go away spammer

Author Topic: ULN2803A COM Pin Function  (Read 3955 times)

0 Members and 1 Guest are viewing this topic.

Offline arixroboticsTopic starter

  • Full Member
  • ***
  • Posts: 119
  • Helpful? 3
    • TalasTronics WCIT KL2008 Fund Raising
ULN2803A COM Pin Function
« on: November 04, 2009, 10:23:50 PM »
Hello all,

Can someone tell me what is the function of the COM pin (pin 10) on a ULN2803A Darlington Driver IC? Here's the datasheet if you need it:
http://focus.ti.com/lit/ds/symlink/uln2803a.pdf

I've seen schematics which connects the COM pin to Vcc, but I don't see why. The drivers can only sink currents, not source any, so I don't think we need to supply Vcc. I've also tried to understand it by looking at the logic diagram, but still can't make any sense out of it.

I've also tried connecting an LED to one of the outputs, and then tried connecting COM to Vcc and also leaving it hanging. Both have the same results; the LED lit.

Then I also tried connecting COM to Vcc, LED's anode to output and LED's cathode to ground. The LED didn't light up, confirming that the driver only sinks current. 

Can anyone help me here? I really need to know this.

Thanks.

Offline arixroboticsTopic starter

  • Full Member
  • ***
  • Posts: 119
  • Helpful? 3
    • TalasTronics WCIT KL2008 Fund Raising
Re: ULN2803A COM Pin Function
« Reply #1 on: November 04, 2009, 10:35:51 PM »
Why do the answers always become obvious only after I've asked other people?  :-[

I've figured it out, with the help from Nuts & Volts for Basic Stamp here;
http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf

Quote
When the devices are used to power inductive loads like
relays and motors, these diodes should be connected to the positive supply that powers
the load.

The COM pin is vital for the suppression diodes to work. I've only tested with LEDs, so that's why there was no problem yet.

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: ULN2803A COM Pin Function
« Reply #2 on: November 05, 2009, 03:19:30 AM »
Hi,

Why do the answers always become obvious only after I've asked other people?  :-[
Because you didn't see p.2 in the datasheet?  ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


Get Your Ad Here

data_list