a common enough phrase for an in-house spec. Such phrases should be cleared up later of course, once the specifier knows what the engineers can actually do and what the options cost.
Some do. Some are 'how can you get it as cheap as possible and still have it sell & not destroy our repuation.' Some are 'we want to beat what's out there, how good could you get it?' Some are 'we want it to do this.' etc etc etc.
1% is doable for some analogue meters. That's enough precision for a lot of tasks.NT
I don't know if you are asking for an expanded scale meter, were the
0V line is 10V and FS line is 15 volts, so the whole range is only 5 volts. That's what I gleaned. First you need to put a series 60K ohm resistor to make the meter read FS with 15V applied. (for accuracy you should subtract the resistance of the meter from the 60K ohms) Then you need to add a circuit shown in these sites.
This last one will probably need a 10K resistor tied from one OUT to the other OUT. Just so there is enough current to make the regulators work properly.
Here's a youtube, I didn't watch it all, but you might learn something.
Mikek
At least the extra option should be there.
The consensus seems to be to use an expanded scale analog voltmeter. If the OP is going to build such a thing, he needs a new scale on his analog meter. That can be done with the free version of the software. (I just checked and it will do an expanded scale).
I usually ignore one line pontifications, but I'm curious. Why does a new meter scale fail to solve the issue? Ummm... what issue?
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
The meter isn't even that accurate. You're not going to be able to read the meter to +/- 0.1V. Also, I coulnd't find where anyone specified the accuracy required.
If you really want 1% accuracy, maybe a 10.0V precision reference, such as the TI REF102: However, there's a problem. The IC requires a V+ of 11.4-36VDC which will probably require a battery or other power supply.
Hmmm... I sorta blundered across this idea: It doesn't expand the entire meter scale from 10-15VDC. Instead, it compresses 0-10V into a small part of the meter scale, and expands the 10-15V over the rest of the scale. However, it requires
+12v and -12v power.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
that's part of the spec
yes
it can
The issue with using an inaccurate zener is that the meter scale most likel y won't run from 10.0v, and will thus need calibrating over its scale. That also means a new scale with marks in new places - a pita to do. Far easier to knock off 10v exactly then just change the numbers on the scale.
NT
We don't know the meter's accuracy. 1% is common enough, but it might be worse. (For all we know it might even be a moving iron thing.)
Even my most rock bottom multimeters, under $3 each new, are easy to read to 1%. Decent meters do much better.
Nice idea, though I don't expect the op needs it. I'm sure one could design a circuit that can use opamps that are happy on the 10-15v and halve V_in.
NT
Metrology is an interesting topic.
When someone asks me how to measure something, my first thought is, "exactly how will the answer to your question improve YOUR life tomorrow?" Sometimes I actually say it. The "jolt" to their thought process forces them to express what they're trying to accomplish...or admit to themselves that they have no idea. I find that almost all questions end up in the "won't affect them at all" category and I can dispense with trying to teach them something they'll never use. If the answer won't change the future, don't bother with the question.
It's similar to pointless social interaction. People you don't know ask questions, like "how are you?" They think they know what you'll say. Respond with something like, "I have this rash on my dick and think I need to find a different street corner... got any ideas?" to see how much they really care about how you are. ;-) But I digress...
If I had an electric boat, the ability to measure voltage to six decimal places would not be my priority. What I want to know is, "can I get back to the dock?" You can do that more accurately with a sharpie and nonlinear meter than you can react to a change in wind direction.
The river current and wind and the shape of the curve of distance covered vs velocity and temperature would be far more important variables to me. Amp hours consumed would be important.
Battery voltage that would be strongly impacted by whether I'm going up or down stream at what net velocity is a poor substitute.
An integrating current meter, a water speed speedometer and a GPS would much more interesting. If you knew the water flow direction and speed, you probably wouldn't have to measure anything else other than what you can get from the GPS. Drifting downstream for a minute would give you those two numbers.
But, since you're dead set on measuring voltage... The basic problem is that the voltage you want to measure is the biggest voltage you have to power your circuit. That's why I split my original post into the concept (battery) and the implementation (zener). A series zener needs to draw ZERO current at 10V and still be 10V at MUCH higher current ratio. You will have to live with some amount of nonlinearity.
You could stack an AA-cell or two on top of your battery system and alleviate that problem.
I maintain that a digital voltmeter is the best option. They're dirt cheap on EBAY.
All the above is based on an extreme extrapolation of very limited information. That's why the system spec is so important.
In fact you could re-zero the meter so it reads 10v spot on. Add an extra mark for where it points when unpowered so you can see if it ever decalibrates.
yuck. 2 resistors beats 2 cells any day.
but noncompliant with the basic spec
NT
Yes, you can , but it's still nonlinear. It's a matter of how much nonlinearity you can tolerate. It's my assertion that the measured quantity is so crude that you don't need to worry much about anything. Just learn what it reads and calibrate your expectations to match.
