Why are LED drivers output voltage not fixed - Page 1
Forum section: electronics / beginners

author: Lotus
author_rank: Contributor
timestamp: August 08, 2016, 10:03:32 pm
content: My led from China comes with an led driver, which has an output voltage between 11-28V.Why is the output voltage not fixed?In local shops in Denmark, led drivers output voltage is fixed at something like 12 or 24V.The leds seem to be in good quality, but I have my doubts about the drivers, and I would like to replace the drivers with a quality brand.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: August 08, 2016, 10:07:59 pm
content: LEDs are current devices, not voltage. With most devices the proper way to power them is to provide a fixed voltage and let the device draw whatever current it needs. With LEDs it's the other way around, you want to drive them with a fixed current and let the resulting voltage be whatever it needs to be.

author: Lotus
author_rank: Contributor
timestamp: August 08, 2016, 10:12:57 pm
content: So if I powered my led from a bench supply and give it 28V it is likely that I damage it?...the leds accompanying driver says 11-28V...

author: Seekonk
author_rank: Super Contributor
timestamp: August 08, 2016, 10:36:04 pm
content: The LED strings likely have built in resistors. That gives a little margin for voltage. You can greatly overdrive an LED and it will still last a pretty long time. Long enough for you to have lost the sales slip. I see a lot of designs that work but are born to fail.

author: mariush
author_rank: Super Contributor
timestamp: August 09, 2016, 01:03:00 am
content: Each led has a forward voltage which varies with temperature of the led and other factors.By default, it could be 3.2v but when it gets hot the forward voltage could drop down to 3v.This means that if you'd power this led with a fixed 3.2v x 100 mA = 320 mW, when the led heats up and forward voltage drops to 3v then led would still consume 300mW so now the current going through the led is 320 / 3 = 106 mA.A LED drive is supposed to let a fixed current through the leds for this reason and the wide output voltage could mean that the led driver starts with the lower voltage and goes up until the current limit is reached and then varies the voltage to keep the current fixed.So, for example, if you put 5 leds with forward voltage of 3v in series, you'd have a 15v led string, so the led driver would try to reach this 15v while pushing an X amount of current through the leds.

author: Brumby
author_rank: Unknown
timestamp: August 09, 2016, 04:01:41 am
content: The term 'LED driver' has a couple of interpretations on what is actually used to achieve the result - but they all target the current parameter - not the voltage.This is the key:Quote from: suicidaleggroll on August 08, 2016, 10:07:59 pmNormally, an LED drivermodulewill have a target current it wants to supply - and the circuitry can do that over a particular output voltage range. By allowing for a voltage range, the driver module can be used with LEDs that have a variety of Vf as well as the situation where you might run 5 LEDs or 8. So long as the voltage drops for all the (series connected) LEDs being driven (at that current) add up to a number within that voltage range, then all the LEDs will be driven by the target current.(Another type of LED 'driver' is an IC designed to control LED operation. Some of these have a lot more 'intelligence'.)LEDs or LED strings that are sold as "12V" or "24V" will have an arrangement of LEDs with current limiting resistors that have been calculated to giveaboutthe right amount of current for the nominated supply voltage. Because the Vf parameter of any LED can vary, there is the possibility that you can get variation in brightness between one set of LEDs and the next ... but since LED strings tend to contain LEDs from the same production batch, this isn't usually noticeable, let alone a concern.

author: sleemanj
author_rank: Super Contributor
timestamp: August 09, 2016, 05:44:22 am
content: Quote from: Lotus on August 08, 2016, 10:12:57 pmObserve a typical "I-V Curve" plot for some LEDsYou can see that below a certain threshold voltage (the forward voltage) the current is minimal, and above the threshold the current grows at an incredible rate and the LED will gladly "eat itself to death".You might say "ok, well I will give exactly the forward voltage then", and one can simply reply you don't know exactly the forward voltage, it changes depending on conditions (temperature etc), and your power supply is not that accurate, try applying Ohm's law with your LED and just the resistance of your connections and see how accurate that constant voltage has to be to keep things under control (eg, if your power supply was 0.1v over, and you only had say 20 milliohms of resistance (wires etc) between it and your led, boom that's 5A and probably a dead LED).Note that this is for bare leds (or combinations thereof), things like self-adhesive led strips typically have current limiting devices (resistors or other) already on-board.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2016, 07:33:01 am
content: Quote from: Lotus on August 08, 2016, 10:03:32 pmAs mentioned above, LEDs require a constant current source to work. The most important rating on that power supply is the output current, which should stay the same for any load with a voltage drop between 11V and 28V at that current. For example, if the output current is 1A and you connect a 20 Ohm resistor to the output, the voltage will be 20V but if you connected a 15 Ohm resistor, the current will be 15V but its only rated to give a fixed current at output voltages between 11V and 28V so any resistor lower than 11R or greater than 28R would give unpredictable results and might even damage it.QuoteBecause the LEDs have built-in resistors so are suitable for constant voltage operation.Quote from: Lotus on August 08, 2016, 10:12:57 pmYes, if there was nothing to limit the current and you connected the LED directly to 28V then it would be damaged.

author: SteveyG
author_rank: Unknown
timestamp: August 09, 2016, 02:12:45 pm
content: Some LED stuff here: