Re: Moving Coil Meter
Patrick, Fri May 13 2011, 08:38PM

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

Re: Moving Coil Meter
magnet18, Fri May 13 2011, 11:42PM

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...

Re: Moving Coil Meter
Ash Small, Sat May 14 2011, 12:25AM


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.

Re: Moving Coil Meter
magnet18, Sat May 14 2011, 01:40PM

chapter 8 might be of some amount of use.

Re: Moving Coil Meter
Proud Mary, Sat May 14 2011, 01:54PM

20 kΩ per volt - i.e. 50 μA FSD - is the established standard for moving coil movements.

Re: Moving Coil Meter
Ash Small, Sat May 14 2011, 05:42PM


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.

Re: Moving Coil Meter
magnet18, Sat May 14 2011, 09:26PM






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.

Re: Moving Coil Meter
Patrick, Sat May 14 2011, 09:30PM

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.

Re: Moving Coil Meter
Sulaiman, Sat May 14 2011, 09:40PM

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.

Re: Moving Coil Meter
Patrick, Sat May 14 2011, 09:55PM

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.

Re: Moving Coil Meter
magnet18, Sun May 15 2011, 04:10AM

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.

Re: Moving Coil Meter
Patrick, Sun May 15 2011, 04:33AM

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.

Re: Moving Coil Meter
Ash Small, Sun May 15 2011, 07:13AM

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'.

Re: Moving Coil Meter
Sulaiman, Sun May 15 2011, 08:45AM

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?

Re: Moving Coil Meter
Proud Mary, Sun May 15 2011, 09:07AM

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.

Re: Moving Coil Meter
Steve Conner, Sun May 15 2011, 09:41AM

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.

Re: Moving Coil Meter
Proud Mary, Sun May 15 2011, 10:53AM


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


I have a wonderful old 18kV electrostatic voltmeter in a mahogany case, and use it where even a few tens of μA 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.

Re: Moving Coil Meter
Ash Small, Tue May 24 2011, 04:13PM

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.



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.

Re: Moving Coil Meter
Proud Mary, Tue May 24 2011, 06:09PM


A sensitivity of 20 kΩ 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:

Re: Moving Coil Meter
Patrick, Wed May 25 2011, 09:56AM

i may not have said this all quite right let me get another post to you regarding this matter.

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 02:54PM

I scored this for a tenner on Ebay last night, and collected it this morning. It was two miles away.


AVO Model 8 Mk III


I'll open it up and have a look at the movement later. I'm particularly interested in looking at the 'concentrator'.

It's giving some strange readings on the Ohm scale, but I'll try some new batteries and see if that sorts it out.

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 04:17PM


Coo, that's a real beauty, Ash, and well cared-for by the looks of it.

You should put a fresh 15V battery in it, before drawing any conclusions about the resistance ranges.

All being well with the new battery, I would resist the temptation to open it and look inside. It has survived all those years without your intervention!

Re: Moving Coil Meter
Nicko, Thu May 26 2011, 04:45PM

Congratulations. Seconded to what PM said (don't mess too much with it - they are subtle and quick to anger). I don't know why but I've fallen in love with AVOs recently. Got one at a boot fair in Tunbridge Wells about a month ago for 13 quid complete with pristine leather case (Its a Model 47 or a Model 40, not sure!). Leads a bit crummy, but otherwise OK. Seems mostly ok - movement has been replaced. Now got 4 of them!



The place to go for info on AVOs and how to look after/repair them is the UK Vintage Radio Repair and Restoration Discussion Forum on "Specific Vintage Equipment" .

They've been extremely helpful with my questions and also maintain a register of 1000's of these things' serial numbers. They will also tell you when yours was made.

HTH,

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 04:50PM


This one has been converted to take an A23 12V battery in series with an AAA 1.5V battery. This gives 13.5V (according to my limited maths)

Is this sufficient, or should I add another AAA to bring it up to 15V?








Also, the battery cover is missing, but I'll make up an aluminium one, I think. (If it works OK once I've replaced the batteries.


Maybe you are right, opening it up could be considered as 'cheating', but I've not been able to find any detail about the iron magnetic field concentrator online. (Some articles refer to an iron core or a piece of iron tubing inside the coil)

I downloaded the AVO service manual, and that seems to state that the concentrator remains in the scaleplate assembly when the coil is removed.

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 05:15PM


Oh dearie me - we can only hope that the person responsible for such an awful bodge job on the batteries hasn't been tinkering elsewhere.

