Please advise a relative newbie!
I'm looking to build a system for monitoring a small DC current not more than 100mA at 5V. I gather that I can do this using an Arduino, along with the the emonTX shield. (Probably using an acs712 sensor as shown by rubensben.)
I have an emonTX v2 knocking around from a previous project however, and I'm wondering if it is possible (or feasible for a newbie) to bypass the arduino and shield, by connecting the acs712 direct to my emonTX?
Re: Kit required for small DC current?
Can this current source share a ground pin with the shield, or does it need to be isolated? If so, how much potential difference can exist (worst case) between the sense circuit and the sensor?
Re: Kit required for small DC current?
Hi emjay. The current source will need to be isolated. I would be looking at the 5A version of the ACS712 module, see here for specifications. According to this the analog output is 2.5V + (185 mV per Amp), so at 100mA this would be 2.5185V. Does this answer your question?
Re: Kit required for small DC current?
emjay was asking, What is the voltage between GND of your emonTx/Arduino and GND of your current source?
Re: Kit required for small DC current?
@roblog,
Does the monitoring require tracking the current value or just knowing presence/absence? The Hall effect device you mentioned is the most sensitive version, but 100mA is only 2% of its range, so that is close to the precision/noise level of the ADC.
Certainly you can remove the signal conditioning components for one of the CT inputs (this is covered elsewhere in the forum) and just about get a presence/absence trigger if you reference both the chip and the ADC to the same Vcc. Seeing a useful smooth range of 0-100mA readings is not feasible this way though.
Re: Kit required for small DC current?
@emjay @Robert, I'm not sure to be honest - like I say I'm a bit of a newbie. The current source would be a 3.7V battery, and the emonTX/Arduino would probably be powered from the mains. How would I go about working out the maximum potential difference from this information? Also, how does this relate to my original question? I'm guessing from its name that the emonTX Shield plays a protective role against high potential difference .. ?
Re: Kit required for small DC current?
Hi again @emjay. Yes I am wishing to track the current value, rather than just a presence / absence. I was concerned that the low amps are a small fraction of the range so thanks for informing me that this approach is not feasible.
I wonder if you or anyone could advise me how best I could achieve this?
Re: Kit required for small DC current?
@roblog,
It's all about if there is a real requirement to have isolation between the monitored circuit and the monitor. The design gets much easier if there is a common potential between the two. Sounds like your battery driven circuit can share a ground signal with the emonTX.
Are you in fact looking at how much charge is taken from the battery over time? There are specialised chips for this - here is a tutorial on coulomb counting.
Re: Kit required for small DC current?
I'm guessing from its name that the emonTX Shield plays a protective role against high potential difference ..
Hi Rob,
The answer to your question about shields...
Shields are boards that can be plugged on top of the Arduino PCB extending its capabilities
https://www.arduino.cc/en/Main/ArduinoShields
Re: Kit required for small DC current?
And a sketch is nothing to do with a drawing, nor comedy. It is the name Arduino have given to the user's program:
https://www.arduino.cc/en/Tutorial/Sketch