True RMS vs averaging DMM

[RogerAF]

Pi is not relevant here.
The limit is an instantaneous value of 15V, which is 2V below the average value. Let's take this peak value of 2V. The waveform of ripple is constituted of a broken segment of sinewave and a segment of exponential. In a decent system, the two segments can be assimilated to straight lines, so we can use a triangle waveform for the actual ripple waveform.
The average value of a triangle wave is half its peak value, so the averaging meter should indicate less than 1V for correct operation. A true-rms meter should register about 1.15V.

P.S: I'm not the teacher here.

Aw shucks! That's why I prefer using a scope instead of a meter.
B.S. You just taught me a lesson!

 

[Tubetec]

Prof. Abbey ,surely your underestimating your contribution to these pages if you say that .

 

[Tubetec]

The Fluke 177 arrived today ,

It had a layer of what looked like concrete dust all over the inside ,so the guy must have been an electricans apprentice who gets to chase out the walls for a few years before he's cut loose , Ive only done basic checks so far but after cleaning the resistance test is showing O/L like it should with no test leads connected, where before it read something around 230kohms, the dust was fairly abbrasive causing some rub marks around the edge of the screen perspex and on the Fluke logo underneath . It also got into the battery compartment and had worn through the paint on the battery back started to scratch into the metal .
Most of the PCB is covered with a white polycarb sheild which very effectively kept the dust away from the important areas like the switch matrix ,(which it would have made a total bollox of)

I dismatled the whole thing and washed the casework in hot water with washing up liquid in it , I found a stainless pot scrubber and plenty of soap used gently to be superb at getting rid of grease and oil marks from the rubber membrane thats around the fluke handled cases . There was also some minor drip marks that could have been either beverage ingress or battery leakage and various debris and dust collected around the base of the 4mm sockets and the pcb , anyway a thourough cleaning out appears to have the unit back to correct operation on the high resistance modes .
Its got its fair share of scuffs scrapes and scars and the dial indicator ledgends are nearly ground off due to the nasty dust , but everything appears to be working so Im a happy man for the small money I payed for it . I might Dymo tap out labels and see how they sit ,OFF ,Vac, Vdc, mV, Ohms, uA and A in scarlet red with white letters just for better idiocy proofing and visibillity as time ticks on .

I'll put the 177 up head to head with the 187 later and we'll see how the calibration holds up,

 

[Tubetec]

As I said there were some abrasion marks on the lens/window of this meter so I checked the price of the replacement just for fun and japes , 24.95 plus shipping from the USA .
I used a product designed for polishing polycarbonate car headlight lens , some of the deeper dings remain on the outside ,but I was able to get rid of all the hazing and much improve contrast with only a few minutes polishing .
Another slight bum note is the A range fuse is toast , a few strands of copper had been used to short it by the previous owner . buyer beware I guess , shame the part costs nearly half what I paid for the meter itself .
Win some loose some . still a good deal on a meter that retails for over 400 euros new .
coming up a few basic tests and comparrisons with the 187.

 

[Tubetec]

Heres the results ,

77IV 177 179
1.5V cell 1.503V 1.503V 1.5029V

Resistor 2.458 2.457 2.4562 Megohms

Resistor 559.2 561.0 560.0 ohms

Cap4.7uF 10% 4.84uf 4.85uf 4.85uf

AC mains 240.4V 240.8V 241.0V

The results seem to suggest everything is in good order with the new meter . The labels work great , theres a funny symbol thats looks very like ohms upside down on the stamper so I used it , couldnt think of anyway of representing continuity bleep in the form of a symbol . Off wasnt going to fit so I just left it out .

 

[JohnRoberts]

Heres the results ,

77IV 177 179
1.5V cell 1.503V 1.503V 1.5029V

Resistor 2.458M 2.457M 2.4562M

Resistor 559.2 561.0 560.0 ohms

Cap4.7uF 10% 4.84uf 4.85uf 4.85uf

AC mains 240.4V 240.8V 241.0V

The results seem to suggest everything is in good order with the new meter . The labels work great , theres a funny symbol thats looks very like ohms upside down on the stamper so I used it , couldnt think of anyway of representing continuity bleep in the form of a symbol . Off wasnt going to fit so I just left it out .

