I need to make sure I understand how the ULN2003 Darlington Array works

[engineheat]

Here is a schematic of the ULN2003 being used to drive the 28-byj48 stepper motor (unipolar).



Here is a schematic of the internals of the ULN2003:


So basically, power (5V) is delivered to the stepper by the red line (connected to the COM of ULN2003). Drive signals from the microcontroller goes into the IN1, IN2...IN4 pins of the ULN2003, which allows current to flow through the respective channels of the Darlington. This is the current that flows through the motor and allows it to turn. This current goes from the OUT pin of the ULN2003 and flows to the GND pin of the ULN2003. 

The OUT pin is essentially the collector, and the emitter is essentially the ground according to the ULN2003 schematic. There is a free wheeling diode at the COM and its purpose is to allow current to flow back to motor in a cycle once current gets shut off in the channel. This is because the motor has inductance and current cannot drop to zero instantaneously.

Did I understand it correctly?

Thanks

[danadak]

Yes.


Regards, Dana.

[engineheat]

So if I think of each Darlington pair as a NPN, then is it in Saturation mode when there is a signal from the microcontroller, and Cutoff mode when there are no signal from the microcontroller?

thanks

[Zero999]

So if I think of each Darlington pair as a NPN, then is it in Saturation mode when there is a signal from the microcontroller, and Cutoff mode when there are no signal from the microcontroller?

thanks

Yes.

[MarkF]

So if I think of each Darlington pair as a NPN, then is it in Saturation mode when there is a signal from the microcontroller, and Cutoff mode when there are no signal from the microcontroller?

Yes.

Be aware.  There are different versions of the chip for different input logic levels:

• The ULN2002A is designed specifically for use with 14-V to 25-V PMOS devices. Each input of this device has a Zener diode and resistor in series to control the input current to a safe limit.
  
• The ULN2003A and ULQ2003A have a 2.7-k? series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices.
  
• The ULN2004A and ULQ2004A have a 10.5-k? series base resistor to allow operation directly from CMOS devices that use supply voltages of 6 V to 15 V.
  
• The required input current of the ULN/ULQ2004A is below that of the ULN/ULQ2003A, and the required voltage is less than that required by the ULN2002A.
  

There is also the ULN2803A in a bigger package with 8 pairs.