Measure current both ways? Need help

computer guy

Hi Guys,

My dad is wanting to measure how much current a battery charger is providing to a battery but at the same time measure the amount of current the battery is providing to the device connected to it.

Is there a way he can do this with an ammeter?


Thank you

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kwinn

A small resistor in series with the battery/load and a voltmeter would do this. The voltage would be positive for one direction, negative for the other.

LoopyByteloose

I suppose you would want two meters placed in appropriate locations to get a simultaneous reading. If you use old conventional meters, some can even show a back flow, like the amp meters that were used on cars.

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Tracy Allen

Look at chips like the MAX472. You put a current sensing resistor in series with the battery supply on the + side, and the output from the chip goes to an analog to digital converter, and there is also a binary "sign" output from the chip that tells which direction the current is flowing. A wide range of currents is possible by choosing the right resistors. Choose the sense resistor appropriate for the amount of current--say 0.2 Ohm as in the attached diagram for measuring +/- 250 mA into a 5 V analog to digital converter. Furthermore, since the output of the MAX472 is a current, it is easy to configure it directly for input to RCTIME or a sigma delta.



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Tracy Allen
www.emesystems.com

dMajo

Have a look at these ICs from AllegroMicroSystems

ACS712 - http://www.allegromicro.com/en/Products/Part_Numbers/0712/index.asp

ACS756 - http://www.allegromicro.com/en/Products/Part_Numbers/0756/index.asp

ACS758 - http://www.allegromicro.com/en/Products/Part_Numbers/0758/index.asp

A1351 - http://www.allegromicro.com/en/Products/Part_Numbers/1351/index.asp

A1360/1/2 -http://www.allegromicro.com/en/Products/Part_Numbers/1360/index.asp

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Tracy Allen

I looked around and see that the MAX472 is discontinued. I still have some left over from a project. Things do change! Maxim recommends the MAX4172 as replacement, It has a voltage output and uses an offset scheme to cover both directions. Now, you have to deal with the surface mount package.

Other companies make chips with the same function. Look for "current sense amplifier", specifically, "bi-directional high-side current sense amplifier". Linear Tech (LT1787) and Zetex (ZXCT1009)) come to mind.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 11/14/2009 10:18:50 PM GMT

computer guy

He doesn't need to log the data. Just an analoge circuit would work fine.
An ammeter, a switch, etc.


Thank you all

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kwinn

There were some old analog meters that had the needle centered in order to read positive and negative but I have no idea if they are still available. A cheap multimeter or digital panel meter with an appropriate sensing resistor is probably the simplest and cheapest.

Franklin

Most analog ampmeters for cars and tractors use a centered needle so you can see both the charge and discharge from the electrical system.
Like this http://www.streetsideauto.com/category.asp?ParentID=29&SubID=994&ptid=3897&brand=BDHZ&LookupType=YMM

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- Stephen

Post Edited (Franklin) : 11/15/2009 4:46:37 AM GMT

mctrivia

most cheap digital meters can go both positive and negitive. You can pick up a low end one very cheap.

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