Recommended current limiting resistor for a 2.3inch 16 segment LED display

[adrianpcb]

Hi guys,
I'm working on a 16 segment LED display from Kingbright (PSC23-11SRWA) with the following characteristics
Forward Voltage = 3.9V @ I = 20mA

My Power Supply Voltage is 24V.

For a single LED segment, the limiting resistor value would be as follows

R = (24V - 3.9V) / 20mA = 1.00kohms, correct?

What about all the segments together?
I saw a similar board with the limiting resistors for each segment as (red,red,red,silver) which is 2.2Kohm. Any idea why we would use such a high value resistor?

My question is, what is the current limiting resistor I should use for this 16 segment LED display?

Thanks

 

[D.A.(Tony)Stewart]

You ought to consider driving it from 5V and subtract the driver drops to leave a resistor drop with 20mA and use Ohm's Law.

24V would be a waste of power.

 

[adrianpcb]

Hi SunnySkyguy,
That would make sense, however, as the incoming power supply is already 24V, I can't change that unless I were to add in a voltage divider.
By the way, from what I saw on the existing board, each LED segment has it's own current limiting resistor (2.2kohm).
From my calculations, I should be using a 1k ohm resistor~1.1kohm but I'd like to know why would someone use a 2.2kohm resistor?
The board is actually the LED display board used to display the floor number in an elevator/lift.
Thanks,

 

[IanP]

Test, if the brightness of LEDs at 10mA (2k2) is enough. If it is, you don’t have to drive them with 20mA (1k) …
:wink:
IanP