ULN2803A

The ULN2803A includes 8 darlingtons.

How much LED's, 20 mA, can i connect to one of the eight darlingtons when ...

1) i connect the LED's in a row ? 2) i connect all LED's parallel to each other ?

Each driver can switch 500mA but the total driven at any one time is about 650mA. http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html

So:- 650 / 8 = 81mA per driver. At 20mA that will be 4 LEDs per driver for LEDs in parallel (with separate current limiting resistors).

In series the current is the same you still have about 80mA but the number of LEDs is limited by the forward volts drop and the external voltage. For 12V and 2V forward volts drop across the LED you might think you could have 6 but there would be nothing for the current limiting resistor (one only). So I would reduce this to 5 LEDs and calculate the resistor value based on a 2V drop.

When i connect 4 Led's in a row then i must use 15V and be able to support 80mA.

When i connect them parallel then i can use 3 V and must be able to support 80mA.

The only difference between the two ways is the needed voltage but not the mA ? Correct ?

When i connect 4 Led's in a row then i must use 15V and be able to support 80mA.

No - 15V and 20mA

The only difference between the two ways is the needed voltage but not the mA ? Correct ?

No as you use more voltage when they are in series you will need less current.

Ooops, i have mean 12 V. I had talked about only 4 Led's.