multi-digit LED display question

Thread Starter

jlon

Joined Aug 13, 2009
13
I'm looking at a 4-digit LED display with a datasheet that has me puzzled:

http://www.lumex.com/pdf/LDQ-N513RI.pdf

As shown in the drawing, there are seven segment LEDs pins, and one for DP, then four common cathode pins....

So this part apparently has only 12 pins to drive all the LEDs. This leads to my question: Is it meant to be multiplexed, since there are way too few pins to drive each segment independently? Would I use a 4-way multiplexer and just run each digit at a 15% duty cycle?

Joe
 

Thread Starter

jlon

Joined Aug 13, 2009
13
Thanks! I meant 25%. And sure enough, way down in the bottom of the datasheet it mentions multiplexed.

So are these things designed then to be bright enough at 25% duty cycle to still be nice and bright? Since designed for multiplexing I'd suspect so...
 

hgmjr

Joined Jan 28, 2005
9,029
The trick is to be sure to refresh all of the positions at least 60 times per second. Then persistance of vision will take care of the rest. Be sure to up the current you use. Since you will be multiplexing the segments they can handle a slightly greater current than you would use if you were driving them continuously.

hgmjr
 

SgtWookie

Joined Jul 17, 2007
22,210
Well, the limit of safe steady current is 30mA, and the typical Vf is 2.1v. I wouldn't go much above 30mA, or you'll shorten their life, even at 25% duty cycle.

Use a current limiting resistor on each anode.
To calculate the resistance needed:
R >= (Vsupply - VfLED) / DesiredCurrent
If you were supplying them with 5v, and you wanted a maximum of 30mA current, then:
R >= (5v - 2.1v) / 30mA = 2.9/0.03 = 96.7 Ohms. 100 Ohms is the closest standard value.

If you're driving it with a uC like a Basic Stamp, PIC, etc - many uC's have a limit of 20mA source or sink per I/O pin. You will need to use transistors or MOSFETs to control the common cathodes, and perhaps the anodes as well, depending on how much current you decide to source to each segment.
 
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Thread Starter

jlon

Joined Aug 13, 2009
13
Thanks for all the help, great folks! 100 Ohms is what I came up with also at 30mA, though I wonder about the mention of 10mA on the datasheet -- is that considering that it's multiplexed?
 

SgtWookie

Joined Jul 17, 2007
22,210
The 10mA is where the 2.1Vf typical and 2.5Vf maximum is determined, the peak wavelength and axial intensity.

The more current you run through the LED segments, the higher the Vf will be. The wavelength can change as current changes; usually right when you put too much current through the LED you will see the color change. The intensity of an LED is more or less linear with respect to current flow; the more current you feed it, the brighter it is.

Unless you have a really good reason for running them really bright, try just running them at 10mA to 20mA.
 

hgmjr

Joined Jan 28, 2005
9,029
I agree that there is no reason to use more current than you need to produce a viewable output from the displays in the typical ambient light conditions. That way you will obtain the maximum lifespan from your LEDs.

hgmjr
 
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