uCurrent (from EEVblog) current upgrade
I have been having a lot of issues as of late using my HP 34401A DMM to measure current. The DMM measures current by passing the current through a resistor to rectify the current into a voltage based on the relationship V=IR. It then measures the voltage across the resistor to determine the current. I used a secondary DMM to measure the resistance to 5.5Ohms
This is quite typical. However, the size of the resistor is somewhat large (5.5Ohms), and causes a drop in voltage across that resistor (also based on V=IR). The more current you measure, the larger the drop in voltage. A 1A current will cause a 5.5V drop!
As my WiFi module was powering up, it would require a larger draw of current due to large inrush, this would cause a sudden spike in current which would cause the a drop in voltage across the current sense resistor which would cause things to break. Took me a full day to figure out that this was the culprit to my problems. Very annoying.
So I ordered a precision current measurement device by the EEVblog author (http://www.adafruit.com/products/882) which works in the exact same way as my DMM, but it passes the current through a much smaller resistor (in the mA setting, it passes the current through a 0.01 Ohm .5% resistor – R1 in http://alternatezone.com/electronics/ucurrent/uCurrent%20Schematic.png). Thus, for a 1A current, we expect only a voltage drop of .01 * 1 = 10mV! Much better.
However, I want to measure at least up to 1A, and the documentation states it only works up to 300mA. If you look at the circuit, it sets up a virtual ground at 1.5V (given a 3V supply), take the voltage difference across the current sense resistor and amplifies it 100x using U1. Thus a 1A current should result in 10mV * 100 = 1V. This should easily be within the output capability of U1. So I posted to the authors forum, and asked. It seems the 300mA limit is because of the switch SW1’s limit. Doh. So I added some solder blobs to force it into mA mode and capable of supporting much higher currents (see photo – 2 solder blobs directly below the LED (D1).
So it works fine, but I am getting an unusual amount of noise out of the system given the precision components and the quality opamp. In looking at the circuit, I would wager that it’s from R2. It is always in there, parallel with either R8 or R1. It’s so much larger than R1 or R8, that it doesn’t really alter the measurement, however, it does add Johnson noise which is amplified as well. The amplifier has a GBW of 6.5Mhz, and no additional filtering is provided, so we can approximate the bandwidth as 6.5Mhz/100=65kHz (not exactly, but close enough). We can use this to calculate the noise from R2 to be 3.2uV (from http://www.sengpielaudio.com/calculator-noise.htm) * 100 = 320uV noise. At 1mA/mV (the output of the device), that’s equal to 320uA of background noise. I’ll take out the resistor at some point to see how it improves the signal, but it’s a bit annoying that it’s there. He could have easily hooked R2 to the 1st terminal of SW1 to avoid this source of error..
