Measuring Current on a 5V power supply

Thread Starter

Wendy

Joined Mar 24, 2008
22,647
I am thinking about building a current fold back for a USB power cube. But first I need to be able to measure the current. A LM324 can do this quite well:
Figure 1.jpg
Figure 1​
I'm not so sure about this layout:
Figure 2.jpg
figure 2​

it is almost 11:00PM my time and I am fried. I'm done for the night. Thoughts, suggestions?
 

Ian0

Joined Aug 7, 2020
3,761
A LM324 can do this quite well:
Figure 1.jpg
Actually that should be -1V = 1A, and to achieve that you would need a negative supply.
I'd also dispute "quite well" when referring to an LM324! An LM324 can't do anything quite well. Offset voltage will become a problem when measuring small currents.
I'd suggest a shunt amplifier such as INA181 or AD8418.
This would work if you want to use an op-amp in inverting mode, provided that the common-mode input voltage includes the negative supply (which it does for an LM324) R1 is the load resistor.Untitled 1.png
 

MisterBill2

Joined Jan 23, 2018
9,843
Now there are IC devices made for exactly that application, boosting the voltage from a shunt. One of them would be a better choice. Not as cheap, though.
 

Irving

Joined Jan 30, 2016
2,311
INA250A4 is a better, if slightly more expensive (£2.90) solution with internal sense resistor, gives 2v per amp output.
 
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