where can i find, or how can i build tiny ammeters?

[jonqrandom]

i'm looking to build something like a puck, but in a cylindrical enclosure - i don't like box mods by and large.

here's where i'm running into issues: i'm after an analogue voltmeter and current-clamp ammeter to build into the mod, but i can't seem to find any at the small size (<2cm) i want. the smallest panel meters i can find to buy are around 5cm wide, way too big for my purpose.

if anyone's aware of a supplier that makes these, that would be wonderful, but i'm thinking i'll probably have to roll my own.

on wikipedia, it describes an iron vane current clamp type of ammeter that can read DC currents of above about half an amp or so from a straight wire without using the Hall effect. a mechanic on some forum described using such a device, and specifically mentioned that it worked by being laid against the wire to be measured and had no "jaws" to clamp around anything.

and this is my second issue: other than those two paragraphs, my searches for "iron vane current clamp" and similar are returning diagrams for moving iron ammeters (an in-circuit type), mechanic's Hall effect ammeter's, linkbait slight re-writes of the wiki article, and other almost relevant stuff.

are there any old-school modders that can direct me to resources about iron vane current clamps, please? i'm embarrassed to admit that i've been drawing blanks for more than a couple of hours, and i don't think my search-fu is gonna cut it here the closest i've come is probably this diagram from a landrover repair blog:



and even though i'm planning on using wire internally that's way over the max wattage the batteries could push, i'm still dubious about the wisdom of coiling any part of that wiring inside the mod. if there's a way i can do that and be sure to be safe - by using thick enough wire with a particular insulator, f'rinstance, please do say so
EDIT: that appears to be an AC source anyway, no idea where it's supposed to be coming from, i thought cars only used DC from the rectifier and didn't pull power directly from the alternator. just noticed that, sorry. duh.

before anyone starts worrying: no, these meters are not supposed to be used in place of a multimeter.

other "before-you-asks" ;-)

it's an aesthetic choice, both regarding the outward appearance and construction; i want to avoid semiconductors. no digital displays, no little all-in-one Hall effect sensor and amplifier packages.

i'm not expecting accuracy from homebrew meters (or any analogue meter at that size), just a rough indication of how the device is faring.

i doubt that an in-circuit ammeter at that size could handle a few amps passing through it, hence why i want a current clamp ammeter.

 

[classwife]

Moved to the Modding Forum from New Members

 

[jonqrandom]

Moved to the Modding Forum from New Members

thank you very much

 

[Alexander Mundy]

That is a tall order.
I started to write below about a small meter I found, but the problem is that even with a shunt you are going to get too much voltage drop with it.
The closest thing I found that looks like it might work is this 10A meter

But it is 2" X .75" and appears to be about 2" in depth.

I am leaving what I started to tell you about for the reference math involved:

First questions are what accuracy level and what range of voltage and amperage?
You can use a piece of wire for a shunt to increase the range of an ammeter or to convert a voltmeter to an ammeter.
At the size you want an analog meter is not going to be very accurate.
Take for instance this meter

"Battery Status Meter 0-500 uA

Incredibly small only 12.5mm diameter DC moving coil indicator"

Going to the datasheet

SP11 0-1 mA +-20% 345 ohms +-15%

1 mA = 0.001 Amp
Now say we want to make it read 10A full scale.
We want 9.999A to go through the shunt and .001A through the meter.
In parallel we need a shunt of (.001A / 9.999A ) * 345 ohms = .0345 ohms
16 gauge copper wire is .00473 ohms per foot so a shunt of .0345 ohms / .00473 ohms per foot = 7.3 foot
Unfortunately with this shunt at 10A you will have 10A X .0345 ohms = .345 V of loss.
A meter with much lower resistance will be necessary.

 

[jonqrandom]

hey, i appreciate the reply, and that 10 amp meter would almost fit inside what i'm thinking about... almost.

so yeah, that tiny meter (actually the round version is exactly what i'm after physically) is so much smaller than anything i came across, that's brilliant, so that and a shunt and the voltmeter's sorted

accuracy wise, i'm pretty much looking for little analog telltales. +-20% is dealable, and i can always calibrate against my own meter, although those will probably drift plenty anyway so it may not be worth it lol.

the whole voltage drop issue over the ammeter and its shunt is exactly why i was trying to find information about iron vane current clamps, as they can measure DC current without insertion into the circuit. problem is, they were superseded long ago by Hall effect current clamp ammeters which is what, as i understand it, you'd commonly find in those modern portable multimeters with the big jaws. Hall effect sensors need amplification though, and i'm going for something without semiconductors... i'm not up for building a mod with subminiature vacuum tubes either, much as that would look awesome, it's beyond me right now.

i think maybe i should just buy a few of the cheapest panel meters i can find and butcher them to learn how to convert them into iron vane type meters. i almost feel sorry for them. if it's doable i can apply the process to one of those tiny meters, so thank you very much for that!

