"Good" DMMs with bar-graphs - crippled by design?

[Doug3004]

I have seen various buying guides and reviews on digital multi-meters that have an analog-style bar-graph indicator on the LCD.
I have at least one myself, tho I did not particularly care for that individual feature.

I've also seen reviews of cheaper meters where the reviewer noted that "the bar-graph was not very effective, because it updated just as slow as the numeric display". Their implication being that whole idea of the bar-graph is that it would simulate an analog meter needle movement somewhat, and intuitively indicate a relative value faster than the 7-segment portion of the display would.

My question here is, how would it be possible to make any one part of a LCD refresh faster than all the other parts? I've not heard of any way to do this, tho I have not looked before today I suppose.

If the meter is measuring voltage for example, and has only one ADC that is supplying the numeric value to be displayed, then the bar-graph cannot obtain the numeric value any faster than the 7-segment portion of the LCD could...

And if the whole LCD is driven by one IC then I would presume that all the elements of the LCD are being run off the same voltages.... and they should have the same refresh rates, since all the LCD elements are run off of one shift-register arrangement.

So then--the only way to make the bar-graph appear to be updating faster than the 7-segment display is to intentionally slow down the update rate on the 7-segment display? And this is a good thing?

 

[Sunnysky]

I recall my Fluke DMM has a fast dot... bar graph display, but still not as fast as an analog meter.

 

[hevans1944]

You make a lot of assumptions and generalizations about the way digital multimeter bar-graph displays work in conjunction with their 7-segment digital displays. I use a (now discontinued) B&K Precision Model 2890A digital multimeter with a 00000 to 51000 count 7-segment display, and below that a 20-bar display that reads from 0 to 5. I have not seen any perceptible difference in the update rates of the two displays. The bar-graph very easily shows "trends" much in the same manner as an analog D'Arsonval meter display, but without any meter "dampening" effect which I sometimes wish it had. Clearly the bar-graph full-scale display resolution is only slightly greater than one significant digit: 0, 0.25, 0.50, 0.75, 1.0, 1.25, 1.50 ... 4.75, 5.0. My eye usually ignores the intermediate bar-graph "values" and quickly "estimates" the deviation from the "cardinal" values of 0, 1, 2, 3, 4, and 5.

My question here is, how would it be possible to make any one part of a LCD refresh faster than all the other parts?

The LCD display has independently addressable sections. How else to distinguish between meter functions and polarity of DC inputs? It is a matter of programming how often any given part of the LCD display is refreshed. Inexpensive multimeters may not have as many display elements as, say, my multimeter, but surely they have at least a polarity indicator and an over-range indicator that are separately addressed from the 7-segment digital display.

 

[(*steve*)]

I was using one of my multimeters a day or ago to check a pot that seemed noisy.

Watching the bar graph as I turned the shaft was far more useful that reading the displayed digits. Of course, it can sometimes help to fix the meter on a single range...

 

[shrtrnd]

If what you want to do was easy, the original equipment manufacturer would have done it. Trying to re-engineer what the designers did, does not seem to me to be a very practical fix. I'm of the mind that if you really want an analog representation of what's happening with a reading, take Sunnysky and Hevans1944's thoughts on using an actual D'Arsonval meter movement.
My DVM was made by Fluke for Square D. Except for Hevans1944 observation that it displays signal 'trends', the
only really useful application I've found for it is its sample & hold capability for peak values. Otherwise I just hook-up my Simpson 260 for analog questions I have.
What's the specific application you have, that you want a faster bar-graph reading for?

 

[Chemelec]

I also Don't like the Bar Displays on these DMM's.
That is why I still have and use Analogue VOM's at times.
I Also still have a HP, VTVM.

 

[hevans1944]

I keep analog multimeters with D'Arsonval movements handy on the bench. I must have used a dozen different types over the years, including the venerable Simpson meters with their taut-band movement and mirrored scale (to avoid parallax errors when reading the pointer position) which I like a lot but don't own one yet. However, I keep coming back to my handy pocket-sized Triplett Model 310, shown below, for quickie measurements or just monitoring a power supply output. The first one I bought did have the clamp-on AC current meter as shown, but not all model 310 meters will accept this adapter and work properly. The mounting holes are there on the case, so you can attach the adapter, but on some recent meters the internal electrical connection is not present. If the meter does not have the "AC AMPS" position on the rotary function switch it probably will not work with the clamp-on AC current measuring adapter, so buyer beware. These meters are an incredible buy on eBay if the movement is intact (no bent needle, bearings okay) and none of the internal range-setting resistors are damaged. I bought a new replacement 310 (without the AC current clamp adapter capability) a few years ago after I broke off the little red thumb-operated range switch knob, but I am embarrassed to say how much I paid for it. Check the on-line new prices for a hint. View the eBay page here.

 

[Doug3004]

... The LCD display has independently addressable sections. How else to distinguish between meter functions and polarity of DC inputs? It is a matter of programming how often any given part of the LCD display is refreshed. ...

Well yes but that is my point... the only way to get the bar graph display to update "faster than everything else" is to.... slow down everything else.

If you wanted the bar graph to not jump around so much, then the last 2 or 3 readings could be averaged,,, but that would make it initially respond even slower.

I saw where a couple of fairly-popular people doing their own online reviews of meters mentioned this, and it seemed a rather odd quality to consider a drawback.
I don't care to mention names of the guilty parties; if you look around they're not difficult to find.

~~~

I have a cheaper analog meter around if I thought I needed one.
Haven't yet needed it in the ~2 years I've had it, but I mostly have been doing Arduino/TTL stuff.

 

[hevans1944]

I mostly have been doing Arduino/TTL stuff.

Yeah, me too. A logic probe and an oscilloscope are my "go to" tools for that sort of thing. The multimeter is mainly for verifying I remembered to turn the DC power supply on and have connected it to the circuitry. I actually prefer my analog Triplett VOM for continuity measurements because (1) my hearing is not so good anymore so it is difficult to hear the high-pitched continuity "beeeep" from my digital multimeter and (2) I can see the meter pointer move and get a quick "eye ball" estimate of the resistance from a fair distance away.

As far as LCD update rates are concerned... LCDs in the past were notoriously slow to respond because of the physical properties of the twisted nematic crystal's response to an applied bias field. This has been much improved over the past forty years or so, to the point where my eyes don't see any delay at all in the rate at which both the bar graph and the five seven-segment digits update on my multimeter. Once you get past a certain speed of response, it is impossible to observe, and irrelevant to consider, what needs to slow down to make the display work. After all, television sets now commonly use LCD pixels and those turn on and off very quickly. As in all time-division multiplexed schemes, everything needs its own time slot, but some things can get more time slots than others for any given refresh interval. It's a Human-Machine-Interface (HMI) design consideration, so feel free to design your own version.