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Moving Coil Meter

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Ash Small

Fri May 13 2011, 07:40PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

After the recent discussion in this thread:

I've started running some preliminary tests, as I've been challenged to build a moving coil meter (or moving magnet meter) and I've decided to take up the gauntlet

I could only find one supermagnet, a broken one I keep in one of my toolboxes, so I placed it in one side of a TV flyback core to create a horseshoe magnet of sorts. I'll sort out something better when I can but this is satisfactory for now.

I then wound a coil around a cigarette filter using some enamelled wire from an inductor from a TV, I think. not sure what the wire guage is, but it's a good starting point for initial tests.

I left the ends long and suspended it above the makeshift magnet as per the photo below, and connected it up in series with a potentiometer (22 k), a DMM and a 12 V supply.

I started turning the potentiometer down from 22k and the coil started to swing.....I turned it up and down a few times and the coil was swinging through 90 degrees.....then the potentiometer burned out......

Anyway, it's proved the concept. I'll refine the setup a bit and run some more tests. I didn't have a chance to ascertain how much current gave full scale deflection, but I do have a starting point now.

Patrick

Fri May 13 2011, 08:38PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Oh, now we all did it, we really put Ash's spine up now.

magnet18

Fri May 13 2011, 11:42PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

I just built one, stationary coil moving the meter.
It works, assuming I want the entire 0-5A span on a 1cm scale.
and theres no breeze
and noone bumps it

I think it might work better if I use a magnetized iron needle instead of one made of emi shielding...

Ash Small

Sat May 14 2011, 12:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

magnet18 wrote ...

I just built one, stationary coil moving the meter.
It works, assuming I want the entire 0-5A span on a 1cm scale.
and theres no breeze
and noone bumps it

I think it might work better if I use a magnetized iron needle instead of one made of emi shielding...

You can either adjust the number of turns or add some resistors. It's all elementary stuff. It's a trade off between power consumed and accuracy.

I'll develop mine a bit further and post results.

magnet18

Sat May 14 2011, 01:40PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

chapter 8 might be of some amount of use.

Proud Mary

Sat May 14 2011, 01:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

20 kÎ© per volt - i.e. 50 Î¼A FSD - is the established standard for moving coil movements.

Ash Small

Sat May 14 2011, 05:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

20 kÎ© per volt - i.e. 50 Î¼A FSD - is the established standard for moving coil movements.

I'll need some better magnets than the ones I'm using. I have a load of supermagnets somewhere, not Neodymium, but a higher temperature spec. which aren't as poweful. It may be a couple of months before I can get them though.

I may get some neodymium ones in the meantime.

I'll need to use thinner guage wire than I'm using at present. I forsee this presenting most of the problems, firstly, due to the difficulty of winding thin wire without ot breaking, and secondly, too thin or too many turns and it will overheat. I'm not yet sure how much of a problem this will be at 50 uA, but any change in resistance due to temperature change will probably affect accuracy. This is where I expect the most problems.

Balancing the movement will also present some problems but I should be able to overcome this. Static balancing is simple enough in principle.

Obviously, the stronger that magnetic field, the easier it will be. The makeshift magnet I used above is no-where near as strong as neodymium ones

Obviously, in a lot of amatear applications, 50 uA FSD isn't required, but that's the challenge I've accepted, so I'll see what I can do.

@ Magnet18, Do you have any photo's of your setup?

EDIT: I'm assuming a larger diameter coil will have advantages, assuming the same number of turns.

magnet18

Sat May 14 2011, 09:26PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

15 turns, 20ga, I need more turns. much more turns.
I'll see what happens when I make some changes, I have some nice thin wire and better material for a needle.

Patrick

Sat May 14 2011, 09:30PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

magnet18 wrote ...

15 turns, 20ga, I need more turns. much more turns.
I'll see what happens when I make some changes, I have some nice thin wire and better material for a needle.

remember the d'Araonsval meter has that arm stretching in a curve across from pol to pole, to linearize the meters scale, otherwise what your showing above is exponetial or logarithmic, i forget which.

im trying to find pics and my book at the moment... to show what i mean.

Sulaiman

Sat May 14 2011, 09:40PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For a moving-coil meter, winding the coil will be tedious,
making the pivot/movement will require machining
making and mounting the springs at each end of the coil for centering/restoring-force/lead-out I want to see !
Seriously, don't start this project!

buy/get a moving coil meter movement
and try to completely dis-assemble then re-assemble the movement
now imagine having to make the parts as well

Make a Tangent galvanometer using a magnetic compass and helmholtz coils
this can be calibrated knowing the earth's magnetic field and the number of turns and dimensions of the coils.

You could also make a taut band mirror galvanometer...very sensitive.

For a compact/rugged unit I would go for a moving magnet design.
e.g. mount a diametrically magnetised disc magnet (I can post a couple if you like)
on a pivoted shaft with the pointer needle.
Wind many turns of wire on a stationary core with the pivoted magnet in the airgap
use a second magnet above or below the pivot points to zero the pointer and act as the restoring force.

An easy to make (large) coil and no spiral coils.

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