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measure negative voltage of battery by using voltage of that battery itself

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zeroskj1988

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hello sir, i want to make a circuit that measure negative voltage of battery.i am using pic adc to measure this voltage, but does not read negative voltage. so i have convert this negative to positive voltage.
problems
1. i don't have negative voltage on the system,to convert this negative volt to positive
if i am opting for differential amplifier with single supply then
2. negative of my positive and positive of negative supply both grounded to earth, so actually i need a negative voltage to measure this voltage
so i am thinking of a circuit which derive a -5v from the battery which is to be measured. i am uploading a image of proposed circuit.
can any body suggest any idea or feasibility of my circuit. i am thanking u i n advance battery voltage measurement.GIF
 

Use a differential amplifier

DiffAmp.jpg

V1 to GND
V2 negative input
Vr positive offset voltage

if all the resistors are equal then the gain is 1, use an offset that is equal or higher than the min negative voltage input, for example if the input can be down to -5 use an offset of +5 (or more), ,then when the input is -5 you will get 0, when it is -4 you will get +1 etc.

The opamp doesn't need negative supply in what I describe but it should be able to output 0v because not all opamp can output 0v

Alex
 
thank you alex for your reply.i have one doubt left as a said earlier negative of opamp supply and positive of negative supply is earthed or shorted,this make differtial voltage pull down below the ground of opamp. that is reason why i am saying i need a negative supply.sorry for any misatkes
 

If you connect the negative voltage to V2 and the offset voltage Vr is higher then the negative voltage will never reach the opamp input so you don't need any negative supply.

Alex
 

i want measure voltage up to voltage up to -60v.i used potential divider 10k-680ohm.to step down voltage.then i connect it to v2-negative. v1 to common positive.but i am not getting value.as expected please help me
 

Referring to the schematic of post #2 if you use 20k for R1 and 1K for Rf , + input to gnd and -60v to the negarive input (R1) then you will get 3v at the output (1/20 attenuation and positive output)

Alex
 

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