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Digital Ampmeter

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trigo(at)mail.telepac.pt
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Posted: Fri Jun 22, 2007 7:56 am Post subject: Digital Ampmeter

I installed a 50 mV shunt in the AWG 6 wire coming from the alternator, to 

measure Amps "produced" by it, and I don't want to put an analog Ampmeter in 

my pannel.

Can someone please indicate me a source and a model of digital ampmeter to 

connect to my shunt.



TIA

Carlos

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nuckollsr(at)cox.net
Guest

Posted: Fri Jun 22, 2007 12:08 pm Post subject: Digital Ampmeter

At 04:48 PM 6/22/2007 +0100, you wrote:



 	  Quote: 	
		  

<trigo(at)mail.telepac.pt>



I installed a 50 mV shunt in the AWG 6 wire coming from the alternator, to 

measure Amps "produced" by it, and I don't want to put an analog Ampmeter 

in my pannel.

Can someone please indicate me a source and a model of digital ampmeter to 

connect to my shunt.



TIA

Carlos


	


   It's not a simple answer. Unlike the ordinary analog instrument,

   a digital instrument cares about how much voltage is on the shunt

   with respect to ground. The instrument is trying to deduce and display

   a voltage on the order of millivolts while shunt is riding above ground

   by about 14,000 millivolts and has a lot of 'noise' on it. The ability

   to ignore the big picture while staying accurately focused on the little

   picture is not a trivial task.



   Most manufacturers get around it by using the hall effect sensor

   which doesn't care how much voltage is on the sensed wire . . . it

   cares only about the magnetic field around the wire that results

   from the flow of the current to be measured.



   If you're dead set against an analog (pretty cheap at $50) like



   https://matronics.com/aeroelectric/Catalog/AEC/9007/9007.html



   then in order to use a digital instrument, you'll need to

   find one that can either run from your 14v bus and resolve

   a millivolt signal riding on 14v common mode or select an

   LCD instrument that runs from a 9v battery (just leave it on

   all the time. Battery lasts for months!) and doesn't worry

   about the 14v common mode thing. An example of the later

   instrument can be seen at:



http://www.mpja.com/download/12205me.pdf



   These are inexpensive but you'll note that the input

   signal must lie within plus or minus 1.0 volts of

   the instrument's ground (Com Mode Voltage).



   Now, there ARE ways that one can craft a shunt signal

   processing amplifier using a device with a very high

   common mode capability. And example of such a part is

   the Analog Devices AD628



http://www.analog.com/UploadedFiles/Data_Sheets/AD628.pdf



   Note that this puppy is designed to live in a very

   large common mode range of over plus/minus 100 volts.

   This is the type of device employed by folks to choose to

   offer digital displays driven by shunts. However, most

   digital suppliers no probably use hall-sensors and step

   over the common mode limits.



   I thought JPI or Electronics International had stand

   alone ammeters and voltmeters but a quick check of their

   websites didn't turn anything up. Perhaps someone else on

   the list will have some suggestions. It seems the trends

   are toward multifunction instrument packages that include

   volts and amps. Stand alone instruments are going to be

   increasingly difficult to find.



   The easiest thing to do is bite the bullet and

   go the stone-simple analog instrument. You can spend

   a lot of time and a lot more money getting to the

   digital solution.



   Bob . . .

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trigo(at)mail.telepac.pt
Guest

Posted: Fri Jun 22, 2007 1:47 pm Post subject: Digital Ampmeter

Bob



Thanks for your quick and complete response.



I have a couple of (electric instruments challenged guy) questions:



If I use the first digital panel meter you suggested 

(http://www.mpja.com/download/12205me.pdf)

do I simply connect power (9V and -) and the 2 wires from the shunt, and it 

will indicate Amps?



The "Full scale range" of 200mV indicated in the specifications, has it 

anything to do with the 50mV characteristic of my shunt ?



Carlos

---

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retasker(at)optonline.net
Guest

Posted: Fri Jun 22, 2007 6:40 pm Post subject: Digital Ampmeter

Since Bob has not answered yet, I will.  The 50mV shunt will drop 50mV 

at it's rated amperage.  You didn't specify what one you have, but let's 

assume that it is a 50mV shunt at 50 Amps.  The voltage drop across the 

shunt will vary from 0V to 50 mV as the current goes from 0A to 50 A.