In the normal course of events, one can compensate for declining battery voltage by means of set zero, but I would advise you to use the full 15 V for re-commissioning so you can test the set zero across its whole range, and perhaps identify any tinkering or other short-coming.

RESISTANCE
Before testing, the pointer should be adjusted to zero in the following sequence :
(1) Set left-hand switch at “ RESISTANCE.”
(2) Join the leads together.
(3) On the “Ω” range, adjust to zero by means of the knob marked “ ZERO Ω.”
(4) On the “Ω ÷ 100” range, adjust to zero by means of the knob marked “ ZERO Ω ÷ 100.”
(5) On the “Ω x 100” range, adjust to zero by means of the knob marked “ ZERO Ω x 100”
To test a resistance, set the right-hand switch at the range required, the leads being connected across the unknown component. Resistance is read directly on the “Ω” range, but indications should be divided or multiplied by 100 on the other two ranges.


It is often possible to identify past bodging and tinkering by examining the fastenings with a jeweller's loup, as the bodger will commonly use screwdrivers or sockets of the wrong size, which leave their own characteristic markings behind them.

Re: Moving Coil Meter
Sulaiman, Thu May 26 2011, 05:42PM

As Small'
It's amazing how many seemingly unimportant details turn out to be very clever bits of engineering
- often too late to be fully appreciated. (oops !)

If in good condition it's a useful meter

if minor repairs (resistors etc.) can get it to a useful state then it would be a nice little project.

If there is bad corrosion or the meter movement (magnet, pivots, coil, springs, pointer et.) needs repair then it can be a significant project, hours of restoration calibrated vs. a $5 dmm.
(that I've previously tested the calibration of common ranges to better than 1%)

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 07:13PM


Well, I've taken a couple of A23's apart and made one 10 cell battery (reads 15.6V on my DMM). I can't get it to zero on the Ohms/100 range(lowest it will go is 50 (equal to 0.5V), so I assume it was adjusted inside to allow zeroing on the 13.5V it was operating on before.

I found the instructions to adjust this earlier, but can't seem to find them now.(I'm sure I will if I keep looking) but this means I'll have to open it up anyway. (The internal adjustment should be set so that the external adjusters are midway when it is zeroed, to allow for adjustment in either direction)

I was on the UK Vintage Radio Repair and Restoration Discussion Forum earlier, but I'm not sure if it was there or somewhere else.

EDIT: The 15V battery is only used by the Ohms X 100 range (according to the circuit diagram), so the above fault isn't due to the 15V battery

EDIT EDIT: It's a resistance problem. I shorted the terminals using some silver plated wire and it zeros. I'll try cleaning the terminals, that should improve things further. Time for some new leads I think.

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 08:55PM

Does it display believable readings in voltmeter mode?

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 09:29PM


When connected to a 12V battery charger with a 21 watt bulb as ballast, it reads 8.5V, my DMM reads 9.17 (there could be a voltage drop due to high resistance in the terminals)

The same circuit gives 1.2 amps on both the AVO and DMM.

I'll set up a better test with a car battery in the morning.

(It's only a cheap 4 amp battery charger.)

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 09:45PM


I'd suggest testing the voltmeter function against ordinary AA 1.5 V cells, rather than introduce the unknowns of a car battery charger into the mix.

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 10:07PM


using a brand new 1.5V duracell LR20 and a 3Ohm nominal resistor (3.3 according to my DMM) it reads 0.5A and 1.45V (but I've not fixed the 'suspected' problem of resistance at the terminals.)

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 10:27PM


That sounds promising, and suggests you don't have a 'sticky' meter movement, an often serious fault. Once you've got your bad connections sorted out, you could do what you can to test the meter on every switch position, as multi-pole rotary switches are very vulnerable to corrosion, man-handling, and the effects of time.

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 10:47PM


I only bought it to strip down and look at the concentrator. It's already cost me more in batteries than I paid for it, and I haven't replaced the leads yet (apparently RS still sell them)

EDIT: I've got the ends of the leads and the terminals soaking in Sainsbury's cola, not as good as 'the real thing', but it still contains phosphoric acid.)

Re: Moving Coil Meter
Proud Mary, Thu May 26 2011, 10:56PM


Invest your time and efforts into reconditioning an Avo and you'll have a useful piece of kit to help you on your way for years to come.

