A mod proposal for the uCurrent - Page 1
Forum section: electronics / projects

author: ang370
author_rank: Newbie
timestamp: March 05, 2015, 05:40:01 pm
content: Hi all,I wonder if someone has already considered the possibility to extend the uCurrent functionalities in order to make it connectable to a pc.I need a simple instruments capable to measure small currents (and possibly power) and save that measure in a log file.Would it be a crazy thing consider such a topology to reach that goal?Another interesting features would be an automatic voltage-scale change, probabily feasible with the microcontroller.?Let me know.Thanks

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 05, 2015, 05:47:36 pm
content: Wow .. and what precision you had in mind for the ADC? Not the crap ADC inside a Arduino, I may hope.And do you have a reference that goes with that?

author: Kalvin
author_rank: Super Contributor
timestamp: March 05, 2015, 06:02:02 pm
content: Maybe the mcu should be able to compute the RMS, average, integrated average, peak min and peak max as well. A 12-bit ADC may not be sufficient, but a 16-bit ADC doesn't cost too much today. However, the ADC should have sufficiently high sampling rate to be useful and give valid measurement result. For example, 1 MSPS should be sufficient for 300kHz bandwidth.For cost reduction one could create a cheaper version with 12-bit ADC and +/- 1% accuracy, and for more demanding measurements a 16-bit ADC and high quality reference is in order.

author: DmitryL
author_rank: Regular Contributor
timestamp: March 05, 2015, 08:09:06 pm
content: Instead of USB and isolation you can use a cheap bluetooth module. So that every hipster with a tablet can use uCurrent!The rest is sort out some benign things, like selection of PGA, input filters, ADC and other boring stuff

author: mikerj
author_rank: Super Contributor
timestamp: March 05, 2015, 08:43:29 pm
content: Or you could use a multimeter with a serial output and get far better resolution and accuracy, albeit slow sampling rates.

author: Neganur
author_rank: Unknown
timestamp: March 06, 2015, 06:30:23 am
content: I can absolutely recommend something like the AD7685 for this, 16 Bit, Vsup 2.3...5 V, 250 kSa/s and SPI output. Have your Arduino talk to this one if that's what you're familiar with.It's quick, has enough resolution and easy to interface

author: ang370
author_rank: Newbie
timestamp: March 10, 2015, 11:40:05 am
content: My first idea is to use an STM32F4 which has 3 ADC (12bit). With each ADC I would read from uCurrent:- Vout (i.e. the current measured),- Vgnd (virtual groud because this voltage is half of battery voltage which is variable) and- the voltage of the device under test in order to compute the power.The ground reference for the STM32F4 would be at the battery negative pole, so using a single cell Li-po I need to scale all the values above to the 3,3V which is the upper voltage for the STM32 ADC.Another need I have is to change automatically the measurament scale since often my application jumps from micro amps (or less) to milli amps and vice versa. Is there any way to change the actual switch?Next if everything works fine I'll consider to move to a more precise ADC and/or a more cool data transmission.

author: Neganur
author_rank: Unknown
timestamp: March 10, 2015, 02:31:29 pm
content: bear in mind the loss of resolution when you use a 12 bit adc, and also the noise. A 12-bit adc corresponds more or less to a 3 1/2 digit DMM.16 bit is a little more than 4 1/2 digit, so 10 times better resolution.I guess 12-bit is sufficient to see 1 mV but you're going to limit your input voltage severely if you use a 3.3 V STM32, the uCurrent can take 5V and you have that from the PC.I suppose you can do the range switching with something like NLAS4684, MAX4684 or maybe even small relays. the switches Dave used have a resistance of below 10 mOhm.

author: sacherjj
author_rank: Frequent Contributor
timestamp: March 10, 2015, 03:03:46 pm
content: I have used my uCurrent with my PC. I connect it to my LabJack. Works great. I also have tons of other channels for other things. Including ADC output, etc.

author: Neganur
author_rank: Unknown
timestamp: March 10, 2015, 03:47:46 pm
content: ah nice item, reminds me of the NI myDAQ orUSB 6000-series from NI, although not so bloody expensive!

author: AG6QR
author_rank: Frequent Contributor
timestamp: March 10, 2015, 05:05:18 pm
content: The uCurrent is nice because it is simple, doing one thing well. And it is versatile, able to send its output to any device that measures voltage: a DMM, scope, logging data acquisition system, Arduino, whatever.If I needed to log the uCurrent output to a PC, I'd probably also have a need to occasionally log voltage and/or higher current levels. So I'd get a general purpose logging meter with a way to send its output to the PC. Why reinvent the general-purpose part that is widely available?

author: ang370
author_rank: Newbie
timestamp: March 11, 2015, 08:44:35 am
content: You are right. I wasn't aware of LabJack, I had heard only about quite expensive NI logging meter.In any case with LabJack it's probabily difficult to automatically switch from uA to mA scale since by now the only way is to use the slide switch.