Need hardware advice: ULN2003A

[Luciano]

The reason for the parallel connection is clear. Here is a real world example using the "N" package at 60 degrees C:

I have a relay that takes 700mA to close and your duty cycle is 100%. If I am using one output, the maximum current supply is about 450mA and that won't be enough. If I ganged two together, the supply will be about 580mA; still not enough. If I gang three together, the supply will be about 660mA; still not enough. If I gang four together, the supply will be about 720mA; this would just be enough. For a buffer, I would gang five together and the supply would be about 750mA.

As you can see, as you increase the number of outputs used, the overall current increases but the individual current per pin decreases.

The propagation delay time of the ULN2003A is from
0.25 to 1 μs. This means that if you connect more
buffers in parallel, the outputs might not commute
at the same time. When you energize the relay coil this
is not a problem because the current grows slowly
in the relay coil. The problem is when you deenergize
the relay, where you could have a situation that due
to the propagation delay you could have only one output
with a 700mA load which is too much for a single output.
Will that reduce the life of the output or damage the output
driver? I am sorry I cannot answer this question.


(Click to enlarge the picture)

Best regards,

Luciano