LED Voltage - Page 1
Forum section: electronics / beginners

author: Erincka
author_rank: Newbie
timestamp: September 04, 2021, 01:39:00 am
content: Hello, what are you using to find out the voltage of an LED? I saw a uni-t ut116a but I do not know if it can find the tension of a LEDThank you

author: jeroen79
author_rank: Frequent Contributor
timestamp: September 04, 2021, 01:46:05 am
content: You run a current through it and then measure the voltage across it.As an experiment, do this with a range of different currents and see how the voltage drop changes.

author: Erincka
author_rank: Newbie
timestamp: September 04, 2021, 03:40:54 am
content: Thank you, I'm afraid of grilling I had a 3V LED I put 5V to try she exploded

author: tanveerriaz
author_rank: Contributor
timestamp: September 04, 2021, 03:48:26 am
content: you need a Constant Current LED Tester or use this

author: james_s
author_rank: Super Contributor
timestamp: September 04, 2021, 04:10:03 am
content: A 1k resistor and a 9V battery are good enough for a quick test. To get a really accurate reading you need to drive the LED at the current you intend to use it at but a lower current is safer for unknown LEDs and will give you an approximate number.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 04, 2021, 06:53:57 am
content: Quote from: Erincka on September 04, 2021, 01:39:00 amThe question is why do you need to know that?LEDs are not like incandescent light bulbs, they do not have a voltage to work at. The voltage drop on a LED varies with the current through the LED and with the temperature of the LED. As a wide range, depemding on color, it can be anywhere between, let's say 1.5V-3V. Again, the voltage drop on a LED is not a constant value, it depends of the current through the LED and of the temperature.A LED should never be powered with a constant voltage.A LED should be powered with a constant current, or else the LED life will be reduced.So, why do you need to know the "LED voltage" for?

author: hagster
author_rank: Frequent Contributor
timestamp: September 04, 2021, 07:59:55 am
content: Quote from: RoGeorge on September 04, 2021, 06:53:57 amAlmost every consumer product has LEDs driven by constant voltage and a limiting resistor. I agree for when you are making ultra bright LEDs, but for that low power indicator LED it is fine to put something like a 200ohm resistor in series.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 04, 2021, 08:05:40 am
content: Yes, and that series resistor prove exactly that: never connect a LED straight to a voltage source.

