# Sensing and measuring current?

Discussion in 'General Electronics Discussion' started by eem2am, Aug 17, 2013.

1. ### eem2am

414
0
Aug 3, 2009
Hello,
We need to sense and measure the current from a battery which could be anywhere between 2.8V and 11V terminal voltage ...as follows...

http://i41.tinypic.com/2hf5teu.jpg

....an ADC of a micro will end up sensing the output voltage of whichever current sense method that gets used.

Can you suggest a relatively cheap current sense method for this setup?
(the ZXCT1009 cannot work down to 2.8V)

The current will be anywhere between 50mA and 2A.

2. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
648
May 8, 2012
If you use a 100mR current sense resistor in the low supply rail you'll develop a voltage drop between 50mV and 200mV. Feed this into an OpAmp with a gain of 100 and you'll have an output voltage swing of .5V to 2.0V. If you increase the OA gain to 1000 you can drop the sense resistor to 1.0mR to produce the same output swing but working with a value this low will be a very big challenge.

Chris

3. ### BobK

7,682
1,686
Jan 5, 2010
If you don't want a current sense resistor at ground, there are also high-side current sensing amps that can do this with the resistor on the high side. Look up "high-side current sensing amplifier"

Bob

4. ### eem2am

414
0
Aug 3, 2009
yes I dont want a resistor in the low side, but regarding high side resistive sensing, there are very few ic's that will work with vin as low as 2.8v.

Here is an idea with the LT1787 current monitor IC, but do you think it will work down to vin = 2.8v?

LT1787 DATASHEET:
http://cds.linear.com/docs/en/datasheet/1787fc.pdf

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5. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
648
May 8, 2012
The LT1787 is rated down to Vs = 2.5V.

Chris

6. ### eem2am

414
0
Aug 3, 2009
Thanks, its just that the ZXCT1009 says it is rated down to 2.5V, and actually what it means is that it needs 2.5V of voltage headroom above the output voltage that represents the current value.
I wondered if the LT1787 has any qwerks like this?

zxct1009 current monitor:
http://www.diodes.com/datasheets/ZXCT1009.pdf

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
648
May 8, 2012
The LT1787 data sheet was very clear and includes typical circuits. They guarantee Vsource lower limit = 2.5V which means it will probably work even lower.

Chris