The digital voltmeter has a 200 mV full scale readout so it will read 0 

to 50 mV.  If you indeed have the shunt in the example above, then the 

meter will read 0-50mV which you can interpret to be 0-50A.



If you have some other range shunt, the you will have to interpret the 

reading.  For example, if you have a 100 A shunt then it will have a 50 

mV drop at 100 amps and the digital meter will read 50 mV when you have 

100 amps flowing.



Dick Tasker



Carlos Trigo wrote:



[quote] 

 <trigo(at)mail.telepac.pt>



 Bob



 Thanks for your quick and complete response.



 I have a couple of (electric instruments challenged guy) questions:



 If I use the first digital panel meter you suggested 

 (http://www.mpja.com/download/12205me.pdf)

 do I simply connect power (9V and -) and the 2 wires from the shunt, 

 and it will indicate Amps?



 The "Full scale range" of 200mV indicated in the specifications, has 

 it anything to do with the 50mV characteristic of my shunt ?



 Carlos

 ---

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nuckollsr(at)cox.net
Guest

Posted: Fri Jun 22, 2007 6:46 pm Post subject: Digital Ampmeter

At 10:42 PM 6/22/2007 +0100, you wrote:



 	  Quote: 	
		  

<trigo(at)mail.telepac.pt>



Bob



Thanks for your quick and complete response.



I have a couple of (electric instruments challenged guy) questions:



If I use the first digital panel meter you suggested 

(http://www.mpja.com/download/12205me.pdf)

do I simply connect power (9V and -) and the 2 wires from the shunt, and 

it will indicate Amps?



The "Full scale range" of 200mV indicated in the specifications, has it 

anything to do with the 50mV characteristic of my shunt ?


	




   No, the instrument cited doesn't have sufficient common mode range to

   use with your shunt. You would either have to convert from a shunt to

   a hall-effect sensor . . .



   -OR- build the signal conditioning amplifier I suggested using the

   AD628 chip



   -OR- you could use a 9v battery to power the instrument, leave it

   hooked up all the time (but easy to replace) and set the instrument

   to read 200.0 Mv full-scale. Now, if your shunt is a 50A shunt

   (50 mv = 50A) then the instrument will now read correctly in amps

   to the nearest 0.1 amps.



   Bob . . .

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nuckollsr(at)cox.net
Guest

Posted: Sat Jun 23, 2007 5:44 am Post subject: Digital Ampmeter

At 10:42 PM 6/22/2007 +0100, you wrote:



 	  Quote: 	
		  

<trigo(at)mail.telepac.pt>



Bob



Thanks for your quick and complete response.



I have a couple of (electric instruments challenged guy) questions:



If I use the first digital panel meter you suggested 

(http://www.mpja.com/download/12205me.pdf)

do I simply connect power (9V and -) and the 2 wires from the shunt, and 

it will indicate Amps?



The "Full scale range" of 200mV indicated in the specifications, has it 

anything to do with the 50mV characteristic of my shunt ?



Carlos


	




   Carlos, I'm not sure we've completely explained all the

   toys and tools in the sandbox for achieving what you've

   stated as a design goal:



   See:



http://www.aeroelectric.com/Pictures/Schematics/DPM_Ammeter.jpg



   This is a schematic for incorporating a low cost, digital

   panel meter into an alternator loadmeter function using

   a shunt.  Note that while the low cost DPMs offer great

   accuracy and value, they all have some limitations that make

   it a challenge to install for your application. Note that

   I show a separate 9v battery. These devices have a very limited

   common mode range for the input signals. This means that we cannot

   ground the power(-) lead for the DPM to power ground and supply

   9V from ships power. The power supply has to "float" with

   respect to the signal leads.



   Using a battery to power this instrument is perhaps not as

   bad an idea as you might think. The DPM draws very little

   current and the battery will last for months. So if the

   battery is easy to reach (velcro to some handy spot behind

   panel) you can consider NOT having an on/off switch.



   You'll want to have a 50A shunt . . . no matter what

   size alternator you have so that the signal going to

   the DPM is 1 millivolt per Amp. When you set the DPM

   up to read in millivolts, then the display will

   be calibrated to read out directly in amps. The

   200 mV input to a 3-1/2 digit DPM has a max reading

   of 199.9 mV so your instrument will read Amps of load

   to the nearest 0.1 amps.



   If you can accommodate the limitations of these inexpensive

   but rather capable readout devices, they can offer some

   dependable, accurate measurement at very reasonable

   prices.



   Bob . . .

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