Pin 9 is the common of the flyback diodes which should be connected to the positive of the load power supply. Thermal restrictions of the IC would not allow this current to continue for very long. Should all 7 Darlingtons take their maximum 500mA, this ground pin would take 3.5A, requiring good PCB design to manage. Pin 8 is the Ground Pin connected to all 7 Darlington Emittors. There is no single power supply pin as power is supplied to each output via the individual loads. When viewed with pin 1 at the top left, the input for channel 1 is on the left, directly opposite the output pin for channel one, and so on down the chip. The ULN2003A is a 16-pin device designed so that its connections are intuitive, and logical for PCB design. The ULN200x family includes options for being driven by other sources. The ULN2003A is specifically designed for 5V TTL logic input, driving up to 50VDC loads.

It is worth noting that if you are using individual commercial Darlington transistors for a project, they are available in both NPN and PNP configurations and a variety of input arrangements to suit TTL, CMOS, and other input voltages. While nowhere in the datasheets are the devices referred to as ‘matched’, this practice ensures far less difference between channels.

The ULN2003A takes that a step further by having all the transistors made from the same wafer of semiconductor material. It’s unlikely that you will have this case, but there will still be appreciable and problematic differences between your channels.Ĭommercially produced Darlingtons are manufactured in such a way that their gains are within the published range for the device, a much lower range than would be if two discrete devices were used. One channel may have devices at the lower end of their gain range, while another channel may have devices at the higher end of their gain range. This means that, in building your own arrays, one channel may end up with a significantly higher gain even if constructed from the same devices. The effect is compounded when the second device is considered. The gains of the individual devices are always stated as within a range, often quite broad. It is quite possible to construct your own Darlington pairs, or even arrays, but there is a challenge when more than one Darlington pair is used.

WHAT IS A DARLINGTON?Ī Darlington pair is the name for two transistors connected as a current amplifier in such a way that the Darlington gain is the product of the individual gain of each transistor of the pair. It does have the ability to control seven separate single-channel loads, or for outputs to be paralleled to increase current capacity. The integrated circuit (IC) is an array of seven Darlington transistors, with flyback diodes common-cathode connected inside, specifically for driving inductive loads. The reason for this is that it has uses far beyond driving stepper motors with microcontrollers. The ULN2003 has been around longer than Arduino or Raspberry Pi, and even predates the now almost unheard of Picaxe. This specialised IC provides a neat and effective interface between a stepper motor and an Arduino or Raspberry Pi.

Darlington transistor array pnp