do we always need LED drivers to light up LEDs - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 06:44:54 pm
content: hello everyone..I have been dealing with LED strips and its drivers for a while now in many of my hobby projects. What I know so far about LED drivers is that it provide a constant current source which is the recommended method to lit up LEDs. But I was often confused with this constant current concept of leds.for eg. the label on my led driver says that it could be used for a 2A max current output. So does it mean that its a constant current source of 2A??If yes then, a LED strip rated for 2A during normal condition cant draw more than 2A when connected to the driver and thus is definitely a constant current source fro the LEDs. But what if only a section of leds are used which draws much less current than the previous case. Then what is the difference in using a specific led driver since one can just use a suitable dc adapter??So does it mean unless if one is not using a LED strip which draws the max current rating of the driver there is need for a specific driver??

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 24, 2018, 07:04:58 pm
content: You need to think about individual LEDs and their care and feeding.A single LED will need a specified range of current to operate. Too little current no usable amount of light will be generated. Too much and the LED will self destruct. The brightness of the LED will vary over that operating range. The voltage required to sustain a given current will vary slightly with temperature and also depends on the construction of the LED.Put several LEDs in series. The current required is the same, but the voltage across the string will be larger, equal to the sum of the individual LEDs. That voltage may or may not exactly match a given supply. A constant current supply adjusts the voltage as required to achieve this current.If you vary the number of LEDs, it doesn't change the required current. So a constant current supply can be used with strings of varying numbers of LEDs, as long as the resulting voltage is within the supplies design range.LED strings are usually not simple series combinations of LEDs. They frequently are arranged in groups, say three LEDs in parallel, and then strings of these three LED groups in series. When using part of one of these strings it is important to cut them on one of the group boundaries.You can use a standard supply to power an LED string, but you must put a series resistance or other means to limit the current to the desired value.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:14:45 pm
content: Thank you very much CatalinaWOW for your detailed reply. So if I change the length of an led strip from 1m to 2m then a constant current supply alters the voltage to accomodate the new extra meter of led strip to match up the power required for the new extra meter ??

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 24, 2018, 08:58:36 pm
content: Within the limits of the supply, it will adapt properly. The limits are broad, but vary from design to design. I think almost all of them will deal with this level of change.One way to think of it, which is correct for some implementations is:There is a lower voltage required to operate the internal electronics. If the string gets short enough that the voltage across the string is below this it will stop working correctly. If the string gets long enough that the supply cannot generate enough voltage it also stops working.The more common commercial implementations have limits with other causes than the simple case described above, but the end result is the same.

author: Kjelt
author_rank: Super Contributor
timestamp: July 24, 2018, 09:18:44 pm
content: I like to make a difference between a led strip which usually has a resistor in series with a few (3 or more) leds and wants a constant voltage. These are the common rgb coloured or white ledstrips you buy on a reel.And ledstrings which can as CatalinaWOW correctly describes be any amount of leds in series (one string) and multiple of these strings parallel. These can often be found mounted on a pcb or alumium bar.The first one needs a constant voltage powersupply.The second the led driver has two parameters:1) operating voltage range, for instance 36-100VDC2) operating current, sometimes this is fixed (300mA or 700mA) sometimes this can be controlled by mains dimmer or 0-10V or Dali or Touchswitch.In the second case you have to match the driver to the strings. If your string is say 11 white leds it is about 34VDC . If the leds can max do a 200mA current your driver needs to be set at 200mA max output current.If your pcb contains 3 of these strings parallel you have a 11S3P configuration and your led driver has to do 32-36VDC and 600mA.Now as you guessed this is a PITA for driver manufacturers to build a driver for all possible scenarios of string configurations. That is why modern led drivers can be configured, per potmeter or through some protocol like Dali or NFC.One thing you also want to remember is that when leds age their Vforward rises. Not very much but enough to go with 11 leds from 34VDC to 37VDC.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 25, 2018, 03:19:03 pm
content: Thank you everyone for your sincere help and I have understood the concept.

author: IDEngineer
author_rank: Super Contributor
timestamp: July 25, 2018, 06:05:25 pm
content: One more thing: Not all LED's with the same part number have the same performance. There is normal variation within lots, and even more lot-to-lot. That's why many LED's can be purchased pre-binned, where their luminosity has been ranked so you can buy the same dash number and get tighter matching between otherwise "identical" part numbers.Without such selection, done either at the manufacturer or by you, a string of "identical" LED's, whether in series or parallel or a combination, can yield very noticeable brightness differences. As standalone parts this is less critical, but in an array of LED's it can be anything from "don't care" to "what a piece of junk".This isn't strictly related to your original question, but since you're talking about arrays and drivers it could come up as you play with various options so now you're prepared if/when you see an unexplained brightness variation.