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David Ashton
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Re: AC meters
David Ashton   8/25/2014 1:22:52 AM
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@traneus ... " it dissipates 4 watts at fullscale deflection"

Sounds about right.  We use cheap moving iron meters for battery monitors in our radio sites.  The voltmeters are 15V fsd and run around 12-14V all the time.  The bare meter sensitivity is around 2-3V I think, so they have a high wattage multiplier resistor built in.   After a few years of service the plastic faceplates go brown because of being in proximity to the hot resistor for so long.  BUT I've never yet seen one go wrong!

PS - keep your meter under lock and key, Max'll be after it for his projects!!

traneus
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Re: AC meters
traneus   8/24/2014 9:03:00 PM
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David Ashton, you are correct.

I have a moving-magnet center-zero DC meter made probably before 1920, likely before 1900. The meter has three terminals connected to two windings (one terminal is common to both windings): a low-resistance, few-turns winding for current measurement, and a high-resistance, many-turns winding for voltage measurement. The meter is so insensitive that it dissipates 4 watts at fullscale deflection! The voltage winding measures 30 ohms, and gives fullscale deflection at 11 volts DC. Perusing an electrical-equipment textbook copyright 1891, showed this was reasonable at that time.

JGrubbs
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Re: New faceplates for analog meters
JGrubbs   8/24/2014 8:38:35 PM
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Max: If you are going to print the faceplates for the meters take a look at "Meter" and "BasicMeter" software from Tonne Software. BasicMeter is free and Meter is $35. You can download the manual from their website. 

 

 

Max The Magnificent
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Re: AC meters
Max The Magnificent   8/24/2014 10:53:22 AM
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@traneus: While you have the scale plate off, check the internal wiring of the meter. Many AC meters are actually rectifier-driver DC meters.

I'm currently sitting in Atlanta airport waighting for my flight to ESC Brasil ... but when I get back I will be taking a much closer look at what's inside thise little AC rascals :-)

Max The Magnificent
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Re: AC meters
Max The Magnificent   8/24/2014 10:51:36 AM
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@David: ...but only if the basic meter movement is of the moving-coil type.  If it is moving iron...

There's always something new to learn...

David Ashton
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Re: AC meters
David Ashton   8/24/2014 4:42:50 AM
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@traneus this is true, but only if the basic meter movement is of the moving-coil type.  If it is moving iron, forget it.  it will either have a coil of thick wire if it is an ammeter, or of thin wire and probably a multiplier resistor too if it is a voltmeter, but even if you take the multiplier resistor out you will have a fairly insensitive meter, needing a fainr bit of current for FSD.  Most AC ammeters are of the moving iron type, they are cheap to produce and rugged.

traneus
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Re: AC meters
traneus   8/23/2014 11:06:32 PM
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Max, don't give up on those AC meters. Most AC meters will read DC, though may be nonlinear. near zero. For your custom-scale purposes, that might be OK.

While you have the scale plate off, check the internal wiring of the meter. Many AC meters are actually rectifier-driver DC meters.

High-voltage meters usually have external or internal "multiplier" resistors in series with a low-voltage low-current movement. Many high-current meters have low-resistance current shunts in parallel with low-voltage, low-current movements.

Max The Magnificent
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Re: faceplates
Max The Magnificent   8/23/2014 1:23:02 PM
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@David: Big subject - I feel a blog coming on :-)

I await in dread anticipation LOL

David Ashton
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faceplates
David Ashton   8/22/2014 7:48:13 PM
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Strange how these threads get onto all sorts of extraneous topics (including food, as MHRackin pointed out on another blog).  Not that I am complaining, apart from the fact that I have better (but not as entertaining) things to do with my time.

But seriously....  Max 30 years ago in Rhodesia I got a job in an Electronics Lab.  The guy I took over from was just finishing a transmission measuring set project and he had some large square meters for them.  He got a firm to produce for him some plastic sticky labels for the new faces.  If he could do that 30 years ago in the backwater of Rhodesia, I'm sure someone could do that near you.  Yes, you could just buy laser labels, but paper tends to discolor fairly quickly and shows the dirt more.  but then you could get a can of photo-protect spray and try that to protect them a bit better.

As MHRackin points out, you can turn the faceplates over - then you don't have to worry about the old markings coming thru if you get them screen-printed.  But is this worth it for a couple of meters - I'd imagine the costs of preparing the screens would be a bit?

AC meters - As MHRackin also points out, (he's a clever guy I must say) you need to look at the inside of a meter before abandoning it. Moving iron meters are generally produced to a range (eg 5 amps) and there is not much you can do about it, but if you find a moving coil movement (and I am not being rude, that's what they are called) inside, then take it apart further and look for shunts or muliplier resistors.  Once you get at the bare meter - the coil connections - you can mess around and find out the FSD and the meter resistance and set about getting it to do what you want.

Big subject - I feel a blog coming on :-)

Clive
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Re: Faceplate re-calibration
Clive "Max" Maxfield   8/22/2014 6:19:31 PM
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@Steve: I'm not worthy!

That's the attitude! LOL

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