Help checking current sense circuit - Page 1
Forum section: electronics / projects

author: Apothus
author_rank: Contributor
timestamp: May 12, 2013, 08:33:30 am
content: I am working on a system that runs off a single LiPo cell that uses the ISL9230 charge controller with Path management.So when the device is charging from a USB port or from a solar panel the output from the charge controller is sourced directly from the supply and leaves the battery to charge provided the source current is sufficient.My maximum system load is ~700mA in the worst case scenario, LCD drawing the maximum, GPS acquiring etc. However a GPRS modem (sim900) can draw bursts pf 2A when transmitting.Essentially what i am trying to do is measure my current requirements as the system operates. Both the current pulled from the source, and how much power is drawn from this system. Especially useful when operating off a solar panel during the day so i can size it appropriately. I would need to be able to log the data over several days which my multimeter cannot do and figured i would utilise the ADC of an Arduino with an SD card to do the leg work.To that end I have delved yet again into the world of analogue electronics! I was hopeful someone could have a look at my schematic and tell me if I am on the right track or would have spent hours soldering something up that would never have worked!A0:Voltage divider to measure panel voltage Vout=3.3V when Vin=20.2VA1:Current output of solar panel with 0.1Ohm current sense resistorGain = 32With a 10bit ADC this should provide 1 count per mAI do not expect to draw more than 800mA as my charge current however 1mA per count saves some clock time with math and figured it would be useful.A3:Measures the cell voltageVout=3.3v when Vin=4.3VShould provide the maximum resolution on the pack voltageA4:Measures both the charge and discharge current with a bias of 0.825V (1/4 VCC)Gain = 9.1 ADC resolution 3.5mA per countThis is not something I have ever attempted before and figured this should help me to see very clearly how the battery is being utilised.So 0 current over the resistor should be 0.825VoutWhilst a Vout=0V would indicate 0.9 amps charging current & the system running from the Solar panelAnd Vout=3.3v would indicate 2.7 amps discharge current & the system running off the batteryAm I at least going along the right track with this design or have I completely failed to understand something?Is there any recommendations people would make to such a schematic? Any help would be appreciated!

author: Paul Price
author_rank: Super Contributor
timestamp: May 16, 2013, 01:41:26 am
content: A4 is missing. Looks like an important test point is MIA. Am I missing the point here or what?Look at it this way. You need only to monitor the Li-ion battery voltage as it is in use and is being charged by a solar cell array. You might make and transmit or store a voltage reading every time the GPS transmitter is operating. Estimate the size of the solar cell array and let your instrument run around in the sun for a day or week or a month. The instrumentation you are powering will either drain the battery down faster than it can be charged, or you have the possibility that the solar cell array is getting enough sun and big enough and is keeping the Li-ion battery near full charge on the average. If you need more solar panel power then boost the solar cell size, or else you might want to make is smaller to keep your instrument in business with a smaller footprint, but consider also the cloudiness of the weather in situ.With a gain of 33, 1 amp across a .1-ohm resistor would give 3.3V. Ok, so you are using a 3.3V MCU/reference for your A2D and you would get 1024 counts for 1Amp or 1.024 counts/mA...close to what you say.