Advice on a LED sign (VMS) Att. LED experts !

[Morcego]

Hi,
I'm currently researching to design a LED sign. This sign is to use outdoor and as 16x160 leds. The Led pitch is 12.7mm. I'm gonna use constant current LED drivers.

My only question is: Should I use multiplexed design or not? My first choice is to use multiplexed and pulse the LEDS at 80 or 90ma for 0.1ms in a 1/16 duty cycle.

The other approach is using standard wiring and drive the LED's at standard 20mA with no multiplex. The problem is:
16*160 =2560 LEDS * 20mA = 51,2 Amp !!! Of course normally only 40% or 50% of the LEDS are on at a given time, but even in this case we are talking about a 20 to 25 AMPS.
(not to mention the 160 (16 bit) shift registers)

Is there any problem (besides reducing LED life) in usig multiplex? My major concern is brightness.

Thanks in advance all.

 

[sreid]

You are likely to get about the same preceived brightness with a 4:1 over drive multiplexed 16:1. Clipped from a web site;

Driving LEDs – Static Drive and Multiplex Drive

The easiest way to drive multiple LEDs, such as display digit segments, is to drive each LED separately, each with a resistor or current source setting the forward current. This technique is called static drive because the LED current is continuous. Static drive is useful when relatively few LEDs are driven, with the sensible limit being about two 7-segment digits. High efficiency LEDs can be driven to high brightness with 2mA, which is available from the output ports of most microcontrollers.

When a lot of segments are to be driven, static drive demands an uneconomic number of drive outputs, 1 per LED. Multiplex, or pulse drive reduces the drive connections by strobing only a small number of segments (typically a complete digit) at a time. The strobing is done at a high enough repetition rate that the eye perceives continuous illumination. However, the LEDS must be driven at a higher current to compensate for the reduced duty cycle.

An advantage of pulse drive is that the human eye behaves as a partially integrating and partially peak reading photometer. As a result, the eye perceives rapidly pulsed light somewhere between the peak and the average brightness. This means that a low duty cycle high intensity pulse of light looks brighter than a DC signal equal to the average of the pulsed signal. Therefore one advantage of multiplexed operation is an improvement in display intensity for a given average power consumption.

The efficiency of an LED typically rises with forward current, presuming constant junction temperature. This is not always the case however, and LED data sheets should be examined (and compared) carefully when choose the optimum peak current (Figure 11). However, multiplying can often provide 1.5 times the light output from the average drive current of the cycle, compared to the equivalent DC level.

 

[itsmoked]

Morcego: The application can also dictate which method you can use. Pick up any multiplexed display and move it rapidly while looking at it.... You will see a blinking strobe effect on all of the LEDS. This is because the aformentioned eye ball integration is rendered useless in a moving situation. So if this is a sign that is moving or the viewer is moving you may have problems with multiplexing.

If you mess with this to determine whether or not there will be a problem remember that humans have a wide range of integration ability so ask several people for their opinions and then compensate a little extra.

For example, I can see lots of things flickering that annoy the heck out of me that other people cannot even detect.

 

[hrc]

Another approach is using PWM to drive the LED's. It reduces the power requirements.