Re: Moving Coil Meter
Ash Small, Thu May 26 2011, 11:00PM


But I've accepted a challenge to build a 25uA FSD movement. I can't back down now!

EDIT: BTW, apparently the movement in an AVO is 35uA FSD, and the lowest range is 50uA, at least it is on the Mk III

Re: Moving Coil Meter
Nah, Fri May 27 2011, 12:11AM

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?

PS Try using Navel Jelly, it is basicly gelled pho. acid and pink too! It is great as a rust remover and dosn't have the temptation to be eaten.

Re: Moving Coil Meter
Proud Mary, Fri May 27 2011, 12:30AM


Do you not perhaps mean 37.5 μA? In any event, the meter is shunted so as to show 50 μA FSD.

Re: Moving Coil Meter
Ash Small, Fri May 27 2011, 12:54AM


But I've accepted a challenge to build a 25uA FSD movement. buying one isn't an option.

I do have some 'pure' phosphoric, but it's 200 miles away. Coke is the next best thing. (after Naval jelly)

PM, you are correct, as usual, 37.5uA, not 35uA. (I was going to post in the morning (in the cold light of sobriety) that I have the utmost respect for yours, and Sulaiman's and Nicko's opinions (Wait til I post details of the other stuff I picked up when I collected the AVO, I'm sure you'll have plenty of advice).

This thread is, as I said before, 'just a bit of fun', but I don't really want to strip a perfectly good meter if I can find the relevant info elsewhere.

Re: Moving Coil Meter
Proud Mary, Fri May 27 2011, 01:11AM


Tesco sell a phosphoric acid based stainless steel cleaner spray called Viakal, and I often use it for pickling copper and brass to expose a fresh surface, which it does very well.

Re: Moving Coil Meter
Ash Small, Fri May 27 2011, 10:52AM

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.

Re: Moving Coil Meter
Sulaiman, Fri May 27 2011, 11:33AM

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?

Re: Moving Coil Meter
Ash Small, Fri May 27 2011, 12:36PM

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.


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.

Re: Moving Coil Meter
Proud Mary, Fri May 27 2011, 12:49PM

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.

Re: Moving Coil Meter
Ash Small, Fri May 27 2011, 02:33PM




"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

Re: Moving Coil Meter
Nah, Sat May 28 2011, 06:38PM

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.

Re: Moving Coil Meter
Steve Conner, Sat May 28 2011, 08:56PM

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.

Re: Moving Coil Meter
Proud Mary, Sat May 28 2011, 11:11PM


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 ±1% ? Very few indeed - you will recall that the standard resistance tolerance in the Thermionic Age when the Avo was queen was ±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.

Re: Moving Coil Meter
Ash Small, Sun May 29 2011, 10:35AM


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 £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.

Re: Moving Coil Meter
Steve Conner, Sun May 29 2011, 11:18AM

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.

Re: Moving Coil Meter
Nah, Sun May 29 2011, 01:48PM

All right, how about a VTVM?

With a 11 + Mohm impendence and impervious to most EMR, lets see how you can beat my mighty HP with your wimpy baklite meter!

Re: Moving Coil Meter
Conundrum, Sun May 29 2011, 02:10PM

I have some nice antique moving coil meters here, anyone interested?
Mainly voltmeters and ammeters, some work and some do not.
Possibly fixable, might just need wires soldering back on.

-A

Re: Moving Coil Meter
Steve Conner, Sun May 29 2011, 02:42PM

That's easy Nah, say I wanted to measure some current. Oh dear, VTVMs don't have current ranges.

Or I wanted to go test my car battery. Damn, there is no outlet to plug the VTVM in.

For some reason, "Valve voltmeters" never caught on in the UK, they were mainly an American thing. As far as I'm concerned they have the same advantages and disadvantages as a digital meter. I've got an old ITT Metrix multimeter that has both a digital display and an analog needle, but it somehow manages to combine the worst characteristics of both instruments.

Re: Moving Coil Meter
Ash Small, Sun May 29 2011, 03:06PM


That's pretty interesting. I might pick up a 12AU7/ECC82.

There is a circuit diagram here:

Re: Moving Coil Meter
Nah, Sun May 29 2011, 03:15PM

How about this...

I need to measure .5 ohms exactly. Oh dear, my 260 and my DVM can't measure low resistances! I want to read 20 kvolts at 100 microamps. Oh dear, my 260 and my DVM can't measure that high, and will load the output! What do I do? I use my HP VTVM with a 30kv high voltage probe! When I connect the leads of my fluke together what do I get? 0.01 ohms! With my handy RCA, I can use the ohms adjust to get 0 ohms! Also, what's this? MY meter is picking up EMF and showing 0.01 ac volts? With my EICO, I can use the volts adjust to get 0.00 volts!

