auricchio]

[
This article says that a meter that claimed to be "True RMS" was not:
http://www.enginova.com/true_rms_volts.htm

Yeah, I found the same article while reading to see if I knew anything about true RMS. I will confess that I may have known something back in my college freshman EE days, but that's 39 years ago...