Analogue moving coil meter range extension?

He's making a one off for use at home, not commissioning a new design of military hardware.

not compliant with the spec

NT

On 9/21/2017 6:35 PM, T i m wrote: > But still, ohms law applies and with just a series resistor to limit > the current for FSD, zero on the meter (re-marked to display 10V) > won't reflect the required 10V.

Now, ya see, I missed that. you want a 10-15 volt meter, not a 0-15 volt meter.

The simplest is a 20K resistor in series with a 10v zener diode. Use a low current Zener so the knee works right at low values. Something like this:

Depending on how non-linear the low end is, you can print a custom dial scale that has been corrected for "this voltage is this mark." Although this one looks to be linear down to 10 uA or better.

simple but time consuming producing a new dial paper

NT

"The Windows programs for drawing analog meter scales" $35 for the full version.

On 9/21/2017 10:59 PM, snipped-for-privacy@gmail.com wrote: > simple but time consuming producing a new dial paper

With the low current Zener, it doesn't look like that will be necessary.

The task is the same no matter who the customer. The more steps you skip, the more problems you have. The more you think like a project manager, the better managed your home projects. Once you acquire the habit, you'll find that it adds little to the time in the beginning and greatly simplifies the end. And it wastes far less time for rocket scientists in residence.

I haven't seen the system requirements, but I'd wager that the precision measurement desired is never entered into any other system/program. The desired functionality could be obtained by a Sharpie mark or two on the face of the meter.

One of the difficult engineering skills to acquire is the ability to match the solution to the actual problem. Overkill is rampant. It's what you do when you don't understand what you need, but you start building it anyway.

If you need precision, you don't want an analog meter.

The meter program is capable of 100 "placed" I.e. non-linear tick marks over the scale. Much like some of the expensive lab grade equipment built with hand lettered scales.

I am reminded of a story in Model Railroader back in the early '50s. One of their staff members was in South Korea during the Korean war. He was bound and determined to build himself a small table railroad. He'd spent several weeks making sure every measurement was right and the corners were square.

Finally, the big day arrived. He placed it in the corner of the living room. Much to his horror, he had a triangular gap along one side.

He asked his Korean attache how could he have possible had that happen. The answer was simple, "Korean house not always square."

"But, your furniture and tables fit perfectly!"

"Korean tables and furniture not always square either."

I guess that depends on your background, viewpoint and goals mike?

e.g. I knew what I wanted, had a rough idea how it could be done and just thought I run it past the panel for some real world *design* feedback (I'm very much a dabbling designer but long term builder / support guy etc).

Understood. I felt I laid out the relevant spec on my initial post, or at least as far as someone who understood humans were concerned. ;-)

"Not a 'repair' question as such but I was thinking of using an old skool 250uA FSD moving coil panel meter to read the 10 to 15V DC range (a lead acid battery charge / discharge indicator), next to a 0-30A Ammeter (inc shunt etc).

I was thinking of using either a voltage reference such as LM4040DIZ-10.0 or could I get away with a simple zener (BZX55C?) with associated resistor(s) please?"

So that was the need, the specs as far as I knew them and a potential solution to help people 'get' what I was considering. What I didn't do was signpost the fact that I was actually using a 0-5V panel meter to display the 10-15V range. ;-(

The 'human' bit: A small (key fact) analogue (key fact) panel meter connected to a lead acid battery (key fact) needs reading and those readings interpreting and so ITRW, we are 'most likely' to not require much in the way of precision or we wouldn't be using those things in the first place. ;-)

So, what I was hoping I was saying is 'Could someone confirm or recommend that 'for the (reading_between_the_lines) reasonably 'soft' requirements, what is likely to offer *a* (there may be many equally competent) real word / human solution?

So, anyone who knows their stuff and who understands people *might* have already answered the question on 'how much money', 'how much effort', 'how complex / simple' would any solution likely to be in their heads.

Understood, but unbeknown to anyone here, that will all be catered for

Again, understood. In this instance the only clue I gave to the potential size of the battery was the scale of the ammeter (0-30A) and that scale was because it is to be used on an electric outboard motor that has a maximum current of 30A and will in fact be running from 3 x

Quite. ;-)

Again, how would knowing what I wanted to use it for, more than I already specified help anyone actually resolve the question asked? I'm not saying they couldn't then rule out or add 'other things' but nothing offered so far has affected the physical outcome of this particular project in a physical sense.

That said, I could have been using a large and expensive panel meter, graduated in .2v increments, rather than one only marked in whole volt increments . But the reading of the meter is my problem, making it read 10-15V is the bit I needed help on (please). ;-)

As an aside here ... I ran a Telephone Help Desk for over 5 years and feel I became fairly good at 'gauging' people. If their opening gambit was 'Your beige box isn't working' I knew to deal with them at a different level (initially at least) compared with someone who said 'Our X.25 PAD No3 doesn't seem to be bringing up a Link LED?'.

Just the same as the PC user reporting that their 'screen is blank' when it in fact has a flashing cursor in the top left corner. It's not what they expected or wanted but it's far from 'blank'. ;-)

Oh, absolutely and you / anyone (who hasn't already dealt with me on here) wouldn't have any idea what I do or do not know and may have already considered. My solution potentially required 2 components (resistor and zener) so it couldn't get much simpler so it's just a matter of selecting the 'best' (all things considered) types of those components, if they are appropriate.

Points noted and generally agreed mike. ;-)

No, it's something I'll glance at either now and again or specifically when I change something and need to see the consequences (like changing 'speed' on my electric outboard.