Also, lets say that I buy a $5 VTVM and a $5 DVM. Oh dear, I can't find a VOM fr $5! Then, lets put them on the 10 volt range and input 200 volts. VTVM- nothing DVM- BOOM, fizz, fizz

Right back atch ya!

Re: Moving Coil Meter
Proud Mary, Sun May 29 2011, 03:21PM


Not wanting to put your nose out of joint, squire, but here is yer actual Avo Electronic Testmeter, aka the Avo valve voltmeter, full of meaty goodness as they say in dog food commercials:





There was also a later Electronic Avometer type EA113 which had a high Zin - compared to your analogue 20 kΩ/volt, that is - but which had little of the Herculean industrial design of the Electronic Testmeter, being stylistically similar to the rather nasty Marconi Instruments TF2600 Sensitive Valve Voltmeter:

Re: Moving Coil Meter
Steve Conner, Sun May 29 2011, 04:54PM


If you're going to build one in the 21st century, at least consider replacing the tubes with JFETs. See the circuit on page 3 here.

They recommend the 2N4340, but any JFET will be better than a tube.

Re: Moving Coil Meter
Nah, Sun May 29 2011, 10:40PM

It will be MUCH cheaper just to buy a ready built VTVM from the 50s. You can pick one up at a Hamfest for $5, or build one for $50.

What happens when you over voltage the transistors or over current them? With tubes they'll be fine for some time, transistors, BOOM.

No, I am not as old as dirt, only as old as hydrogen.

Re: Moving Coil Meter
Ash Small, Mon May 30 2011, 12:05AM

I think you have a valid point, Nah, but I'll let Steve answer. I hope Chris doesn't lock this thread, it's starting to get interesting

Re: Moving Coil Meter
Steve Conner, Mon May 30 2011, 06:59AM

Well, granted, the FETs will go boom far easier than tubes. And I guess if Avo made a VTVM, they can't be all bad.

But, VTVMs aren't as common over here, nor are hamfests for that matter. (if you're looking for one, the British name is "Radio rally".)

And, I'm thinking of the hassle of getting a really good 12AU7. The VTVM is very demanding of the tube's DC performance, and some random one pulled from a radio might give disappointing results.

For some reason, while I've managed to scavenge about 2 dozen 12AX7s, I always struggled to find the T and Us. I needed a couple of good 12AU7s recently and ended up paying nearly £20 each for JAN Philips 5814s at Watford Valve.

A 6SN7 or its Russian equivalent can also be used.

Re: Moving Coil Meter
Ash Small, Mon May 30 2011, 11:25AM

Steve, the ECC82 is the european equivalent.

Apparently it was used in virtually every valve TV ever made.



Loads on Ebay cheap. for example £6 buy it now, £3 postage for a Phillips one. others are cheaper

eg 10 for £1.20, ends tomorrow

Re: Moving Coil Meter
Proud Mary, Mon May 30 2011, 12:01PM

You can download this very comprehensive volume

Rider, J. Vacuum Tube Voltmeters, John F Rider Publishing Inc, New York, 1951, 430 pages, as a free pdf here:

Re: Moving Coil Meter
Nah, Mon May 30 2011, 02:07PM

12AT7 tubes are almost identical and can be found in any AA5 radio (you being in europe.......)

The 6SN7 is a great tube and is still cheap. I think the octal tubes are better than the 9 pins

Re: Moving Coil Meter
Steve Conner, Mon May 30 2011, 09:43PM

No they aren't, the 12AT7 has considerably higher mu, about halfway between the U and the X. I guess it would work in a valve voltmeter though.

I always forgot which was which until I realised they go in alphabetical order, the T is the ECC81, the U is the ECC82, the X is the ECC83.

The All American Deathtrap is a story for another day. They were imported to the UK to electrocute the unwary. To convert to 240V, a resistive line cord was added to drop the excess voltage, this also sets the carpet on fire and makes the death look like an accident

Re: Moving Coil Meter
Proud Mary, Mon May 30 2011, 11:19PM

In 1936-38, Osram produced a dedicated valve voltmeter triode, the A577.

