Reading 12 V battery voltage on Microcontroller Arduino Mega 2560.

[QutabBaig]

Hi

I wish to read a 12 V battery voltage on Arduino Mega 2560 ADC pin. To do that, there are two methods.

1. Use a voltage divider circuit.

2. Use an op-amp that would attenuate the voltage to keep it as low as 5 V like in given figure I m reading 14 V of battery by scaling it down to 4.19 V. My controller will read this 4.19 V as 14 V and I will do programming accordingly.



Now the issue I m having is if I go with second method, do I need to worry about current. Is there any possibility that a large current may flow through op-amp towards controller and ending up burning my op-amp and controller? Is there any op-amp which can sustain high currents?

Pl assist

 

[barry]

How much current can you push through a 100K resistor with 12volts?

 

[QutabBaig]

Ohms law. 12/100 k = 120 uA

My application is Battery Charging through Solar Panels. So naturally current depends on rating of solar panel and other daylight conditions. I need to read voltage from Solar Panel as well.
If its a 40 Watt panel with 12 V output, it could give up to 3 A current.

Please correct me if I am wrong.

 

[barry]

As you calculated, you can't push any more than 120uA through a 100K resistor at 12 volts, REGARDLESS of what the current rating of your source is. If that weren't the case, people would be electrocuting themselves with AAA batteries.

 

[QutabBaig]

Okay.

Another question: Which method is better to read voltage. Via voltage divider circuit or attenuating voltage gain via amplifier?

Appreciate your kind assistance

 

[barry]

It depends on the input impedance of your AD. If it's low impedance, the voltage divider method will contribute some error.

 

[QutabBaig]

What is input impedance of ADC?
What and how does input impedance of ADC effect my reading?
If voltage divider method is not reliable should I opt op-amp method?

I m sorry for my stupid questions. My electronics is not very good.
Pl assist

 

[barry]

I sure don't know what your impedance is; read the data sheet. The input impedance of the the ADC is paralleled with the resistor of your divider. So, sure, use the opamp.

 

[D.A.(Tony)Stewart]

Simplest solution is 30k from Vbat with 10k to ground gives 6:1 passive divider using 2.56 Vref thus full scale is 0 to 15.36V with a resolution of 2560/4096*6= 3.75 mV

Passive R divider is still low current using 10k across ADC port thus 6:1 divider using 50k series to Vbat is 15V/60k =250µA max.
If this is too much to your liking, buffer it with a NON-Inverting unity gain OA, not the inverting one you chose.

background info and other methods...

You have many options for this ADC and no OA buffer is required.


• 10 bit ADC
• 0V - VCC ADC Input Voltage Range
• 2.7V - VCC Differential ADC Voltage Range
• Selectable 2.56V or 1.1V ADC Reference Voltage
• 4 Differential Input Channels


You cant use single ended input and scale 14V=4.19V because this exceeds internal Vref options


Remember that Vref internal is the full scale ADC output $03FF.