Already Done.

No need mike. It will likely only be in circuit when required (testing) and the current drawn when it is will be insignificant relative to the other loads and battery capacity (93Ah).

Check.

Cool. ;-)

Check.

Check (~20k?)

Bench PSU + DMM, check.

Check.

Hopefully not. It will only ever be 10.5V or higher or 14.4V or lower ... or disconnected completely (points already considered etc).

Quite. ;-)

Check. I did suggested a BZX55C in my OP but not being a designer I would have no real idea where to start re finding the 'right' component (by that I mean there is probably a 'go-to' device that 'most people' would typically use under these circumstances, like they might a 7805 or 555) ;-)

Ok?

Unlikely (see all the above). ;-)

Chances are I'm not so ...

Quite ... and with simple 1V increments over a 10-15V range should give me a better 'feel' of the voltage than trying to read a similarly sized meter graduated in .2v increments in the 0-15V range all whilst going along a river in a small folding boat. ;-)

Very much so, thanks mike. ;-)

Cheers, T i m

It can and maybe should be, in an ideal world. I say that because I hate having to deal with 'fools who rush in' or those expecting me to make technical / choice decisions on their behalf (like getting them some 'size 10, brown, lace up leather shoes'). ;-(

'May have'.

Agreed. I like to understand as much about something as I can. However, my ability to do so, time available to do so and the pertinence of doing so don't always allow me to do so or to do so to the level I would like. Sometimes you just have to get stuck in. ;-)

Agreed 100%.

Quite ... however, this is a 'discussion' group and so does have an element of that, along with the hard science / fact stuff.

Part of the 'human' bit is not making a post (especially the initial one) too long (I fail most of the time) as it will put people off reading it and if there were any further points that need refining they could be done with a simple Q&A.

e.g. I initially outlined the fairly close detail at a level relevant to my needs and understanding (IMHO anyway). I didn't expand on what I was going to actually use it for because it didn't really matter to the question. I left it open to the reader to ask any supplementary questions as they felt relevant. ;-)

Cheers, T i m

Wow, what a long thread about such a simple problem. Here's a web page describing exactly the circuit you need for an expanded scale voltmeter

Another method that I've used in the past to make an expanded scale meter is to suppress the mechanical zero. You still have (in your case) a 0-15V meter, but the needle doesn't get up to the "0" mark until 10V is felt across the circuit. After that, the meter responds normally, up to 15V full scale. So,with this approach, you don't need any extra circuitry to make an expanded scale meter. This means that you'll have to open the meter and (CAREFULLY!!) move the mechanical zero tang. First, apply 10V to the circuit. Then, turn the mechanical zero tang so that the pointer goes down to the "0" mark. Reassemble the meter movement, and you're all done.

To make this approach work, your meter movement must have enough mechanical adjustment range to suppress the pointer enough so that it needs 10V across the circuit to make the meter read "0" V. Typically, D'Arsonval movements have about 20-30% range on the mechancial zero adjustment, so yours might be a long stretch for this method. Never know till you try!! Dave M

I forgot to mention that for the zener solution, you really need to select as low power a zener that you can find, such as 400mW or lower. The 1N4104 is a 10V, 250mW zener, with a test current of 1uA, and a max current of 25ma, well within your requirements. In stock at Mouser.com.

Cheers, Dave M

In college (about 1969), I build an "electronic slide rule" in the form of an analog computer inside a brief case. Inside, it was two log amps, a multiplier, range switch, and an anti-log amp. Input was via two Bournes Helipot 10 turn pots (linearity 0.1%). Output was on a mirrored scale analog panel meter about 6" in width. Accuracy was 3 decimal places in the lower part of the meter, and 2 decimal places over the rest of the scale. However, I had to use temperature compensation tables for the pots, multipliers, and amps to obtain that level of accuracy.

What level of precision were you expecting?

The TL431 is actually a comparator with its own built in 2.5V reference.

In closed loop operation, that's as low as the cathode goes - you can saturate it in open loop operation, but "Vce-sat" is around 2V.

You can use any zener under 10V to do a 10V zero offset (just divide down the input voltage until that 10V input matches the zener knee). So, with an arbitrary zener (I'd use a TL431 as a 2.5V reference, they're convenient and accurate) and a trimmer acting as voltage divider, it just remains to make a range-setting resistor in series with the meter.

The important thing, is to doodle up a really nifty scale for your meter that indicates 10V to 15V, with clear markings, along the pointer's arc, maybe with a bunch of subdivision marks. Last time I did that, it took a plotter and a bit of custom software to draw the arcs and labels, then some fiddling with the faceplate of the meter to affix it (warning: you need to worry about laser-print ink, paper, glue compatibility).

fails to solve the issue AND adds pointless cost.

NT

Oh, you've found a zener that's 10v +/- 1%?

NT

f military hardware.

Rubbish. For a large project all the specs have been calculated to meet the requirements, the contract gone through by a team of lawyers, and the task of the lec eng is to produce a circuit that meets all the given specs unde r all specified conditions.

For a one off at home the requirements are often not pinned down firmly, as in this case. There are no lawyers, no contract, and the elec eng's job is to trade off cost & complexity versus specs to find what turns out to be t he most agreeable compromise for the client, oneself. In most cases this is a device that is much less tightly specced.

NT

Indeed. This can be done in such a way to make it tidy. But the use of a zener means the scale will have to be calibrated, which is a waste of time, especially to get a nice neat result.

I see it's foolish claims day today.

NT