Led in circuit ? - Page 1
Forum section: electronics / beginners

author: Nhan95
author_rank: Contributor
timestamp: April 10, 2015, 01:39:22 pm
content: For 2 ways to limit the current through the LEDs in circuit. Which way should we use ? Why ?

author: retrolefty
author_rank: Super Contributor
timestamp: April 10, 2015, 01:43:27 pm
content: First way. Second way does not ensure equal current through leds due to variation.

author: Fat
author_rank: Regular Contributor
timestamp: April 10, 2015, 01:51:45 pm
content: Quote from: retrolefty on April 10, 2015, 01:43:27 pmI agree. Individual resistors on each is a better way.

author: Ian.M
author_rank: Super Contributor
timestamp: April 10, 2015, 01:52:56 pm
content: Up to about 250V or so (depending on the LEDs If rating), either will work with those component values as the 10K resistor cannot pass more than 25mA which a typical single LED can handle.However when you wire LEDs in parallel without individual limiting resistors, one LED is likely to have a slightly lower Vf than the others, so will hog current and be brighter than the others. It will also run hotter, which will further lower the Vf so it will hog even more current. The result is visually unappealing, especially in 7 segment or bargraph displays, and if enough LEDs are paralleled with a low enough single current limiting resistor the result can be catastrophic - thermal runaway resulting in severe over-current and a blown LED, which if it fails open circuit tends to result in a cascading failure burning out more LEDs faster and faster until either all are dead or one stays shorted when it fails.A prudent designer always uses one resistor or current limiting circuit per LED (or series string of LEDs) and avoids parallel connection with a single (or no) limiter.

author: mariush
author_rank: Super Contributor
timestamp: April 10, 2015, 02:00:30 pm
content: The first one is the proper solution, each led in theory receives the same amount of current.In practice, each led will be a tiny bit different, with just a bit different forward voltage, so the brightness of each led will be a bit different... but the difference is so small your eyes won't notice it.So retrolefty's statement is not quite correct but it's good enough. His intention was good, it was sort of a simplification.In the second drawing, no mattery how many leds are there, the whole circuit receives a finite amount of current (the resistor limits everything), so the more leds are there, the less bright they will be. Also, if one led blows up, all the other leds would suddenly get a bit brighter because the current consumed by the blown led now spreads evenly among the other leds. If too many leds blow up, it could happen that the rest of the leds receive so much current they would also burn up and die, and you eventually get this "cascade" effect ending up with all leds being damaged.As an analogy, think of it as having 4 x 25w incandescent bulbs and you force 100w to the circuit. If one bulb blows, then the 3 remaining bulbs will each be forced to consume 33 watts instead of 25w. If they're good enough, they may last a while until one of them gives up and now the remaining two bulbs each have to consume 50 watts.Also as an observation, the resistor value is a bit high but I guess it depends on your power supply and the forward voltage of the led.You use the Ohm's law formula to figure out what amount of resistance you need.Voltage (V ) = Current (I ) x Resistance (R )So let's say you have a blue led with a forward voltage of 3v and you want to run it at 10 mA (0.01 A) using a 5v power supply.Power supply voltage - Forward Voltage LED = Current LED x Resistance5v - 3v = 0.01 x R ===> R = (5v - 3v ) / 0.01 = 2/0.01 = 200 ohm.

author: rolycat
author_rank: Super Contributor
timestamp: April 10, 2015, 02:41:34 pm
content: The second method would be appropriate if you only needed one LED to light at a time.