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Help to make an old analogue factory 500 amp meter become a sound meter

DeanO1000

Sep 19, 2016
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Hi, I have an old factory powerhouse Amp meter which measured max 500 Amps, and would like to upcycle it into a sound meter, so the needle twitches in reaction with decibel strength. I have placed a 9v battery across the terminals which works the needle. It is probably possible to do. Any ideas please? Best regards.
 

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Minder

Apr 24, 2015
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Unfortunately It appears to be a moving iron meter, otherwise you would have been able to convert it to a high impedance voltmeter, I think you will find the impedance to low to be useful in the role you want it in.
M.
 

DeanO1000

Sep 19, 2016
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Thanks for your input M. Not sure what you mean about iron meter. It is basically a wound coil around a magnet. I removed some of the coil to increase the sensitivity of the needle which worked. Can a sound sensitive circuit have an output of 9v plus to make the needle move? As you may be aware by this thread, I am no electronics expert! Thanks again.
 

Minder

Apr 24, 2015
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If it is moving coil there should be a number on the dial somewhere that states mA/volt.
If so you can convert to voltmeter, which is what you need by the look of it.
This meter was originally designed for a low resistance shunt in parallel.
Actually I enlarged it and it does say Moving Iron.
You would need to find out the basic sensitivity. (Ma/volt) of the meter.
M.
 
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hevans1944

Hop - AC8NS
Jun 21, 2012
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Since it IS a moving iron meter, instead of a moving coil, removing turns of wire from the coil should not increase the "sensitivity" but instead decrease the inductance of the coil, as well as decreasing the magnetic field produced when current flows through the coil, and hence the reactive force of the iron vane (attached to the needle) that responds to coil current.

It is highly doubtful that the original installation passed anything near 500 A through this meter. It was probably connected to a calibrated shunt whose voltage drop was then applied as excitation to the meter. Or it may have been connected to a current transformer. In either case, the non-linearity at either end of the scale is indicative of a meter designed to measure AC. The iron vane may not respond to higher audio frequencies, but it should be okay for the lower bass frequencies up to 100 Hz or so.

So, go ahead and "experiment" with your 9 V battery and find out how much voltage and current you need to deflect the needle as far as you want for the effect you are seeking. Then buy or build an audio amplifier capable of producing that much voltage and current. If that is a problem (getting sufficient voltage) you can connect a 6.3 VAC filament transformer "backwards" to the amplifier output, driving the 6.3 VAC secondary winding with the audio amplifier and driving the meter from the primary winding. The meter won't require much current, so the smallest filament transformer you can buy or salvage should be fine.

This should make a nice steam punk "hi-fi" installation. I am envious.
 

davenn

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Sep 5, 2009
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It is highly doubtful that the original installation passed anything near 500 A through this meter. It was probably connected to a calibrated shunt whose voltage drop was then applied as excitation to the meter.

THAT ... is the important statement
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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THAT ... is the important statement
Well, It is obvious to those of us "skilled in the art," but the OP in post #3 admitted having no such skill, so I thought I would mention it. I would like to know what he meant by the statement "I removed some of the coil to increase the sensitivity of the needle which worked." What exactly did he do? What was the "sensitivity" before and after and how was this measured? I am all for experimenting and measuring to determine the operating parameters of strange equipment, but will-nilly poking around with a 9 V battery and making modifications is not the way I would do it. Just sayin'... Good thing this wasn't a 50 μA D'Arsonval meter movement or it would be history by now. Iron vanes are somewhat more tolerant of overloads if you don't fry the coil.:eek:
 

Audioguru

Sep 24, 2016
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The very old meter is probably too slow to show audio levels and it will probably overshoot then bounce.
 
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