Can't argue with that assertion, but, if you can provide a circuit that is precisely linear and accurate over the WHOLE range using two resistors, I'd like to see it. I'm betting it's more than two resistors.
Yep, that's a problem with any customer expectation. I try to adjust expectations to meet the ACTUAL requirements before delving into complex solutions to implement a nonsense requirement. Yes, the customer is always right. My job is to alert him to the consequences of his demands.
Another non-complient solution is to use the two resistors and a zener to make a 9V-15V meter and hope that most of the nonlinearity of the two resistor solution happens in the 9-10V range. That way, the characteristics of the zener knee are important over a 5:1 current range instead of an infinite ratio. I'm far too lazy to do the math on how nonlinear it will be.
There's also an elephant in the room. Unless you start with a meter that happens to have scale markings ranging from 10 to 15, you're gonna need a new scale no matter what you do with the electronics. If you're not gonna do that, might as well just mark calibrations on the face with a sharpie. Makes no sense to go to great lengths in one part of the project only to create something that still doesn't do what you want.
What I meant is divide the input to your opamp so it's away from the rails and you don't need any rail boosting.
NT
(I've been a bit preoccupied elsewhere but still following the thread with interest and will reply individually when I get the chance)
That was part I had already considered in that: The meter is fairly small. The meter is glued shut. The meter is only marked in whole volts.
So, as you say, afa this experiment is concerned it was a cheap toe-dipping into the concept of meter range enhancement, partly because I think analogue display still have some merit and partly for the S&G's. ;-)
In this case the actual requirements were fairly simplistic as it was an experiment. A feasibility study if you will. As you say, a DMM can be bought for next to nothing (and I have several and one I'm already using) but that wasn't the (entire) point.
;-)
With a very small buck converter you could get a higher voltage supply to ensure a voltage reference of some sort had sufficient overhead to work correctly when close to the 10V threshold?
ITRW, as long as it's starting to work reasonably by 10.5V it shouldn't really be an issue.
I do like the idea of printing my own scale and if this basic one works, I might go for a bigger / better meter that I can open etc.
Unless it's already marked 0, 1, 2, 3, 4 and 5 'Volts' and you know just have to add 10 to all of them. I would imaging the needle pointing up the middle to 12.x would be a sufficient comforter. ;-)
Only if I can get the thing open and it wouldn't be worth damaging it to try.
My only hope is that it works 'better' than a similarly sized and accuracy meter that is marked 0 to 15 and using it over a tiny proportion of the range.
As has been suggested, if you aren't happy to try to KISS then you are then going to be worrying about thermal stability and scale linearity and I'd like to leave that for the Mk2. ;-)
Cheers, T i m
Hmm, that's crafty. ;-)
I'm trying to picture how the voltage rise would still be linear but I think I can see how it would be.
The -ve of the meter is held at some voltage threshold low enough to ensude a low change in difference between the voltage measured and the ground. A variable (to start with at least) resistor set as a pd then sets the upper voltage to give FSD at 15V.
Yeah, that sounds like a interesting project for a Mk2 meter. ;-)
I saw the link to some free software for doing such but not had chance to try it yet.
Ok. ;-)
Cheers, T i m
Thereyougo...opamp is a lot more complex than two resistors...or a couple of AA cells.
so it's away from the rails and you don't need any rail boosting.
Cheers, Dave, I'll have a look.
As has been mentioned elsewhere, if we used a lower voltage zener (and as high a tolerance as can be found in the right spec and the right price *because*) to set the voltage on the -Ve leg of the meter and then tweak the voltage up via resistors (variable initially) to give a FSD at 15V then that should be sufficient?
I can't help feeling I've missed something with that solution though, like the linearity ... ? ;-(
Cheers, T i m
If it's not an accurate meter just add a zener, job done.
NT
I'm pretty sure we all know that, and what the options are, and what the pros & cons are. And that adding AA cells rather than 2 resistors on the input is not an option with any upside.
NT
Well, I'm not sure if it's electro mechanically any more or less 'accurate' than any similar style meters and being smaller with a shorter / lighter needle it's possible it could be better than some ... but regarding the potential display markings and readability, you may well be right. ;-)
Cheers, T i m
If it's only marked in whole volts, reading it with accuracy won't happen, so it won't be especially accurate.
NT
How so? I suggest that you are mistaking "accuracy" for "precision". They are not even a little bit the same thing nor follow necessarily one from another.
A gauge or meter that reads (is marked) in whole numbers but is dead-on is far more accurate than a gauge or meter marked in thousandths, but is always two units off. Although the latter meter is extremely precise.
I have a metered Iso-Variac that reads in volts measured in tens, and amps measured in tenths on a 0 -1 or 0 - 4 scale. But they are 4" meters so that even very slight needle movements are visible. The human brain is pretty good at interpolation.
Peter Wieck Melrose Park, PA
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