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Moving Coil Meter

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Ash Small

Fri May 27 2011, 10:52AM

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

I'll get some Viakal later. There are a few cleaning products on the market that contain phosphoric, I just happened to have some cola in the house. The leads and terminals look a lot cleaner thismorning.

It looks like I don't need to strip the AVO to examine the concentrator core, at least not at the moment, as I found the images below, after lots of googling.

It would appear that the core remains stationary, so I now have a basic design to work from.

Time to order some magnets and start work on a coil former.

I'm still trying to find some suitably shaped N52 neodymium magnets as the N52 is 1.44T whereas the more common N38 neodymium is only 1.22T. I'm also thinking of trying to shape the magnets themselves, and maybe using neodymium for the core as this should result in a stronger field than using iron pole pieces and core.

I'll run some tests first though, without pole pieces or core.

Sulaiman

Fri May 27 2011, 11:33AM

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

I think that in your design the thing to remember is
for a linear scale you need a uniform magnetic field vs. a linear spring/'restoring force'
and the force F=B.N.I.L (flux density.number of turns.current.length of coil in magnetic flux)
so on average, larger is easier. You will have to protect against wind/draughts etc.
a small ac shaded-pole induction motor would yield suitably shaped magnetics.

For 'academic' reliability you could use nickel (or ferrite) as part of the magnetic path as it can be forced to saturate, providing a constant magnetic field over time.

The core can be stationary or IF it's magnetically uniform it can move with the coil.
You also need to either design for fixed orientation (horizontal or vertical)
or have everything balanced so that as it is tilted, gravity doesn't move the pointer!

Are you sure a $4.99 dmm won't do?

Ash Small

Fri May 27 2011, 12:36PM

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

Thanks for your advice, Sulaiman, especially the bit about the core. I'd sort of worked out that it could be stationary, or move with the coil. I think both are used in commercial meters. My reasoning at present is that if it is stationary, it 'could' be formed from a third neodymium magnet, and this 'should' increase the strength of the field even further. I aim to test this by experiment.

Sulaiman wrote ...

Are you sure a $4.99 dmm won't do?

This has become an exercise in electro-magnetics. I'm just out to see how sensitive I can make a movement. I've two aims now. Firstly, to build a 'crude' device, that is suitable for basic amateur construction and is 'usable'. Secondly, to see just how sensitive a movement I can make, and whether I can make one better than the 37.5uA movement in an AVO.

I'm planning on using a coil around 3/4 inch across, and using some cubic magnets around 3/4 to 1 inch cubed, but shaping them (if I can do so without heating them too much) for the pole pieces and core. (I think this is the approximate size coil that AVO use, but I've not opened mine to check yet) This should keep things 'realistic'. I think going much bigger could be seen as 'cheating'.

I'm also not planning on using jewels.

Proud Mary

Fri May 27 2011, 12:49PM

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

If I had to construct a meter from scratch, I think I'd go for the very Victorian tangent galvanometer.

It uses the Earth's magnetic field as the needle counter force, rather than a troublesome-looking spiral torsion spring and Hooke's Law.

Ash Small

Fri May 27 2011, 02:33PM

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

Proud Mary wrote ...

If I had to construct a meter from scratch, I think I'd go for the very Victorian tangent galvanometer.

It uses the Earth's magnetic field as the needle counter force, rather than a troublesome-looking spiral torsion spring and Hooke's Law.

"Scientists say that magnetic north, which for two centuries has been in the icy wilderness of Canada, is currently relocating towards Russia at a rate of about 40 miles a year. The speed of its movement has increased by a third in the past decade, prompting speculation that the field could be about to "flip", causing compasses to invert and point south rather than north, something that happens between three and seven times every million years."

I'm not sure how accurate that would be, considering the rate at which the Earth's poles are moving

Nah

Sat May 28 2011, 06:38PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

It appears that you have not thought about my opinion. Please do tell if you think I'm wrong or not, as I am always open to change.

