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Getting around the burden voltage in measuring low currents

Current is usually measured as a voltage drop across a known resistance. David L. Jones notes that this ‘burden voltage’ can be a significant disturbance in low voltage and low current circuits. His solution is the µCurrent. He provides a description and theory of operation as well as a circuit diagram for the device at his EEVBlog.

There are three parts of the circuit. The coin cell battery status monitor LED uses a ‘power supply supervisor’ chip that makes it easy to check the lithium coin cell to make sure it is providing the needed voltage. Another part of the circuit splits the battery voltage in order to provide the reference op amp with the supply voltages it requires. The reference op amp simply converts the signal from the dropping resistor into a voltage that can be read on a DVM in a convenient way to determine current.

I guess that these days the input impedance of voltmeters isn’t that much of an issue. It was that long ago (it seems to me) that a 20k ohm per volt meter was something significant. Vacuum Tube Volt meters were the cat’s meow when it came to measuring voltages and current. That was when you needed a vacuum tube to get a high impedance amplifier to isolate the input and get enough output to drive a D’Arsonval meter.

an aside: I am in the process of redoing the post categories and immediately after a first pass to try to make sense of things, along comes another interesting topic that doesn’t quite fit. Where do I stash a ‘build your own’ and ‘circuit theory’ and ‘measurement theory’ ?? I’ll have to think about this. There are tags for individual posts so I’ll have to use them until I can figure out a better solution.

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