[Argus25]

Hi,

I have attached a circuit of the basic arrangement, you would just string a 1k to 2k pot across the supply to create Vin. If you want the voltage control range less simply lower the value of R3.

The 10k resistor with the * near it would just be adjusted so the 1k pot was about in the center of its range. This could supply a 2A motor without problems, it could also run on higher voltages like 20V

The circuit has filtering (anti hunt network) to reduce that chugging effect. When the 1k pot is set right, any drag on the motor is compensated so it doesn't slow down or speed up. The 0.6R current sense resistor was originally a coil of copper wire, you could use a small wire wound resistor of a similar value. One problem, there is often a lot of noise voltage from a brush DC motor.

You can't replace this OP amp with a generic part, the circuit is for a Norton OP amp, these work on difference currents at their input (not voltages) so its best to stick to the LM3900 op amp.

The darlington transistor arrangement could be two transistors like a BC639 & BD139, or you could use a darlington like a TIP140 or similar and save on parts and a resistor.

You can see that as the motor current increases on load the voltage across the 0.6R current sensing resistor increases, this is fed to the + of the OP amp and increases the motor drive.

You would need to build it and tinker with it to suit you exact motor.

The power supply connections to the OP amp are not shown.