Steve Conner

Sat May 28 2011, 08:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Nah, every really good electronic workshop should have both a digital and an analog meter. The analog one can give a reading in conditions that would fool the digital one, and vice versa. For instance, I wouldn't trust a DMM around a tube radio transmitter or a Tesla coil.

And in Britain the analog one has to be an old Avo Model 8. Your side of the pond, it would be a Simpson or Triplett.

The other Avo models had much less sensitive movements. The Model 8 was the expensive, delicate one for working on tube electronics and so on. It was such an industry standard that many old British schematics simply say "Voltages measured with Avo Model 8". It also has the handy 2500V range, more than any DMM I'm aware of.

Maplin still sell the weird 15V battery. RS sell the test leads, and our technicians at work still have a Model 8 that looks like it gets used.

I had one, but I dropped it and ruined the movement.

Proud Mary

Sat May 28 2011, 11:11PM

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

Nah wrote ...

I would suggest just buying a good used 70's Fluke.

While the meter you have is great, it isn't ever going to be too accurate. (Stupid voltage drop) A good hamfest $50-75 fluke will last forever and have more than a 20k per volt impendence. (Correct me if I'm wrong, I'm just using the standard for meters) The chronic display problem is solved by taking apart the display and cleaning the contacts with alcohol. Mary, what is your opinion on this?

I'm afraid I've not been to a hamfest, and so couldn't comment on what meters they might have for sale there.

In general, I agree with Steve McConner's comments above, to which I'll add that DMM's create an illusion of super accuracy - but are all those sig figs in the digital readout really as significant as they appear to be?

How many real world thermionic valve circuits require voltages set to within Â±1% ? Very few indeed - you will recall that the standard resistance tolerance in the Thermionic Age when the Avo was queen was Â±20%, and 5% - the gold band - was regarded as an exotic, costly rarity only to be used in critical applications like gain setting and oscillators.

An analogue differential voltmeter has a theoretically infinite DC input impedance, since it draws no current at null. Just the job for measuring contact potentials and other such stuff. That's what I use mine for.

So it's horses for courses, laddie, knowing the right tool for the job, knowing the strengths and weaknesses of different measuring systems, and knowing how to interpret the results you get at the end of it.

Ash Small

Sun May 29 2011, 10:35AM

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

Nah wrote ...

I would suggest just buying a good used 70's Fluke.

I agree that buying a second hand Fluke is an excellent idea if you require that accuracy, but, as has been pointed out, DMM's have their drawbacks (interference, etc)

While my DMM is relatively cheap compared to a new Fluke (I paid Â£35 for it ten years ago new), I don't trust it, especially on the capacitance and transistor gain ranges, also, as PM pointed out, most of the 'sig figs' serve no purpose on any range.

I bought one of the old yellow Megger meters secondhand at the same time (I believe the GPO, or British Telecom as it became, used them), but I'm not sure if I still have it (a lot of my stuff went 'AWOL' a few years ago, and I still have other stuff in storage).

The purpose of this thread is twofold, as I've said elsewhere, first, to see if I can build a cheap but 'usable' meter, and secondly, to see how accurate a meter I can build using neodymium magnets, while keeping the size reasonable. I do appreciate though, that there will be some voltage drop across the coil.

I'd also like a meter that I know I can fix if I destroy it while working on other HV projects. This is probably the best justification I can give for continuing with this project.

Steve Conner

Sun May 29 2011, 11:18AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, you won't destroy the Avo unless you accidentally drop it down a flight of stairs. Electrical overload just pops the cutout button. It actually pops out on a spring IIRC, it's fun to watch.

You can destroy one electrically, but you really have to try. I've heard an urban legend that involved measuring a 750V third rail with the meter accidentally left on the current range. The innards were vaporised leaving an empty Bakelite enclosure. The usual voltage-checking device used by railway electricians was three 240V light bulbs in series.

Curse you and Nicko, you've given me GAS to own another Avo meter or three.

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