From: "Tom Faloon"
Subject: Re: ULN2804A Darlington Driver, please help!
Date: Tue, 8 Oct 2002 19:35:15 +0100
NNTP-Posting-Date: Tue, 8 Oct 2002 18:31:40 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
If you are only driving LED's don't worry about the COM pin. You can leave
it disconnected, or connect it to V+.
The COM pin is the common for internal protection diodes, which are required
if you are driving inductive loads.
If you intend to drive an inductive load, such as a relay coil, with ANY
output, then connect COM to V+
You must connect a resistor in series with a standard LED to limit the
current through it. Connect the LED's anode to V+ and a resistor between the
LED's cathode and the driver output. The resistor value will depend on V+,
on the LED volt drop, on how much current you want to flow in the LED, and
on the Darlington's VCE sat (The volt drop across the darlington output when
it is switched on. About 0.9V or less if the current is small.)
To calculate a value for R
R = (V+ - LED Volt drop - VCEsat) / current (in amps)
e.g if the supply is 5V, VCEsat is 0.9V, and you want 3mA in an LED with a
drop of 1.6V, then:
R = (5-1.6 - 0.9)/(3E-3) = 833 Ohms. Use any available value, close to this.
(820R, or 910R it's not too critical.)
LED volt drop varies a bit with current & colour, but is typically in the
range 1.6V to 2.4V
You can get figures, and the maximum current rating, from the manufacturer's
I know you are experimenting, and you are probably aware that the ULN2804A
is a huge overkill if you are only driving LED's. Each output can switch up
to 500mA at 50 Volts. There are simpler /cheaper ways of driving LED's. Very
often a logic gate will suffice. Most logic gates can sink at least 3 or
4mA., some very much more.
Old Lady wrote in message
> I am experimenting with the ULN2804A darlington driver. I want to use
> it to turn on a few LEDs depending on the logical (non TTL) state at
> the inputs.
> This seems a very simple task, but I don't understand how to use the
> COM pin.
> If I connect a LED between V+ and the output of the driver, the LED
> works good, but I don't know if this is the correct way to connect the
> LEDs and how may I use the COM connector pin.
> Any help? Any working schematics to connect LEDs is available on the
> Internet? Google search doen't helped me too much!
> Thank You!