Lighting: Why Constant Current LED Control?

What is Constant-Current technology?

Without getting too technical, it is the most power-efficient way to close-loop the current that the LEDs in each cluster see. More importantly, it is the best and most reliable way to protect LEDs in the changing voltage environment that we find on boats. To maximize brightness and ensure a long life, we want to drive the LED at its design-forward-current regardless of input voltage, and we do not want to over-drive the LED whatsoever. The integrated circuit we employ in our constant-current products can provide this constant-current to the bulb from 10V – 30V. What this means is that the bulb is absolutely as bright at 10V as it is at 30V, and the LEDs are being protected regardless of the voltage.


This is the pencil pusher answer so if you don’t like those, look away now and go on about yer business.

LED strip lights are usually arranged in numerous parallel segments each composed of a few series LEDs, that’s 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.

Now some more. LEDs are diodes and have a forward voltage spec, let’s 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 if you drive that with 12V you have 2.1V remaining.  If you don’t have the least 9.9V they may not even light (well more like 8 but let’s not confuse things too 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 LEDs. 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, let’s say that the required forward current is 120mA.  We know there is 2.1V 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.  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.

For us 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 let’s use 13V, the LED still uses 9.9, leaving 3.1V across the 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 it’s 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.

I’ve seen some 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 differently to allow a more constant current in the loop across a narrow range of voltage. Those are better for use in an RV but will be more expensive.

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.

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. That’s 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, LEDs, 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.

by Steve Cook 2003 U320 40′