EEVblog > EEVblog Specific

EEVblog 1448 - Convert a Fluke 77 IV to True RMS for 10 CENTS

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Scherms:

--- Quote from: bdunham7 on March 28, 2022, 11:35:40 pm ---
--- Quote from: Scherms on March 28, 2022, 10:24:33 pm ---Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

--- End quote ---

I've never seen that specified and I'm not sure how it would matter...

--- End quote ---

Automotive ABS signals are AC and are often complex... but this ain't 'rocket surgery'!

EEVblog:

--- Quote from: Scherms on March 28, 2022, 10:24:33 pm ---
--- Quote from: golden_labels on March 28, 2022, 05:53:58 pm ---That was a whimsical comment about the video. That was a different situation. You bought a broken DMM and got it fixed. Not bought a working DMM and made a $0.10 modification that makes you lose a third of the device’s value.

--- End quote ---

Not so, many previous owners of expensive older Flukes like me have often traded functionality for outright performance. If a simple mod can propel a compromise purchase into 'the best you can get' then WOW!

--- End quote ---

It's a "simple" mod technically, but it requires a full recalibration of the meter and the gear to do that.

Kleinstein:

--- Quote from: bdunham7 on March 28, 2022, 11:35:40 pm ---
--- Quote from: Scherms on March 28, 2022, 10:24:33 pm ---Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

--- End quote ---

I've never seen that specified and I'm not sure how it would matter, but my go-to for such things was the Fluke 98.  The problem with TRMS is residual counts and small signals--you will lose the bottom 5% or so of every range if you convert.

--- End quote ---
When using the TRUE RMS chip to calculate the simple rectified value the limitation at small signals may be similar. Part of the problem is the rectifier that just gets ever slower the lower the amplitude and there can be some offset in when to change the sign. The main way to get around that problem is to get a meter that does digital RMS with sufficient resolution of the ADC.  Some of the modern cheap meters do that, but with a limited bandwidth - but at least a constant bandwidth.

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