simulate this circuit – Schematic created using CircuitLab

For a specific project , I need to create a Voltage-Current controller circuit that will be able to control the flow of Current and the voltage by using 2 different potentiometers for each.

Here is the schematic I have tried....

TIP122 Darlington for increasing and controlling current, till that it is working fine but

the IRFZ44N MOSFET part doesn't work properly for voltage regulating and I am unable to find a way to fix it.

But this doesn't seem to work.

can anyone please point me out where i have made a mistake? Explanation with circuit schematic will be much preferred and appreciated.

i need one potentiometer to control voltage and another one for controlling current


I don't want to use any types of IC for this project so please avoid suggesting one till there is another way/ways left.

Thanks for your precious time.

  • 1
    \$\begingroup\$ I think you are missing what ohms law is about - if you control the voltage to the lamp, the current is determined by that voltage and the lamps resistance - you cannot control voltage and current independently. There is no point in your experiment. \$\endgroup\$
    – Andy aka
    Nov 21, 2020 at 9:59

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

Figure 1. Original and redrawn circuits.

The convention is that schematics are drawn with higher voltages at the top and ground at the bottom and reading left to right. That way we will generally be able to trace current flow from top to bottom and signal flow from left to right. We can also use the GND symbol to eliminate a bunch of wires to reduce clutter. Which is easier to read and understand?

Once we untangle your schematic it becomes very clear that there is no way for current to go from the 9V+ terminal to the lamp. Neither Q1 or Q2 have a positive supply connection.

If you are updating your question then please add in some explanation of how you expect your design to work.


simulate this circuit

Figure 2. OP's update.

Once we redraw the circuit with conventional layout it becomes clear that Q4 is upside down and cannot conduct to pass current to LAMP3.

  • \$\begingroup\$ I want to control flow of current and the amount of voltage using 2 regulators (1 for each) \$\endgroup\$ Nov 21, 2020 at 10:08
  • \$\begingroup\$ I know that. You said it in your question. You need to edit your question and (1) Fix the circuit. (2) Explain how you think your circuit is supposed to work. \$\endgroup\$
    – Transistor
    Nov 21, 2020 at 10:48
  • \$\begingroup\$ Edited please recheck \$\endgroup\$ Nov 21, 2020 at 11:12
  • \$\begingroup\$ See the update. Can you see how a proper schematic helps understand the schema? You didn't address my comment (2). How do you think your circuit is supposed to work? \$\endgroup\$
    – Transistor
    Nov 21, 2020 at 12:14
  • \$\begingroup\$ please, i am not used to draw circuits in that way which you are talking about. Please and please try to deal with that. \$\endgroup\$ Nov 21, 2020 at 12:24

You cannot control the voltage and current in a resistor independently. That is why your circuit does not work as you intend it to.

I presume you have heard of Ohm's Law. If you apply it to your load, you will see that once you have set the voltage across the load, there is only one possible current. Voltage over resistance equals current.

If you set the current, there is only one possible voltage. Current times resistance equals voltage.

You cannot control the voltage and current in a resistor independently.


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