Originally Posted by lornaschinske
I have a question about the LED strips... when one goes out, do they all go out? Or do they stay lit? Other question... When you have one led go out can you replace it or do you wait unitl enough burn out and replace the whole thing? . . . . .
1. It depends on the design - most likely LED assemblies designed for efficient 12-volt operation will lose 3 or 4 together, but not all of them at once.
2. It is unlikely you can change an individual LED. If they are soldered onto a circuit board, anyone who can do that type of work should be able to change one. But rope/strip light strings are likely to be permanently "potted," requiring you to put up with a few LEDs out until you decide to change the entire unit.
LEDs run on very low voltages. Typical indicator LEDs are 2.1 volt, .02 amps maximum current each. That makes them 0.042-watt units. A single indicator circuit designed to run at full brightness at 14.1 volts might have a simple 600-ohm resistor in series to limit the current. That means the resistor draws 12.0 volts and the LED draws 2.1 volts at 0.02 amps. The resistor burns 0.24 watts as heat in order to protect the 0.042-watt LED, and uses 85% of the power drawn. Producing heat instead of light is the main complaint with incandescent lamps.
Let's say instead we have 5 LED indicators in series. They cumulatively draw 0.02 amps at the sum of 10.5 volts. But the resistor would be only 200 ohms to work at 14.5 volts. That means the resistor draws 4.0 volts and the LEDs draw 10.5 volts at 0.02 amps. The resistor now draws 0.08 watt, and the 5 LEDs draw 0.21 watt, or 0.29 watts total. By comparison, 5 single LEDs on 5 individual resistors would draw 1.41 watts. The series string uses about 80% less current from the supply to have 5 LEDs at full brightness.
The problem with using only resistors to limit current is that changes in voltage as the batteries charge and discharge change the amount of current the resistors allow the LEDs to pass. And LEDs usually only burn out on over-current, not old age.
I used to design 12-volt indicators on projects using 1200-ohm resistors, so that each LED had 50% current at 14.1 volts, and 100% current at 26.1 volts. That way, I did not have to worry about indicator burn-out. I was concerned with equipment status, not illumination.
Now let's look at the bane of LEDs, supply voltage fluctuation.
In the first example, if the voltage rises from 14.1 to 16.1, the voltage dropped by the individual resistor rises from 12.0 to 14.0, which results in 0.233 amps drawn, or 16% over-current. In the series string example, if the voltage rises from 14.5 to 16.0, the voltage dropped by the resistor rises from 4.0 to 5.5 volts, and the current provided rises to 0.0275 amps, which is 37.5% over-current. If one of the LEDs can't take that it burns out, and all 5 LEDs go dark when the circuit opens.
On the other hand, If the supply batteries discharge to 10.6 volts, the current to the single LED in the first example drops to 67%, but the string current in the second example drops to 02.5% and the five LEDs appear to be out. The less power resistors consume as current limiters, the more changes in voltage affect the LED operation.
There are special LED driver chips made, which are as complex as a miniature computer power supply. As a back-yard design for efficient lighting, I would probably use a common 9-volt regulator chip, and wire indicator LEDs strings of 4 in series for 8.4 volt operation. Each string of 4 would use a 30-ohm resistor in series to limit current to 0.02 amps by dropping 0.6 of the regulated 9.0 volts. The LEDs should have the same maximum brightness as the supply voltage is varied from the minimum needed by the regulator IC (probably about 10.5 volts), up to the capacity limit of the regulator IC, maybe 30 volts.
When you buy an LED taillight, flashlight, or rope lights, you have no clue how simple or sophisticated the current limiting circuitry is. That means you don't know if they will go 'pop' if you have a flakey alternator, or if they will be happy with any voltage you throw at them.
I have used indicator LEDs in my example, because I have worked with them, and they are fairly common. Lighting LEDs are being made now drawing several watts, some have special cooling heat sinks (think police/fire light bars) and special regulators. While 2.1 volts is common for indicators, LEDs in various colors might draw somewhere between 2 and 5 volts each. (Series strings should be made up of identical LEDs.) But the circuit design principles are the same. Having LEDs in series adding toward the supply voltage saves power, but requires more sophisticated current regulation.