I think I am over complicating this but need help with it please.
I need to measure PV voltage, wind voltage and a number of other values all on the same circuit. See simplified version below. All the TPn locations need reading.
The PV panels - I would like to read the voltage of them but the controller is pass-through positive and not ground. Some are common ground.
I would like to measure the wind voltage (it will be rectified DC) but will sit before the MPPT controller. Again, I don't know if the controller is common ground or positive.
I want to measure a number of other things but they are all common ground so should be able to do it with a number of ads1115
Option 1
I don't know - hence the cry for help
Option 2
Have a separate self-powered 328 mini on both PV and wind and using an opto-isolator I can feed serial values into the main controller.
Anything better?
I've scratched around a lot now to find a solution and can't find one that helps me understand so many variables.
I've discounted linear opto-isolators as what I have read on here, they are not great for this purpose.
There are some chips available to do all of that, with isolation. They're mostly used on big battery banks, so you have one chip per cell monitoring that cell. Rare for AA cells, but vital on cells big enough to need a forklift to pick them up.
But putting those chips together is about as complex as your second solution anyway.
Look into current sensors that are isolated. Sparkfun and Adafruit have breakouts for them. Use those where required or feasible. Then the only remaining problem is measuring voltage. You can just use simple voltage dividers so long as the Arduino's ground is at the lowest potential in the circuit. If you have a 12V panel and a 24V panel joined at the positive side, then put the Arduino on the 24V panel's negative. You will still have problems if the 24V panel could get shaded while the 12V panel is making power.
What about this: Take your voltmeter and measure the voltages first? With respect to common ground, to +, to whatever? Then you know what you have and make an educated guess on what to buy (if anything)
@MorganS -
I've seen a few things in my travels which I have thought might work but there did seem some complexity.
I've got a few isolated PSUs like the these which I could use to power the "sensor" Arduinos.
Alternatively I could power them from a simple zener dropper and, well, when the voltage is to low then it's not a viable source for me anyway.
As for the current sense - yes, I am using ACS758s everywhere with the exception of the main battery load - that will be a shunt as there is around 208A max draw from them when the inverters are at full tilt.
I've looked for an isolated current and voltage sense but found none.
Just use two analogue inputs for supplies without a common positive.
Measure the positive terminal with a voltage divider connected to ground, and measure the negative terminal with a voltage divider connected to Arduino's VCC. Then subtract the two A/D values.
For a more stable readout, connect the negative divider to 3.3volt, and use that also as Aref for the A/D.
But that will interfere with your ACS sensors.
Leo..
@Leo -
Would you mind sketching that please and showing the numbers?
I've drawn it out and works with simple numbers - Vpn=20 and dividers each 50% but it does not make sense to me.
I suppose you understand how to use a voltage divider to measure a higher voltage than Arduino's 5volt (Aref).
Take a 10k resistor between voltage source and pin and 10k from pin to ground as an example (1:1 divider).
It takes 10volt on the source, to get 5volt on the divider, to get an A/D value of 1023.
You know that 1023 must calculate to 10volt (and a value of zero must be 0volt).
Now move that 10k pin that was connected to ground to the 5volt pin.
If the source is 0volt, then there is now 2.5volt on the divider, resulting in an A/D value of ~512.
If the source is 5volt, then there is 5volt on the divider (both sides are 5volt), resulting in 1023.
If there is -5volt on the source, then there is 0volt on the divider, resulting in zero A/D.
You have now made a setup that can measure -5volt to +5volt.
Can help you with resistor values and code if I know the exact voltage limits.
Leo..
Electrically isolated opamps like these http://www.ti.com/lit/ds/symlink/iso124.pdf
can solve most of your problems with common positive grounds.
They work like normal opamps, but the input and output are isolated from each other, so you can have as many differant grounds as you like.
