Need hardware advice: ULN2003A

[Christopher4187]

The example was to provide you an explanation of using parallel outputs on the ULN2003A, not to select a better device to do the job.

[sayzer]

Ok. I see.

So what did you do?

[Christopher4187]

To go back to the origninal question I had, I wanted know if using a ULN2003A would OK without using a clamp diode to suppress the flyback voltage on a relay that needs a current of 77mA to close.

The concensus is, for the most part, I should be OK. The only correction I need make is to connect pin 9.

In addition, to be sure of myself, I will set the PIC up to cycle the relay every second and let it run for a week. The cycle of the relay is something like 6-8 times per day and if I let it cycle for one week, it should get something like 300,000 cycles. I think if it passes that test, there should not be a problem with reliability.

[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

[sayzer]

Really interesting stuff I have been learning ....

[Christopher4187]

@Luciano,

If your field is large, I guess it could. The problem is that the clamp diodes within the ULN2003A are most likely not matched and you are forcing that large field into one diode for a very short time. Maybe one way to mitigate that problem would be to put discrite resistors on each output to ensure the current does not go above an acceptable level. I don't have formal education to confirm this but it sounds like it may work.

[Christopher4187]

But why would TI say you could gang together outputs. I mean, it's in the data sheet.....should we believe everything we read? :-)