Mhos the reciprocal of Ohms is the measure of conductivity.

JR

 

[Tubetec]

Im well aware of what Mhos are ,
When I try and do out a table of figures it tends to get scrambled up when I post it ,
Hopefully all is clear now.

 

[user 37518]

Pi is not relevant here.
The limit is an instantaneous value of 15V, which is 2V below the average value. Let's take this peak value of 2V. The waveform of ripple is constituted of a broken segment of sinewave and a segment of exponential. In a decent system, the two segments can be assimilated to straight lines, so we can use a triangle waveform for the actual ripple waveform.
The average value of a triangle wave is half its peak value, so the averaging meter should indicate less than 1V for correct operation. A true-rms meter should register about 1.15V.

P.S: I'm not the teacher here.

Yes, a True-RMS meter would just do peak-voltage/sqrt(3) = 1.15V as you mention.

The thing with averaging meters is that some just rectify the waveform, use a low pass filter with a very low cut-off frequency and multiply by 1.11, since that is the RMS to DC ratio in a fully rectified sine wave, so I guess that a possible reading for an averaging meter is just 1.11V which is not very far off from the True-RMS value.

 

[user 37518]

Pi is not relevant here.
The limit is an instantaneous value of 15V, which is 2V below the average value. Let's take this peak value of 2V. The waveform of ripple is constituted of a broken segment of sinewave and a segment of exponential. In a decent system, the two segments can be assimilated to straight lines, so we can use a triangle waveform for the actual ripple waveform.
The average value of a triangle wave is half its peak value, so the averaging meter should indicate less than 1V for correct operation. A true-rms meter should register about 1.15V.

P.S: I'm not the teacher here.

The thing with averaging meters is that some of them (not very sophisticated ones) just rectify the waveform, use a low pass filter to extract the DC component and multiply by 1.11, since that is the RMS to DC ratio in a rectified sine-wave. So a possible reading in an averaging meter is just 1.11V, which is not that far-off from the True-RMS value.

 

[abbey road d enfer]

The thing with averaging meters is that some of them (not very sophisticated ones) just rectify the waveform, use a low pass filter to extract the DC component and multiply by 1.11, since that is the RMS to DC ratio in a rectified sine-wave. So a possible reading in an averaging meter is just 1.11V, which is not that far-off from the True-RMS value.

Of course, the little calculation I did was based on supposedly correct meters. What a D'Arsonval galvanometer does is actually quite difficult to emulate electronically. Most of the handbook rectifier circuits are a degenerated variation of a quasi-peak rectifier.

 

[Tubetec]

The interesting thing is the 77IV non RMS meter is around 630 euros while the 177 which has true RMS and more or less the same features found on the 77 is 430 euros .

 

[user 37518]

Of course, the little calculation I did was based on supposedly correct meters. What a D'Arsonval galvanometer does is actually quite difficult to emulate electronically. Most of the handbook rectifier circuits are a degenerated variation of a quasi-peak rectifier.

Yeah, your suggestion to keep it lower than 1 V is certainly a good safety margin. I guess that the original question is not that good if someone expects an exact answer, since averaging meters can vary extensively in these sort of scenarios. Once you use a non-sinusoidal, all bets are off in an averaging meter.

 

[abbey road d enfer]

Yeah, your suggestion to keep it lower than 1 V is certainly a good safety margin. I guess that the original question is not that good if someone expects an exact answer, since averaging meters can vary extensively in these sort of scenarios. Once you use a non-sinusoidal, all bets are off in an averaging meter.

Since the original problem was a purely theoretical one (student exercise), I believe it's correct to assume the various meters are ideal, which is certainly not a reasonable assumption in the real world.

 

[user 37518]

Since the original problem was a purely theoretical one (student exercise), I believe it's correct to assume the various meters are ideal, which is certainly not a reasonable assumption in the real world.

I understand.