electronic circuit design

I would like to measure the voltage and the current from the solar using a micro-controller. For the voltage measurement I'm going to use a potential divider but for measuring current I don't know how to really connect the circuit so as to sense that current. Please help with a circuit schematic that shows how to measure current from solar

solar output voltage usually will be less and if it exceeds then you req potential divider. for current you do get current sensor which is made from induction coils. Do purchase the sensor. it wil give you current interms of voltage.

The simple way to determine current is to measure the voltage across a small shunt resistor added in series with the output from the solar panel.

If you want to avoid the resistor and its power loss, you can use a hall-effect current sensor.

An induction coil (current transformer) will not work with DC.

Thanks Msharmavikram and Crutschow. But please can you help me on how exactly will I be able to measure that voltage drop considering that the voltage from the solar might exceed the maximum voltage which the micro controller can measure. Please help with some circuit schematic on how I could successfully connect mu PIC to measure that potential difference. Thanks

just use a pair of resistors to divide the voltage to say .5 X Vs. A very sophisticated way would be to use a zener diode to " subtract" some voltage, which would give you a more sensitive range. For instance , your micro can handle 0 -> 5V measurements. If the solar cell produces 50 - 70 V, depending on the state of the sun and load. So you can use a pair of resistors to reduce the voltage so that 70V -> 5V so 50V goes to 3.57V and fiddle the figures in the micro so it displays correctly (if used). Or you can use a 40V zener in series so the 50-70 goes to 50-40 = 10 to 30. now using two resistors you can knock the 30V down to 5V, so 10V becomes 1.66V, so you have spread the scale a bit more.
Frank

a voltage divider circuit( potential divider) is one of the best option. create a voltage divider circuit and tap the voltage across the needed end. while making a voltage divider circuit make sure u use a preset(or Pot) as one resistor. by doing so u can control the sensitivity of measurement.

E.g. with frank's point:



D2 protects the ADC input. Sensor draws ~5mA at 70V.

Plot of solar voltage vs sensor voltage:


Convert ADC input back to solar voltage in software. With a 10-bit 5.5V max ADC, using 0-5V as your valid sense range (5V-5.5V is leeway for D2 clamp response), your solar sensor resolution would be:

(1 / 2¹⁰) * (70 - 50) * (5.5 / 5) = 2.14mV

Increase R1/R2/R3 to lower sensor current draw but mind ADC input impedance. If impedance is too low you can also use op amp powered off of solar cell or microcontroller supply as buffer, mind the op amps ratings and max output swing and you can use it's + supply voltage to clip sense voltage and protect ADC as well, e.g.:



That draws 0.5mA from cell at 70V, and is forgiving to ADC input impedance, tradeoff is complexity and op amp power supply. Run DC sweep sim to see output voltage and current drawn.

J

Thanx to you all. I have attached a document with a circuit that I want to use to charge series 2x9V 250mA Ni-MH batteries. So please help me with any additions or subtractions to the circuit. I want a charging system with MPPT and which can stop charging when the batteries are full.