In the world of LED displays, “resiliency and redundancy” is a buzz word especially for critical applications such as live broadcasts television, or digital out-of-home–where every minute of up-time is sponsored. The power system, and in particularly the power supply unit (PSU) is an area susceptible to failures and continues to be an area of innovation for designers.
Even with a MTBF (Mean Time Between Failure) of 207,000 hours (MIL-HDBK-217F (25c) on a Meanwell SP320-5–a commonly used PSU–there remains a slight probability of outage due to normal wear and tear, operating in non-lab environments, outlier events, or unpredictable external factors such as a power surge or weather related events.
Most displays on the market today (2013) are not equipped with any smart power system. Thus, if and when a PSU goes down, the LED tiles they drive will inevitably go dark. Most designs allow each PSU to drive 3 to 6 tiles, based on the pitch, tile size and power consumption of each tile. This means up to 6 tiles would go out in a chain of dark line across the screen.
The lack of resiliency is apparent on industry leader Daktronic’s product webpage:
“Fully Sealed, 2-to-1 Power Supplies
We dedicate one fully sealed, internally mounted power supply to every two digital billboard modules, making installation easier and rear servicing more convenient. Your message remains readable even if a power supply fails because the failed power supply only affects two modules. Other LED digital billboard distributors may have a single power supply delivering power to large groups of modules. When a power supply fails, it affects a vast section of a digital billboard and the advertisement becomes unreadable.”
This reveals that if a PSU goes out, 2 LED tiles will be affected on a Daktronics system. This is an advantage over others that drive more than 2 tiles per PSU resulting in a larger area being affected.
With resiliency implemented, any dark spots on a screen can be eliminated. As soon as one PSU goes down, the backup source is immediately activated. This switch over is instantaneous and without any visible delay. The probability of a black out, approaches zero as it’s related to the probability of two or three PSUs going out at the same time. Even in the extreme unlikely event that multiple PSUs go out simultaneously, the feedback and intelligence function will reduce the power draw requirement of the affect screen area proportionally, so the area is dimmed, but never goes completely dark. This concept will be covered in upcoming white papers.
Every PSU is paired with a backup unit. This is the most straightforward approach, but the downside is that weight of each LED panel is increased considerably. Assume there are 4 PSUs per a 3×4′ (1×1.2m) panel and 4 lbs per PSU on average would result in a total increase of 8 lbs per panel. Cost of additional PSU also needs to be factored, in but the benefits should outweigh the cost incurred from down time on the screen in the field.
Another concept that goes hand in hand with “resiliency” is the “feedback” mechanism. As a PSU goes out, the feedback is activated, and the service team will be made aware of it’s existence. The location of the down PSU is also identified for a scheduled, proactive, targeted and swift maintenance event.