LED Lighting Basics

LED strip lights are usually arranged in numerous parallel segments each composed of a few series LEDs, thats why you can cut them to length. Joining strips together should not be a problem as long as you have the current available to drive it. Due to the way they tend to be used in an RV (simply attached directly to the 12V), you should have the current to drive them.

How they work

LEDs are diodes and have a forward voltage specification, lets say it is 3.3V typical. Forward Voltage is the ‘negative voltage’, used by the LED when it’s on. If you have 3 of them in series that’s 9.9V (3 * 3.3), if you drive that with 12V you have 2.1V (12 - 9.9) remaining. If you don’t have a least 9.9V they may not even light (well more like 8 but lets not confuse things to much)

Next is what an LED really wants, that’s current. If we increase the current, the LED will be brighter. Likewise, if you decrease the current, the LED will be dimmer. By picking the correct current, you have full control over how the LED appears (lumen, CRI, color, etc).

In strip lights that current is generally controlled by those little black resistors you see in the strips. A basic 12V strip tends to have one for every 3 LED’s. That resistor is sized to provide the proper Forward Current such that the LED provides the specified output that you likely chose it for in the first place.

So for this example lets say that required forward current is 120mA. We know there is 2.1V (the remaining voltage) across this resistor (at 12V input), so if we want 120mA of current in our loop, 2.1 V / .0120mA ~ 175 Ohm resistor. For the geeky that is a combination of Kirchhoff’s Voltage Law which states that in any ‘loop’ of a circuit, the voltages must balance and Ohms law which states Voltage = Current * Resistance

The assumption here from the LED strip manufacturer is that you are driving that LED strip with a constant voltage of 12V, and the strip then provides the proper current to the LED’s. This would be the case in a normal application where an AC/DC supply is driving the lights. That AC/DC supply will have a maximum current rating with dictates how many strings can be strung together. The strips resistors are proving the constant current in the loops.

So quick recap up to this point. We purchase a strip that has 3 leds’s and 1 resistor in series, The LEDs use 3.3V each (Vf), and the resistor was sized for 2.1V (based on a 12V source input) resulting in 120mA current across each of the LEDs. The LED output (color, lumen, etc) are based on that 120mA

Practical use in the RV

In an rv/automotive application, the voltage is not really 12V, more like 12.7 or 13V in normal operation and well outside of that during certain conditions / operations.

So lets use 13V, the LED’s still use 9.9V, leaving 3.1V across that same resistor, so now the current is 3.1 / 175 = 177mA. On the spec sheet where I pulled this information (one of Cree’s 5050 LED’s), the maximum allowed current is 180 mA, so its just OK in terms of being in spec. That extra current in the resistor is thrown off as heat, depending on what resistors used, some can get quite warm. The extra current in the LED makes them brighter.

Single resistor strips will dim and brighten to the input voltage. The voltages in the RV are also high enough that you will shorten the life of the strips, for an LED thats the number of hours before it drops below, say 80% of its initial light output.

I’ve seen strip lights which have multiple resistors per series loop, these most often specify automotive use and have a 9-14 voltage input range vs a straight 12V. Those are wired slightly different to allow a constant current in the loop across a narrow range of voltages. Those are much better for use in an RV as they will not get brighter or dimmer, in response to the varying battery voltage. They will not have a shortened life due to the higher than 12V inputs, and will not get hot around the resistor. The downside is they will be more expensive, and some RV’s go just by price. They don’t understand they are not using them properly, why they flicker, or why they seem to fail so quickly.

I’ve also seen some with no resistors, those require a constant current power supply to drive them since there is no current resistor in the strip itself. Not as common in a strip light, but very common on puck lights where a buck driver is in place to provide the current across a very wide range of input voltage (9V-30V).

The numbers

Also to note, 5050 or 3528, etc actually refers to the physical size of the LED package, and the specs are dependent on the vendor of that package. Not all 5050 will have the same forward voltages and currents, the numbers above are just ones I pulled from one spec sheet. Thats why a strip of 5050 from vendor A may last and a 5050 strip from vendor B does not, its all how much wiggle room the LED and resistor(s) have to a given the supply voltage.

Anyway LED’s in general are a pretty forgiving bunch which is a good thing since they tend to be abused. You can run them at a range of voltages and currents with the result being how bright they appear and how long they last, running them hot will shorten the life.