 

[jonqrandom]

i think i may have just run into (at some of) the information i needed. here's hoping!

the electromagnetism section of the wiki article on electric fields explained something for me: i was under the impression that a DC voltage moving through a wire only produced a magnetic field for as long as the current varied, hence why AC can maintain power transfer between two inductors and DC can't... of course i had it backwards.

with apologies to everyone who already knows this: DC produces a stable magnetic field, while it takes a varying field to induce current in another conductor.
i saw this image and had a combined eureka/headdesk moment lol:

now some of the diagrams i get googling "iron vane" are starting to make sense to me, i might actually have a hope in hell of building one of these ancient devices!

i still need to do a lot more reading before i understand how the effect is exploited in practice, but at least i have an idea of what i'm looking for now

 

[twgbonehead]

Moved to the Modding Forum from New Members

Because there isn't an Amish forum?

 

[jonqrandom]

silicon's baaad, mmmm'kay.

 

[twgbonehead]

jonq,

I hope you take my previous post in the spirit in which it was intended. I did not mean to denigrate your efforts, but it really seems to me like you're trying to re-invent the buggy-whip.

But since I do know the answers to some of your questions, I will provide you with some more info; I hope this helps you understand what you're trying to do a bit better.

Although that meter diagram you showed used an AC source, it would work with a DC source as well. Iron is attracted to either pole of a magnet, so whether it's a DC field or an AC field, it will get attracted.

I don't know if you ever played around with electromagnets. You can put a DC or AC field on them, and they will attract ferrous objects (for example iron).
The more current that you put through the electromagnet, the stronger it gets.

However, if you put a DC field through an electromagnet, it acts like a magnet. Another magnet will be either attracted or repelled from the end of the electromagnet (depending on whether you're putting them together N-S or NN/SS. Another magnet will not be attracted or repelled from an AC electromagnet, since the switching of the poles will cancel each other out (in other words, it is rapidly going from being attracted to being repelled, and these cancel each other out - almost, but that's too much detail for here).

An analog meter works on this basic principle. You have an electromagnet, which develops a magnetic field whose strength depends on the amount of current through the coil. You have a needle, which has one end which is either ferrous or magnetic, and which is attracted to (or in some cases also repelled by) the electromagnet. You also have a spring, attached to the needle, which exerts a force AGAINST the force exerted by the magnet.

A little current provides a small force, which moves the needle a little. More moves more, etc.

The difficulty is that a spring and an electromagnet have very different characteristics.

A spring is usually pretty linear. Think of the old-fashioned postal scales, which are basically a platform suspended on a spring.
Put a 1-ounce envelope on it and the spring stretches by .2 inches (for example). Put a 2-ounce envelope on it and the spring stretches by .4 inches, and so on. The "dial" on a postal scale is pretty much linear; every ounce is represented by another .2 inches of spring stretch.

A magnet has very different behavior, though. The closer you get to a magnet, the stronger the pull, and this is usually NOT a linear function. Think about trying to pick up a nail with a magnet. As you bring the magnet closer, at first the nail doesn't move. As soon as it starts moving, though, it gets a stronger and stronger pull, so it jumps right up to the magnet. The pull of the magnet (unlike the spring) depends BOTH on how much current is flowing through it AND how close you are to it, and since making the magnet stronger also pulls it closer, you have a very non-linear (and often unstable) relationship; in the worst case the needle will not move at all until a threshold is reached, and then it will immediately peg all the way on full-scale.

You can avoid some of this by using a magnet on the needle, and using repulsion instead of attraction (with a DC electromagnet). This is always stable (since higher magnet current causes greater distance). However, it will be very non-linear; the difference between 1A of current and 1.5A of current will be much larger than the difference between 10A and 10.5A. Therefore, unlike the postal scale, your dial could not be linear, as you go up higher and higher the ticks need to be closer and closer together.

An analog meter is carefully designed with a geometry to compensate for these issues. It's a very complicated process. (not to mention you really need a jeweled movement to get something that works properly).

HTH!

 

[jonqrandom]

twg; don't worry, dude, i took it as joke elsewise i wouldn't have liked your post, neh? i shoulda probably added a smiley at the end of mine, i guess.

yeah, i'm not trying to invent anything, just re-use old technology because it suits my use-case and design aesthetic - which is eccentric. i'm aware

the information you've given there is very useful, thank you ever so much for taking the time
what you describe with regards to field strength sounds similar to the inverse square law IIRC, is that about right?

one of the handy things about the fact i'll have to open up the panels and mess with their internals anyway is that i'll have the opportunity to put my own dial-faces (almost certainly the wrong term lol) in - remember when non-linear dials used to be a thing, on stereos and suchlike? so i can test it with known(ish) currents, record the deflection, and draw a dial or knock one up in inkscape or similar.

again, thank you very much, that was very helpful

 

[iamthevoice]

Found several models here: http://www............com/promotion/industry-business_ampere-meter-digital-promotion.html

 

[jonqrandom]

well, thanks for replying, but your link was to a page of digital meters. was that what you meant to post?