Author Topic: DIY Ampermeter  (Read 7717 times)

Offline DavidA

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DIY Ampermeter
« on: December 26, 2010, 05:59:35 PM »
How about this 'rugged' approach to meter construction.

50 Micro amp to 10 amp conversion. :zap:

Dave

Offline one_rod

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Re: DIY Ampermeter
« Reply #1 on: December 29, 2010, 05:32:50 PM »
Can I ask for a little explanation here?

I'm guessing that the copper windings are acting as a shunt.
If so how do you get away with using an ammeter as the indicator. Should it not be a microvoltmeter?

At work we use shunted DC ammeters that read up to 10,000 amps. Although the meters are scaled in kA they are actually reading the voltage generated across the shunt.

Or (quite likely) am I just missing something simple.  :scratch:

Second and fairly obvious question: The apparatus next to the ammeter. Parallel, separated plates, insulated spacers and a coffee jar lid.
Home made wet cell?   Z.F.M.?    Electoplating set up?

Come on, don't tease.


one_rod.
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Offline picclock

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Re: DIY Ampermeter
« Reply #2 on: December 30, 2010, 05:20:26 AM »
@ one_rod
Ammeters are just coils of wire wound on a spring loaded bobbin placed in a magnetic field. Just cos it says ammeter doesn't make it any different to a voltmeter.

The wire coil has resistance, and if you supply the correct voltage it will behave as a voltmeter ~ for low current designs typically around 50mV will give full scale deflection - so a 50mV meter. So now its a 50 uA ammeter and a 50mv voltmeter. If you want to get more technical, when measuring current the meter drops some voltage, this is called the voltage burden of the meter. If you put a resister in series with the meter you would increase the voltage burden for measuring current, or you could start calling it a voltmeter  :smart:

However, IMHO, best way for making a shunt is to use studding. Connect the meter via wire and solder tags with nuts to a section of the studding. Connect current source to ends of studding. Adjust separation distance of tags until calibration is achieved.  This gives two chunky screw on terminals at the ends and simple calibration. As a guess I'd say M3 studding for 10A. When done reduce length of studding to separation distance + 10% for future calibration. This will give minimum voltage burden.

 :offtopic:  Really impressed with your welded Halloween sculpture, very very impressive - I think you may have missed your vocation  :med:

Best Regards

picclock



 
Engaged in the art of turning large pieces of useful material into ever smaller pieces of (s)crap. (Ferndown, Dorset)

Rob.Wilson

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Re: DIY Ampermeter
« Reply #3 on: December 30, 2010, 07:48:49 AM »
Whats It for David  :scratch:   


Rob  :D

Offline DavidA

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Re: DIY Ampermeter
« Reply #4 on: December 30, 2010, 07:58:48 AM »
One_rod,

As picclock says,  All analogue voltmeters only really measure current.  So they are actually ampmeters with the volt markings simply a convenience;  saves you calculating voltage from Ohm's law.

I made up this 'device' as part of a 10 Amp supply circuit to run my experimental HHO cell. The thing on the right is the plate assembly used in the test. Fits in a coffee jar.  To calibrate the meter I placed a normal multimeter (on the 10 Amp range) in series, increased the load until I got 5 Amp on the multimeter then adjusted the tap position on the coil to give me half scale deflection on the Micro ampmeter..  It does it's job. Also allows me to sample the current for use in computer data logging.
The voltage drop with bare copper wire is very small so the coil doesn't even get warm.

Dave.

Rob,
Does that help ?
« Last Edit: December 30, 2010, 08:03:11 AM by DavidA »

Offline one_rod

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Re: DIY Ampermeter
« Reply #5 on: December 30, 2010, 06:42:24 PM »
Thanks for the explanation chaps.

As I suspected;  "missing something simple"  :loco:

We buy a lot of ammeters and voltmeters at work. Never thought of them as being the same equipment with a different label. 




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Offline latheman

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Re: DIY Ampermeter
« Reply #6 on: January 19, 2011, 03:18:17 PM »
Never thought of them as being the same equipment with a different label.


er, not quite. Moving coil meters, to give them their correct name, all work on current in that the movement is proportional to the current flowing through the device. (Actually, the current induces a magnetic field in the moving coil which acts as a limited movement motor.) Anyway, because the coil has resistance, it follows that for any current flow, there will be a voltage developed across the coil: typically a coil may measure 2000 ohms, and have a maximum current capacity of 50 micro amps, hence the maximum voltage will be 0.1v for a full scale deflection. Therefore, provided these maxima are not exceeded, the meter will read both current and voltage simultaneously with the scale being written to suit. Usually, for a meter to read voltage, they are specified as being so many ohms per volt, eg for this example, 20 000 ohms per volt and this does give an easy way to calculate the resistance of the particular range being used.

In order to read current in excess of the, eg 50 micro amps, it is usual to place a low value shunt in parallel with the meter. If therefore it was required to read 1amp, the meter would take 50 microamp whilst the shunt would take 1 amp less the 50 microamp, hence the value of the shunt would be 0.099995 ohms, a lot less than that of the meter. Note that the voltage across the combination would remain the same, eg 0.1V which might, or might not be significant.

In order to read voltage in excess of, eg 0.1v, it is necessary to use series resistors to limit the the current flowing through the measuring circuit, so for a, eg 10v range, the total resistance would be 200 000 ohms, of which 2000 ohms would be in the meter with 198 000 ohms in the external circuit. If you wanted to measure say 1v, then the series resistance becomes 18 000 ohms. Note that the maximum current through the combination remains 50 micro amp which might be significant in a high impedance circuit.

In brief then, to measure amps, you use a parallel low value resistance, whilst to measure volts you use a series high value resistor.

Hope this helps,

Peter G. Shaw