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UNL2803A relay driver pin connection

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sean87:
Hi all,

I want to use UNL2803A in my little project. Everything is good so far, but before I really put the relay driver in use, I want to know what is the PIN #10 Task and where should I connect it? In datasheet it is referred as "Common free wheeling diodes".

Also here is a picture of my breadboard, take a look please. The chip on the left is PICAXE 20X2 and the right chip is UNL2803A.

Thanks!

ivan747:

--- Quote from: sean87 on June 26, 2011, 11:24:22 pm ---Hi all,

I want to use UNL2803A in my little project. Everything is good so far, but before I really put the relay driver in use, I want to know what is the PIN #10 Task and where should I connect it? In datasheet it is referred as "Common free wheeling diodes".

Also here is a picture of my breadboard, take a look place. The chip on the left is PICAXE 20X2 and the right chip is UNL2803A.

Thanks!

--- End quote ---

There are some diodes called the flyback diodes that go connected in parallel with your load. In this case your load goes from the positive of your power supply to the ULN2803.

When you have an inductive load such as a relay or a motor, they generate a very high voltage across their coil (in case of a relay, the coil is anything but the switch itself). This high voltage can destroy your ULN2803.

So normally, when you're dealing with inductive loads, you should add a diode like this


But the ULN2803 is nice enough to add them inside it for you


The diodes inside the are connected together and they have to be connected to the positive supply pin to ensure the relays don't fry your chip. The diodes go out on pin 10.

So it should look like the attachment

Bold fonts are given for quick reference and not for yelling at people.
Ivan

Uncle Vernon:

--- Quote from: sean87 on June 26, 2011, 11:24:22 pm ---Hi all,

I want to use UNL2803A in my little project. Everything is good so far, but before I really put the relay driver in use, I want to know what is the PIN #10 Task and where should I connect it? In datasheet it is referred as "Common free wheeling diodes".

Also here is a picture of my breadboard, take a look place. The chip on the left is PICAXE 20X2 and the right chip is UNL2803A.

Thanks!

--- End quote ---

Pin 10 should go to your positive supply voltage,  this places a freewheel diode across each output to snub back EMF from inductive loads such as relays.  Your design will work without a pin 10 connection  but your chip may not last very long.

sean87:
Thanks a lot for nice information. I think I got the idea.

Just one question remains. I have a 5V DC and a 12V DC adapters which powers my circuit. Obviously the 5V is for PIC and some LED's and 12V is for relays.

Now where should I connect the PIN #9 of the UNL2803? Should it go to GND of my 5V adapter or the GND of 12V source? Or no difference?

Thanks again.

ivan747:

--- Quote from: sean87 on June 26, 2011, 11:59:47 pm ---Thanks a lot for nice information. I think I got the idea.

Just one question remains. I have a 5V DC and a 12V DC adapters which powers my circuit. Obviously the 5V is for PIC and some LED's and 12V is for relays.

Now where should I connect the PIN #9 of the UNL2803? Should it go to GND of my 5V adapter or the GND of 12V source? Or no difference?

Thanks again.

--- End quote ---
You must join the grounds of both, otherwise they circuit will be incomplete since the current will not be able to flow through anything connected to the other supply. If you are dealing with LEDs you must add a resistor in series, specially with a 12V power supply. I recommend 1K for 12V and 330 ohms for 5V.

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