Need hardware advice: ULN2003A

[sougata]

Hi,

1. The peak cuurent of a single array is 500ma and if your application needs for you can parallel the outputs. Keep in mind not to exceed the overall dispation of the device. Thus it is technically possible to parallel outputs for higher peak current at a lower duty cycle . For constant driving (100% duty cycle) it doesn't make much sense to parallel outputs as it will increase the overall dissipation of the device.

2. When you turn off a relay the voltage collapses much rapidly than the magnetic field in the coil and is manifested as a large reverse peak voltage [read BACK EMF]. This could kill the transistor array as it is likely to exceed the VCEO (collector emitter voltage) of them. So the clamping diodes come handy to eat this up. You can find in the datasheet that the clamping diodes can handle upto 2.5A (Iok). But to complete the circuit you need to connect them to the other pin of the relay coil which is most likely to be the relay supply. Thus connecting pin 9 to the supply completes the figure. The peak current through the clamping diodes would be high but for a short time and would not let reverse voltage buildup (limited to diode voltage).

So Chritopher it is always a good idea to connect the clamping diodes (pin 9). It does not mean essentially that your transistors will fry immmediately but may reduce operation life.

As for personal experience I use the ULN2803 in my designs. They are neat, saves PCB space and reliable. I parallel the ouputs in my LED Matrix design with no problem as the duty cycle is much less. In your application I suggest the use of pin 9 and you can rest assure that for a coil current of 77ma your product will not fail. You can parallel the outputs if you like. Even if you leave your unused input floating the transistors are unlikely to switch on as they have inbuilt base to emitter resistors. But as Luciano suggested it is a good paractice to gnd your unused inputs.