author: mariush
author_rank: Super Contributor
timestamp: September 04, 2021, 09:10:04 am
content: In some scenarios with basic circuits, the resistor can be missing, but in fact there is still some resistance, either introduced by the thin wire between the power supply and the LED or in the battery itself.For example, a 3v CR2032 battery can not supply a lot of current and has a reasonably high internal resistance, so if you connect a basic led directly to such battery, the led is unlikely to be damaged.You can't do the same with a AAA or AA battery, because those have much lower internal resistance... you have around 10 ohms for CR2032, but less than 0.5 ohm for AAA batteries.A LED starts generating light once the voltage goes above some threshold.There's actually two thresholds, one where the LED starts producing very little light but the amount of light will increase as the voltage increases by very small amounts, and once the voltage hits a second threshold the LED opens up fully and accepts any amount of electricity through it and that's where it can burn out if the energy isn't limited by various methods (adding a resistor in series would be the easiest).So for example, let's say you have a red led which advertises a forward voltage of 1.7vFor this led, the first threshold may be 1.5v and between 1.5v and 1.55v the led may generate a tiny bit of light which increases as you get close to 1.55vBut as soon as you hit that second threshold of 1.55v and go above it, the led would open fully and let as much energy go through it as available and that can damage the led.Unfortunately, you can't rely on that very narrow, because that region "drifts" a bit with temperature. A cold (let's say 10 degree Celsius) red led may have that narrow region between 1.50v and 1.55v but a led that's warm at around let's say 60 degrees Celsius may have this region drifted to 1.48v...1.53v.You can use Ohm's law : Voltage = Current x Resistance to limit the current going through a ledSo for example, assuming that red led has a forward voltage of 1.7v and you want to limit the current to 5mA and you have a 5v power supply then you can add these values in the formula :Input voltage - ( number of leds x forward voltage of led) = 0.05A (5mA limit current ) x ResistanceSo from here, you have Resistance = [ 5v - (1 led x 1.7) ] / 0.05 = 3.3/0.05 = 66 ohm - this is not a standard value, so I would go to 68 ohm which means the current will be a bit less than 5mAThe easiest way to figure out the forward voltage of a led (if multimeter can't do it) would probably be to use two linear regulators.The first can be configured to limit the current but don't adjust voltage in any way. For example, you can configure the first linear regulator to allow 10mA to go through, regardless of voltage. I'm choosing 10mA because it's a safe value and majority of modern leds will light up visibly at less than 10mA and only very small surface mount leds can't handle even 10mA.The second regulator you can configure as basic adjustable linear regulator, which will output the voltage you set and will only be able to output as much current as the first regulator gives it, MINUS the amount of current this second regulator uses.Cheap basic regulators like LM317 or *1117 or *1084 will consume 2-5mA internally, so you may choose to bump that 10mA value to 15mA or something like that, to account for the current consumed by second regulator.So basically, you can have a potentiometer to control the output voltage of the second regulator, and you set it to the minimum, for basic cheap linear regulators that's gonna be 1.25v, because that's the voltage of the internal reference. Some more fancy linear regulators can have 0.6v or even 0.4v internal references.Then, you gradually increase the voltage by turning the potentiometer and you'll see the led light up fully once you're over that 2nd threshold, above that narrow region.You can now use a multimeter to measure the voltage on the output of the second regulator.Here's the datasheet of LM317 :https://www.onsemi.com/pdf/datasheet/lm317-d.pdfGo down to page 9 and in the bottom corner you'll see Figure 26, how to configure the linear regulator as a current regulator.It also gives you the formula on how to configure the output current : current = internal reference (1.25v) / resistorSo from there, resistor = internal reference / current ...Let's say I want minimum 15mA, then resistor = 1.25v / 0.015 = 83 ohm so I'd probably use a standard 82 ohm resistorFor the second regulator, the example circuit on the first page of the datasheet can be used, with the mention that I'd lower the R1 value to 100 ohm or 120 ohm, which means the regulator will consume a bit more current (a bit over 5mA) but will be better overall.Instead of a resistor, you can use a potentiometer where R2 is located and that's how you can adjust output voltage.So let's say you use a 1K potentiometer, and a 100 ohm resistor for R1, thenyour minimum voltage will be according to the formula : Vout = 1.25 x (1+0ohm/100 ohm) = 1.25vandyour maximum voltage will be Vout = 1.25 x (1 + 1000/100) = 1.25x11 = 13.75vbut keep in mind the linear regulator can only output up to input voltage - ~ 1v..1.5v ... so if your circuit is powered by a 9v battery, you'll get max 7-8v out.

author: james_s
author_rank: Super Contributor
timestamp: September 04, 2021, 05:32:32 pm
content: Quote from: RoGeorge on September 04, 2021, 06:53:57 amHow else do you calculate the required series resistor to limit the current? How do you know how many LEDs you can connect in series on a given supply voltage? How do you calculate the minimum number of cells for a battery powered device so that the available voltage never falls below what the LED needs to light? The forward drop of the LED is the second most important spec after the maximum drive current.

author: tooki
author_rank: Super Contributor
timestamp: September 06, 2021, 11:21:24 am
content: Quote from: RoGeorge on September 04, 2021, 06:53:57 amYeah, but it’s been the convention for decades to specify LED forward voltage at 20mA. The only exception to this (which wasn’t a consideration until the last 10-15 years) is high-power illumination LEDs (for backlights, room lighting, etc), whose Vf is specified at the stated nominal current or power.With that said, modern indicator LEDs are so efficient that 20mA is blindingly bright. You can always tell when someone blindly calculated for 20mA because you then have those crazy bright blue power LEDs that keep you up at night…