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

Can I achieve from this circuit? below attached Why I am doing? I need this to calculate the voltage of the battery with cc load and without load enter image description here


1 Answer 1


The circuit looks reasonable. Q2 is used to control the constant current source. Q1 turns the constant current on and off.

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.

The combination Q2 and R1 is the constant current source, adjusted by Vref. But it will need some adjustment to get an accurate 100mA. The Base-Collector voltage drop across Q2 will be about 0.6V, as will the Emitter-Collector voltage drop across Q1. The voltage across R1 will be approximately (Vref - 1.2V). So Vref has to be over 1.2V and less than the battery voltage.

Choose R1 so that it is capable of passing 100mA at (Vref - 1.2V). You may end up putting a voltmeter across R1, then tweaking Vref to give the right voltage to give 100mA.

Note: There's nothing in your diagram to say where Vref comes from.

  • \$\begingroup\$ Vref is coming from DAC of microcontrller or can i give external also? \$\endgroup\$
    – mani
    Oct 31, 2018 at 13:00
  • \$\begingroup\$ @mani Nothing wrong with using a DAC (so long as it really is a DAC, not just simulated using PWM). Or an external adjustable voltage source. It's up to you. \$\endgroup\$
    – Simon B
    Oct 31, 2018 at 14:07

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