Question about LEDs in parallel and their current draw - Page 1
Forum section: electronics / beginners

author: skinny
author_rank: Contributor
timestamp: August 26, 2015, 11:58:55 pm
content: Hello All,I have two identical LEDs connected in parallel to an adjustable power supply. The datasheet for the LEDs give the forward voltage and forward current as 3.1V and 20mA respectively. I have the power supply set to 2.9V and the LEDs are pulling approximately 10mA. They are moderately lit up as I expected.Here is where the funny business starts. I would expect that if I lift one of the LEDs out of the circuit that the current being pulled would drop by 5mA (exactly half), but it doesn't. The current drops about 3.5mA. If I replace the LED it goes back to pulling 10mA. If I remove the other LED, it drops in the same manner.The question is, what am I missing? Why does the single LED pull around 6.5mA instead of 5mA?Thanks for any help you can provide!

author: Chris C
author_rank: Frequent Contributor
timestamp: August 27, 2015, 12:08:11 am
content: Did you check the power supply voltage after pulling the LED? Some, especially those intended for higher currents, have difficulty maintaining constant voltage with low-current loads. And since LEDs have such a sharp voltage-current curve once they start to conduct, a relatively tiny change in voltage can result in a surprisingly large difference in current. (And that's why you should not be trying to drive them with exactly their Vf straight from the power supply. Put resistors in series with each one, sufficient that it drops a higher voltage, like 5V, down to the required current.)

author: skinny
author_rank: Contributor
timestamp: August 27, 2015, 12:37:55 am
content: Thanks for the idea Chris, but I checked the power supply voltage upon pulling the LED and it stays true.

author: knks
author_rank: Regular Contributor
timestamp: September 03, 2015, 11:41:57 pm
content: Hmm I'm actually surprised the current through each LED changes at all. Are you saying:- you have 2 LEDS in parallel - each has voltage drop 2.9 V and 5mA current- you have one LED only - voltage drop is 2.9 V and 6.5mA current- same results with another LEDHow precise are your voltage and current measurements?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: September 04, 2015, 12:45:27 am
content: My guess is you're not measuring the voltage accurately enough. You don't care about the voltage being output by the power supply, you need to measure the voltage drop across the LED itself (bypassing the wire running from the supply to the LED, breadboard, etc.) with pretty good accuracy, with the other LED in parallel and without. I bet when you remove the other LED you'll see a rise in voltage across the remaining LED that's responsible for the rise in current. As was said earlier:QuoteThis is why LEDs are not often driven directly by a voltage source as you're doing here. They're either driven by a current source or they're driven by a much higher voltage and use a series resistor to set the current.

author: thejoggingmat
author_rank: Regular Contributor
timestamp: September 04, 2015, 09:06:18 am
content: My best guess is that the LEDs can draw only a current of 10mA at 2.9V.If you look at a typical IV curve for an LED, the amount of corresponding current use increases sharply right before its maximum tolerable voltage. So you might probably want to power them both at ~3.2V, try your experiment again and see.Of course, this is my educated guess as a hobbyist so I might be wrong. But it does sound logical.

author: MikeW
author_rank: Regular Contributor
timestamp: September 04, 2015, 09:20:35 am
content: Quote from: skinny on August 26, 2015, 11:58:55 pmThis is it.With LEDs it's not a precisely linear relationship between forward voltage and forward current.

author: samgab
author_rank: Frequent Contributor
timestamp: September 04, 2015, 10:53:37 am
content: What about a layout like this:Because of that relationship between current and forward voltage in LED's it's best to have a resistor in series with the LEDs or a constant current power supply, as was already mentioned.Play around with this wizard, if you haven't already:http://led.linear1.org/led.wizPS, or this layout: