This circuit was found in a 1965’s General Electric documentation. It is intended for low voltage brush motors. The SCR must be according to the engine controlled.
This amplifier with very high input impedance was suggested by Texas Instruments in a manual of...
Using only a ULN5804 integrated circuit, stepper motors up to 500 mA can be controlled from TTL...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....