constant current circuit load and without load

How can I control the constant current block?
What I need?
I need to calculate voltage with constant current load (I = 100maand after some time I need to calculate voltage without load (I = 0), for that I using BJT to control the constant current block from micro controller to turn off and on.

Can I achieve from this circuit?
Why I am doing?
I need this for to calculate the voltage of the battery with cc load and without load

below is attachment,


Let me see if I understood, you want to measure the voltage of the battery with and without load, being that the load is 100mA fixed.

I think the current limiter circuit can be static, and you can turn it on and off with your microcontroller remotely.

To measure the voltage, with or without load, it would be enough to use a resistive divider, and an ADC input.

See if this helps: [Active Inrush Limiter Circuit] |500x178



You can use a resistor with a value that can boost the 100mA load when the battery voltage is low, so the current limiter transistor (Q1) will act as a resistor when the battery voltage is high.

Otherwise, the transistor will dissipate the heat of this load, say it is 12V under 100mA, results in 1.2W. Your transistor must have ability to dissipate this power. and a sink, of course.

If the 12V battery can operate up to the lower limit of 10V, then we could define a load resistor that waits around 8V, to consider the voltage drop on the transistor series.

Being 8V and 100mA, we have V = R * I, R = V / I, R = 8 / 0.1, R = 80

Nearest commercial value for the resistor: 82 ohms.

Resistor power: P = V * I, P = 8 * 0.1, P = 0.8W

In this case, it is best to use a resistor of 2W or more to avoid long term damage.

@rtek1000 thank you so much ,

here i have to mention few things ,

Vref is given from DAC of microcontorller and battery is 4.2V.

If the base of Q1 is pulled low, it turns off, and the whole constant current turns off. If the base is pulled high, it turns on.


Sorry, the reference I made about “Q1” was in relation to the image I posted myself, it is not the “Q1” of your schematic.

Please, see if the attached image helps you understand what I was straining to say:


Sorry, the 1k resistor needs to be connected above the 15R resistor, not below the shape that is in the image.

I think you can try these circuits transistorized in some simulator, so you can be more productive ;)

Thank you ,i will try this and let you know