Reverse wiring LED Sign

[powerplay]

An commercial building LED Sign required an connection to the LED Driver already on site. if the connections to the sign were incorrect, would there be any damage? ...I was under the impression if the 12 volt DC Driver was connected wrong, the Light Emitting Diodes would simply not work.... any possiblities of causing damage?

 

[GoldDigger]

An commercial building LED Sign required an connection to the LED Driver already on site. if the connections to the sign were incorrect, would there be any damage? ...I was under the impression if the 12 volt DC Driver was connected wrong, the Light Emitting Diodes would simply not work.... any possiblities of causing damage?

Sure, depending on just how the LEDs were wired and what the applied voltage was.
A single LED with the correct voltage applied through a linear current limiting device, hard to cause any problems. But a sign with lots of diodes in an array, possibly in series parallel combinations, maybe. A lot depends on whether the "Driver" is really a driver, intended to connnect directly to the LEDs or actually a power supply to active electronics in the sign.

 

[gar]

131103-1918 EST

powerplay:

You need to understand basics.

First, start with any diode. A diode is basically a one way device relative to electrical current with respect to certain limitations. Ideally in the forward direction it has zero resistance, and infinite resistance in the reverse direction. The reverse direction is when a plus voltage is applied to the cathode relative to the anode.

In the real world there is a finite non-linear forward resistance, and for many diodes nearly infinite resistance in the reverse direction up to a breakdown voltage. In the reverse direction you must stay adequately below the maximum rated voltage, In the forward direction current must be below a level that would overheat the diode junction.

LEDs have a forward drop of a volt to several volts under normal operating conditions. Reverse breakdown may not be higher than 5 V.

Yes, it is important how you connect an energy source to an LED.

This is not the whole story, just the beginning.

What you call an LED may be an assembly of many individual LED devices, and electronics, or at least series resistance to limit current. You need to know much more about the thing you call an LED, and what you call the driver.

More later.

.

 

[gar]

131103-2045 EST

powerplay:

To continue.

I use a high brightness LED for a pilot light. With a 25,000 ohm series resistor and 9 V DC applied to the series string the LED voltage drop is 1.7 V. This means the forward current is (9-1.7)/25,000 = 0.3 mA.

If the series resistor can tolerate the power dissipation, then with 120 V DC applied the current would be about 5 mA. I don't remember the reverse voltage rating, that doesnt't matter, but suppose actual breakdown is 15 V, and I apply reverse polarity, then LED power dissipation would be about 15*.005 = 75 mW. If this is too much power for the LED then it would fail.

To avoid this problem I could just put a regular diode, like a 1N4148, reverse biased across the LED. Now with a high enough power rated series resistor I could probably apply 360 V peak AC or DC to the series combination. The resistor would need to be at least a 360*0.015 = 5.5 W rating.

Suppose your LED sign to be added has its own built-in current limiting and is expected to be driven by a DC voltage source. If this has no internal means to protect against application of reverse polarity, then application of reverse polarity may cause damage. If a simple series input diode is included, then a reverse polarity up to the reverse breakdown of the series protective diode could be applied without damage. If a bridge rectifier was at the input, then either polarity of DC or an AC input could be applied and the sign would be protected against reverse polarity, and the sign would function.

You need to find out the limitations of the devices, and how they work.

.