The recommended circuit diagram is a splendid lesson in minimalist design:

Re: Moving Coil Meter
Ash Small, Mon May 30 2011, 11:35PM

The All American Five
Cautions, Precautions and Troubleshooting.



Wikipedia-The All American Five

Re: Moving Coil Meter
Nah, Tue May 31 2011, 12:02AM

The AA5 is the flagship of Darwin's army.

anyhow, yea, I forgot that they were different, I just remembered that I found a 12at7 in a tube tester socket for a 12ax7........

Um....... What does this have to do with Ash's plan..............?

Re: Moving Coil Meter
magnet18, Tue May 31 2011, 12:26AM


OOH, I shall keep an eye out for one of those, tubes with an everyday function are hard for me to come by.

I find the approach interesting, by varying the grid voltage they change the current on the ammeter, which can be calibrated to a voltage scale, if I read the circuit correctly.

Was the idea to make sure that there would be no current drawn by the act of measuring since the grid is operated purely on potential?

Re: Moving Coil Meter
Steve Conner, Tue May 31 2011, 06:17AM

Yes, that's exactly the idea of the VTVM in general, not just that one circuit. The grid theoretically draws no current, so you can get a perfect measurement of voltage. Of course in practice it draws a tiny bit, but in a tube designed for electrometer use it can be very small indeed. Bob Pease wrote about this, he estimated about 1nA for the 12AX7s used in the Philbrick tube op amps. He also mentions that the LMC660 can beat it easily.

Nah, if Ash needs some Alnico for his magnet, he can smelt it over the flames from an AA5. No, sorry, it is completely off topic.

Re: Moving Coil Meter
Ash Small, Tue May 31 2011, 08:57AM


It's interesting nontheless, and the discussion on suitability and availability of valves for a VTVM is very relevant.

I've never bothered learning more than the basic principle of valve technology before, but this thread (along with the 'magic eye' LCR mater I recently aquired, and another item I've not yet posted about) is motivating me to learn more about the subject.

While FET's could be used to amplify the signal for a meter, a valve is not going to fail short-circuit, so as long as the circuit is designed with the valve(s) as the 'weak link' it will be virtually indestructible (I think!). (the AA5, especially the ones with the resistive cord for 240V, is an example where this wasn't the case)

As for the subject of magnets, There was one 1" cube N38 neodymium magnet on Ebay recently that went for nearly £10. I'm after at least three, probably five, and preferably N52 (or at least N45 or N48), and preferably unplated (as I'm going to 'attempt' to shape them), if anyone can help.

And thanks to everyone who has contributed so far.

Re: Moving Coil Meter
Proud Mary, Tue May 31 2011, 11:59AM

Accessories for valve voltmeters included an RF probe, which would have a miniature diode valve at its very tip, sometimes a wire-ended EHT rectifier like EY51.





Where very low circuit loading and high frequency response were needed, UHF acorn triodes like 955 were mounted on the very tip of a probe, so there was no wire at all between the circuit under test and the meter input circuit.

Re: Moving Coil Meter
Steve Conner, Tue May 31 2011, 06:05PM

Yikes! Rare earth magnets are first cousins to lighter flints, they could make a lot of sparks, maybe even catch fire if you try to cut or grind them. I think the stuff is incredibly hard and brittle: the finished magnets are heavily nickel plated to protect them.

If you let two really big, powerful neo magnets smash together under their magnetic force, from far enough away that they can get up a good speed, their lighter-flinty properties may allow them to explode with a big flash and bang. I have read of this happening on a guitar pickup makers forum, where they are used to charge the smaller Alnico magnets in the pickups.

It could be an urban myth, but then again it might not, and a lump of burning Neodymium in the eye could really ruin your day. So careful now, as Father Ted would say.

Re: Moving Coil Meter
Sulaiman, Tue May 31 2011, 06:26PM

I've never had an interrest in thermionic valves - not much that solid state can't do better.
As for vtvm input current, in the 70's I was using commercial parametric op-amps of 10's of fA input bias current (Teledyne I think) ... no idea what's available now.

