I want to purchase some transistor array ICs, mainly to save space on these rather expensive pre-sensitized boards I'm getting.
I am stuck on one thing: the COM pin.
Base is input from 4017s, Emitter goes to Ground, and Collector pins go to cathode of LED through a resistor. What is COM? I'm assuming its something I can leave "open."
The chip I'm referring to is TI ULN2004AN (https://www.jameco.com/Jameco/Products/ProdDS/700630.pdf)
Thanks guys!
-M

 

If you're operating the 4017's at 5v, then use the ULN2003AN's.
If you're operating them at over 6v, then use the ULN2004AN's.

COM is useful when driving inductive loads; connect it to the +V source for the load for protection of the output transistors.
COM is also useful for a TEST function; connecting COM to ground will sink current from every output of the IC. If you have much of a load on the outputs, then don't use COM for a test function.

Note that the ULN2x0xA drivers are rated for 500mA current sink per output. However, a more realistic limit is 350mA. Beyond 350mA, power dissipation (heat) becomes a problem. If you are sinking that much current, don't have more than 2 outputs sinking current at a time.

 

so COM is like a diode that protects from blowback from relays and such?

 

Yep, each ouput also has a diode connected to COM.
You would normally connect it to the highest positive supply that any load is powered by.

If the array is only driving LEDs or filament lamps, you can also use it as 'lamp test' - grounding it will ground all the outputs via the diodes.

 

Why are questions asked that are answered on datasheets? Don't people look at datasheets?

 

Because datasheets are new to a lot of people. All that information can be intimidating, and people who are new to them just want to make certain they are understanding what they are reading.

 

I forget what it was like when I was a nOOb such a long time ago. There was no internet, just experienced engineers and books to guide me. I got datasheets and databooks from semiconductor manufacturers and carried them home. Now the kids simply download them.

The teachers didn't change over the years since teachers now don't know much like the teachers I had long ago.

 

I try very hard (particularly on this site) to keep in mind that I was once a n00b, too.

Sure, the same types of questions get asked repeatedly, and to people like you and I who have been doing this stuff for decades, much of it is elementary. Not to someone who's never encountered it before. It can take quite a few reads through a datasheet and asking a lot of questions before a person can understand a given part.

Then one fine day, the "brain light bulb" goes on, and it starts making sense.

 

To Audio Guru :
alright calm down. I'm not an engineer I have HAD NO formal training in electronics. I probably do more trail and error in the studio with this stuff than alot of other trollers on this site so don't tell me its my own laziness that I ask these questions, I just find data sheets very cryptic. Maybe I can't decipher every thing on the pdfs but I bet i'm doing things alot more creative than you in the long run.
Peace,
M

 

Woah... How do you put down someone in a post that ends with "peace"?

It doesn't matter who is more creative.. jeesh. I cant even read. I use voice recognition, so I dont read datasheets either. and Im cooler than ALL yous.

Ok, I do read datasheets, My problem is, I read too many and everything gets jumbled.

I think asking questions is a very good way. But I think you should still have a printout of the datasheet in question, so you can figure out how the answers you receive here could be extrapolated from the datasheet.

Once you learn the way of the datasheet, young Skywalker, you too can be someones Yoda.

 

I was asking a question ABOUT the data sheet, so yes I did have it in front of me. I have saved tens of data sheets onto my hard drive that I have printed out and have on my studio wall. It just really burns me up to think that people assume someone is trying to get some easy way out by asking a simple question. Isn't that what this forum is for?

 

Where can I find a forum where we can talk about fairly complicated circuits and without nOObs asking the most basic questions?

 

what is your definition of fairly complicated circuits? I working to build circuits that drive home made synthesizers and light arrays using LFSR driven multiplexers. It gets fairly complicated. I am not jumping for joy when I make a blinking led you assuming cartoon of a duck. I have an idea for you : let people learn at their own pace and don't be so arrogant as to judge them for it.

 

Why are little kids always rude?

 

a) I'm not a child (you obviously enjoy your saturday morning cartoons more than I do).
b) I am responding to quite a blatant display of rudeness on your part.
I'm not going to continue this conversation. I actually have to leave the house and go see humans, novel idea huh?

 

C'mon guys, let's play nice.