Current Buffer

The ULN2001A, ULN2002A, ULN2003 and ULN2004A are high voltage, high current darlington PIN CONNECTION arrays each containing seven open collector dar- lington pairs with common emitters. Each channel rated at 500mA and can withstand peak currents of 600ma.

WHERE TO USE:

These versatile devices are useful for driving a wide range of loads including solenoids, relays DC motors, LED displays filament lamps, thermal print- heads and high power buffers in the situations when the input signal is not providing enough currents. E.g. we need to drive a relay or DC motor according to the signal given by some IC.

HOW TO USE:

Suppose we need to drive DC motor and the motor needs a specific amount of current. It’s in the range of 30-100 mA. But such high currents cannot be delivered by simple ICs or parallel port etc. Hence we need a current buffer for that. So we select Uln-2003 in this case.

The pin connection of ULN 2003 is as follow:

The ULN 2003 is an inversion buffer and hence output is taken from pin 9 and the output pin.

If we are driving motor according to the signal from some IC which is not able to provide enough current, then

• Firstly we input that signal to one of the 7 input pins of ULN 2003, say pin 1.
• Then output I taken from the Pin 9 and the corresponding output pin i.e. pin 16 as shown below:

• We also connect a high current power source of appropriate voltage between pin 9 and pin 8.

HOW IT WORKS:

When the input signal to the IC ULN 2003 is HIGH, then the output of the IC at the corresponding output pin is LOW. But if input signal to the IC ULN 2003 is LOW, then the output of the IC at the corresponding output pin is HIGH.

Hence in the above circuit, when input at pin 1 is HIGH, then the DC motor would start running as at the positive terminal of motor we have a HIGH and at the negative terminal we have a LOW (pin 16). But if input signal is LOW, the DC motor would not work as we have HIGH at both terminals of the motor.

LIMITATION: Using this kind of buffer we can’t drive the DC motor in both directions without using much extra circuitry. For performing all 3 functions on motor i.e. run clockwise, run anti-clockwise and stop, we can use another IC L293D which is a bi-directional buffer.