Re: Moving Coil Meter
Ash Small, Tue May 31 2011, 06:45PM

You can't cut or grind them due to the heat generated. Above 80 C and they are no longer magnets. Nickel isn't that hard (not as hard as chrome), I'm planning to try using aluminium oxide paper wrapped around a former, with plenty of water to keep things cool. (Apparently you can drill them with diamond tipped drills, but I don't want to risk overheating the magnets)

Your comments remind me of when I was working at Harwell on their syncho-cyclotron. A weld failed on what was supposed to be a stainless part, I went to get a supermagnet from my toolbox, one of their engineers had 'assumed control' by the time I got back. He insisted I give him the magnet, an unplated one, and it flew out of his hand and smashed into what turned out to be tool steel that someone had managed to weld to the stainless. The magnet smashed and shrapnel went eveywhere (I had warned him! and subsequently told him he should have let me test it! he was in a state of shock. You'd think a Harwell physicist would know about supermagnets.). It's the same magnet that I used in the first post in this thread. (I usually wrap them in insulating tape when testing steel, just in case.) (In case anyone is wondering, 300 series austenitic stainless, which is what the part was supposed to be (316) isn't magnetic)

I actually made some neodymium pickups once (well, not neodymium, but a rare earth type that withstands higher temperatures) and I could measure the current on the milli or micro-amp range on my DMM when passing a screwdriver over it. It was pretty crude, with thickish wire. I used several 1/4" diameter ones stacked together. I was planning to build an electronic ignition system at the time, but I've not got round to it yet. The principle is the same as guitar pickups.

Re: Moving Coil Meter
Nah, Tue May 31 2011, 07:26PM

A normal $5 vtvm has a impendance of 11megohms

Ash, valves or tubes short circuit all the time. I just had a shorted tube on the tube tester some time ago. The most infamous is the heater cathode short, which is kind of hard to notice, and can cause 60hz hum to leak into the audio. A full cathode to plate is rare, the most you'll get is most likely a grid to plate.

Re: Moving Coil Meter
Steve Conner, Tue May 31 2011, 07:38PM

11 meg is the standard for DMMs, too. At 11 meg, 10nA of bias current will give you an 0.11V error, that is why it's important to minimise it.

The LMC660 has a headline spec of 2fA input bias current. Bob Pease was involved in those Teledyne parametric op-amps, he has a whole chapter of a book somewhere about them. You could buy three for the price of a VW Beetle, but the contents were about the same as a transistor radio, with some real black magic that, according to Pease, even the designer didn't fully understand. Teledyne Philbrick made a huge profit on them.

The LMC660 outperforms them, but you have to wash your PCB in the dishwasher.

All of this is a long way from the average 12A*7 you might find on eBay.

Re: Moving Coil Meter
Proud Mary, Tue May 31 2011, 08:35PM


Isn't 10 MΩ more common?

Re: Moving Coil Meter
Nah, Tue May 31 2011, 09:19PM

Mary is right, 10 is the standard.

Many vtvms have better numbers than 11, I have one that goes at a dc impendance of 21 mohoms. There is more to tubes than the 12ax7. There are millions of tubes, some which are far better than a audiofool run of the mill dual triode.

vtvms using special probes can measure up to 500mhz.
A normal dvm can go up to 20khz

Re: Moving Coil Meter
magnet18, Tue May 31 2011, 09:34PM


Yes, but solid state doesn't have the same warm glow and the quaint simplicity that tubes have

Re: Moving Coil Meter
Proud Mary, Tue May 31 2011, 10:13PM

A like-for-like comparison between thermionic valves and solid state devices (SSD) is really a bit silly.

Valves were superceded because they became uneconomic. Every small town in England once had a repair shop
to service consumer electronic goods - TVs, radios, gramophones, cine projectors - because valves in
their hot confined spaces failed constantly while the parts around them slowly barbecued and
changed their value, or gave up the ghost in puffs of smoke, and pools of wax.

Thermionic technology lingers on where replacement with SSD technology is uneconomic - oven and radar magnetrons,
power klystrons, electron-impact photon sources, CRTs still in much of the world, gas switches like power thyratrons and pseudospark devices, and perhaps still in Russia some exotic 'hard' circuitry intended to function in the presence of EMP and intense ionizing radiation.

As a simple hobbyist with nothing more in mind than scientific amusement, I am not constrained like the factory owner to make a profit, and so can return to the technology of the past to see what entertainment it still might have to offer in the way of curious knowledge.

Re: Moving Coil Meter
Nah, Tue May 31 2011, 11:48PM

We all know that Mary, we just like arguing!

Re: Moving Coil Meter
magnet18, Wed Jun 01 2011, 12:22AM

Russia still uses tube in a lot of things, mainly because it's cheaper to replace a tube every now and then than a whole large item that still works otherwise.

It's a good thing the soviet union was around or we wouldn't have nearly as much old stuff to play with