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Moving Coil Meter

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Patrick

Sat May 14 2011, 09:55PM

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

Sulaiman wrote ...

Seriously, don't start this project!

I think theve got their heart set on this matter, many of us think this is an unessacary waste of time and effort which they could spend on their other stuff.

But its not like I havent felt similarly with my HV probe obsession, though if I could by them for $6 instead of $2400 I would just buy the dam things.

magnet18

Sun May 15 2011, 04:10AM

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

Posts: 624

I'm honestly probably going to end up just shelling out the cash for a bunch of these things, but it's still fun to play around with building one.

Patrick

Sun May 15 2011, 04:33AM

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

magnet18 wrote ...

I'm honestly probably going to end up just shelling out the cash for a bunch of these things, but it's still fun to play around with building one.

Just buy the analog meters from ebay, radio shack or whatever source, then use a color printer to print out a scale you like with the graphics and scale you want. you can make them look cool.

Ash Small

Sun May 15 2011, 07:13AM

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

That's very good advice, Sulaiman. When I first suggested Magnet18 should consider making some, I had in mind a fairly crude moving magnet design, which was no where near 50 uA FSD.

This was after he'd said that ~$9 each was way too much for him.

I'm viewing this as 'just a bit of fun'. I don't even need one! .....And I still have my other projects.

However, I've accepted the challenge to try and construct a 50 uA FSD meter, and I'm assuming that a moving coil meter is probably the best way to accomplish this.

I did strip, repair and rebuild a cheap meter when I was a kid, as I mentioned in the other thread. I also suggested that this would be a good starting point for Magnet18.

I don't expect to make much progress until I obtain some decent magnets. I have some somewhere, but I think they are 200 miles away and they are not neodymium, but another rare earth type which are not as strong but will withstand higher temperatures. They are also 'rejects' that I obtained as samples.

Working with the very fine guage wire is a bit of a challenge, as is winding the spring and making the movement, which I plan to do only using basic hand tools.

Even a crude moving magnet design would be quite useful for amateur HV projects where 50 uA FSD isn't required and where one wouldn't wish to risk destroying a more expensive meter. At the end of the day if you build it you can repair it!

Anyway, I'll run some more tests when I locate some better magnets and post results. As Magnet18 said, 'It's fun to play around with building one'.

Sulaiman

Sun May 15 2011, 08:45AM

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

I have some of these

Glue a thumb-tack to each face for a quick bearing
us a second one for zero/restoring force ... almost done!
Get the core & winding from a small ac shaded-pole ac motor (microwave oven fan etc.)
and there's not much left to do!

I still think that a tangent galvanometer is a better hobby project
with a magnetic compass and a Helmholtz coil you can
- measure the Earth's magnetic field at your location (requires a milli-amp meter)
- easily calibrate your meter
- have a calibrated magnetic field generator
- replicate the earliest type of laboratory-grade current meter.
.........

EDIT: if you construct the two coils for the Helmholtz coil well you will also have;
- a reference inductor
- a reference variable inductor
- two coils suitable for resonance and coupling experiments
- a 'wireless-power' experimentation setup
- two really 'neat' bracelets (uber-geek-cool energy-harvesting?)
- a loop-antenna for radio reception
.........................

With a small mirror on the movement and a laser pointer you could have a 'cute' meter
.
.
.
What more could one desire?

Proud Mary

Sun May 15 2011, 09:07AM

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

Sooner or later, most amateur constructors come to realize that it is usually cheaper to buy something ready made than to make it yourself. We don't enjoy the economies of scale of the mass manufacturer, and can not spread the cost of highly specialised tools across thousands of production items.

We get the best value for our money when we design and build things not readily available in the marketplace.

Steve Conner

Sun May 15 2011, 09:41AM

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

Yes, but it's fun and educational!

Another interesting type of meter is the electrostatic voltmeter. This might be easier to make and more useful to a high voltage hobbyist than the electrodynamic kind. It is basically a capacitor with a spring balance to "weigh" the electrostatic force between the plates. It works equally well on AC and DC.

Proud Mary

Sun May 15 2011, 10:53AM

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

Steve McConner wrote ...

Yes, but it's fun and educational!

I had a paragraph on satisfaction which I cut out before posting for fear of over-stimlulating the starved and suggestible.

Steve McConner wrote ...

Another interesting type of meter is the electrostatic voltmeter. This might be easier to make and more useful to a high voltage hobbyist than the electrodynamic kind. It is basically a capacitor with a spring balance to "weigh" the electrostatic force between the plates. It works equally well on AC and DC.

I have a wonderful old 18kV electrostatic voltmeter in a mahogany case, and use it where even a few tens of Î¼A would pull down the voltage to be measured. It's hard to beat for high impedance DC measurements like GM and prop tube supplies, but you must have room for the 10" x 10" x 10" wooden case.

Field mills are sometimes used to estimate HV DC sources such as VdGs and C&W, but don't seem to have been touched upon in 4HV. There's a good DIY field mill design here:

See also:

Tant, P. Bolsens, B. Sels, T. Van Dommelen, D. Driesen, J. Belmans, R. Design and Application of a Field Mill as a High-Voltage DC Meter
Instrumentation and Measurement, IEEE Transactions on Vol 56 Issue: 4 Aug. 2007 pp 1459 - 1464

Abstract

Field mill instruments are often employed for the measurement of electric fields, electric charges, voltage potentials, and atmospheric effects. This paper discusses in more detail the application of a field mill for measuring dc voltages in high-voltage laboratories. Some specific problems, which arise when voltages in the kilovolt or megavolt range have to be measured, are emphasized. Safety, signal transmission, and electrode design are dealt with. Field meter theory is summarized and generalized in the function of this application. The design of a practical field-mill-based electrostatic voltmeter is described along with some useful enhancements. Finally, test results of the voltmeter are presented.

Ash Small

Tue May 24 2011, 04:13PM

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

Well, I've some spare cash now, and before deciding on which magnets to order, thought I'd do a bit of research.

While I've seen the arm that you mention on meters before, I can't find any mention of it in any of the articles I've looked at.

Patrick wrote ...

.
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.

The Weston revision of the d'Arasonval meter is what I'm basing mine on. (What we generally call a 'moving coil meter')

According to Wikipedea, a typical meter has 100 uA FSD, with a voltage drop of 50 mV at full current.

Others here have stated 50 uA. Would anyone wish to add anything to this, or to elaborate a bit?

I've read that the coils are usually wound on an aluminium former, as this acts as a damper, as it is highly conductive. Is anyone able to add to this? (that is all I found on the subject)

Also, some articles mention an iron core, to concentrate the magnetic field. Others mention an iron tube. Is anyone able to add to that?

I'll be ordering some magnets this week, and running some more tests when they arrive, but If anyone can advise on the above points I'd appreciate it. Thanks.

Proud Mary

Tue May 24 2011, 06:09PM

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

Ash Small wrote ...

According to Wikipedea, a typical meter has 100 uA FSD, with a voltage drop of 50 mV at full current.

Others here have stated 50 uA. Would anyone wish to add anything to this, or to elaborate a bit?

A sensitivity of 20 kÎ© per volt has been the industrial standard for good quality moving coil meters since the 1940s.

One only has to think of the legendary AVO Model 8, that great workhorse of the Thermionic Age:

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