ULN2803 - this works to light an LED but why!!

Using Arduino General Electronics
#1

So I've been playing round with my Arduino and a ULN2803. Got the motor to spin round.

I was having an issue with a stepper motor so I was investigating how the ULN 2803 works.

I can turn on pin 1 and the LED lights.
I can turn on pin 2 - connect an LED and it lights.

I do not have a connection to pin 10.

When I looked at a motor tutorial, it had a connection to pin 10. Can someone explain what pin 10 is for and how do I use it?

This is all new to me - so please keep it simple.

Thanks

#2

Pin 10 connects the internal flyback diode. It only has any effect when you have an inductive load.

#3

Indeed, this chip is really just a bunch of darlington transistors, no power pin is needed,
the emitters are commoned and this is usually connected to ground, the bases and collectors
are all separate pins (the bases have a resistor network to simplify drive requirements).

The remaining pin connects to the cathodes of 8 diodes whose anodes are each
connected to one of the darlington collectors - purely to facilitate switching inductive
loads like relays which would otherwise need a discrete flyback diode each.

The full story is in the datasheet which gives the equivalent circuit for each channel: http://www.ti.com/lit/ds/symlink/uln2803a.pdf

#4

Thanks. Now looking at steppers!

#5

Oh, Hmm, I did not realize that the Com pin on these wasn't necessary. Are there any situation where you would not want to connect the Com to VCC?

#6

mirith:
Oh, Hmm, I did not realize that the Com pin on these wasn't necessary. Are there any situation where you would not want to connect the Com to VCC?

When the diodes aren't needed and VCC is a long way away?

#7

Only needed for inductive loads, as has been already mentioned. If the loads
are resistive, LEDs, bulbs, then its not needed.

#8

If you are driving LEDS or lights pin 10 can be grounded through a pushbutton switch as a lamp test feature.

#9