LED current draw, series / paralell - Page 1
Forum section: electronics / beginners

author: Gazz292
author_rank: Newbie
timestamp: May 15, 2018, 07:15:33 pm
content: i'm probably just being stupid here, but i'm wondering about led current draw,I've been playing about with a sequential turn signal kit for my car, bought a cheapo set off ebay, and the supplied flexible led strips were far too dim for me to put in my car.So i decided to just use the controllers and make my own led boards up, the mosfets used on the controller boards are rated for 2.5 amps,So i got some amber led's (decent ones from RS components, not ebay) they have a forward voltage of 2.1v (max 2.3), and say to use 50ma current for max brightness (6000mcd)The voltage the controllers will see varies from 12 to 14.8 volts,So i put 5 led's in series for each section of the sequential bar, used an online led series/parallel calculator that said for the highest voltage (14.use a 100 ohm resistor, and the led's will pull 50ma total from the source.Why only 50ma? i must be missing something, i thought they'd pull 250ma,if only pulling the power of one led, then do i assume that they will be a lot less bright than they could be... but of course running them all singularly off 14.8 volts needs some rather large and hot resistors.

author: Bassman59
author_rank: Super Contributor
timestamp: May 15, 2018, 07:26:16 pm
content: Quote from: Gazz292 on May 15, 2018, 07:15:33 pmAll of the LEDs are in series?Kirchoff's Current Law.Follow the current.

author: glarsson
author_rank: Frequent Contributor
timestamp: May 15, 2018, 07:31:44 pm
content: 5 LED at 2.1V is 10.5V. With 14.8V that leaves 4.3 for the 100Ω resistor. 4.3V over 100Ω is 43mA. 18Ω might work for you.Note that the current will vary depending on the "12V". 83mA at 12V up to 238mA at 14.8V. Probably not satisfactory.You have two options. Reduce the number of LEDs in each string so the voltage over the resistor doesn't vary as much ordrive the LED strings using a current generator set to the required current.

author: Nerull
author_rank: Frequent Contributor
timestamp: May 15, 2018, 08:07:53 pm
content: The same current flows through all elements in series. 50mA will flow through each LED.If 250mA flowed through each LED you would just blow them all up.

author: sokoloff
author_rank: Super Contributor
timestamp: May 15, 2018, 08:13:45 pm
content: Quote from: Gazz292 on May 15, 2018, 07:15:33 pmCurrentis the same for anything in series (as the above posters indicate).Powerfor each LED will be given by the voltage drop across the LED times the current (P = E * I).Power for LED1 will be ~2.1V * 50ma or ~105mW. Power for LED2 will also be 105mW. Ditto the others...You're pulling a total power of 525mW (the "power of 5 LEDs", not the "power of one LED") in the LEDs and a total power of ~14.2 V * 50mA = 710mW, meaning 525 in the LEDs and 185mW in the resistor.

author: Gazz292
author_rank: Newbie
timestamp: May 15, 2018, 09:41:20 pm
content: Thankyou, i think i understand,i'm going to be re-making the led strips using more led's, atm it's just a single row of 3 lots of 5 led's,the sequencer turns the first set on, then the 2nd set, then the 3rd set, all are on for a couple of hundred milliseconds, then they all turn off and start again, giving the impression of the lights pointing in the direction i'm going to turn.... they are powered by the pulsing voltage of the cars indicator/turn signal relay.. which i have no control over it's rate of flashing.the modules can run 5 sets of led's, and i did have that at first, but the last set of leds in the sequence were a mere flicker before the indicator relay shut off power to start the sequence again half a second later, so i removed the last 2 sets.I plan to make the new led bars 2 led's wide, so will be wiring them in series and parallel, i can go down to 4 series (x2 parallel) led's in each set if i use the 4th chanel on the sequencer to get the overall length of the indicator strip i need,90% of the time the indicators will be used with the engine running, hence the voltage will be around 14.8 volts,But i noticed that when they do the yearly MOT test, they test the lights with the engine off... i originally wanted 6 leds per set, but that would make them too dim running on battery voltage alone... and hence if i needed to use them as 4 way / emergency flashers.

author: sokoloff
author_rank: Super Contributor
timestamp: May 15, 2018, 10:13:19 pm
content: Quote from: Gazz292 on May 15, 2018, 09:41:20 pm14.8 is a little high for charging a 6-cell lead-acid battery. I see 14.2 +/- 0.2 or so typically.Quote from: Gazz292 on May 15, 2018, 09:41:20 pmYou might need to program a mode that is constant for the MOT test.I'm not sure if UK vehicle code allows sequential/animated turn signals. If you plan ahead for it, it might be easy to work a constant mode into the design if you need it for the test.

author: Audioguru
author_rank: Super Contributor
timestamp: May 15, 2018, 11:12:35 pm
content: It is illegal to tamper with lighting on a vehicle in North America, especially since you do not have a clue about how to do it. The LEDs probably shine in a narrow angle straight ahead and cannot be seen a little off angle. They might be too dim or too bright.

author: Gazz292
author_rank: Newbie
timestamp: May 17, 2018, 03:14:21 pm
content: yes we are allowed sequential indicators, my insurance company even has a drop down selection in their modifications form online for upgraded/sequential indicators.The led's i'm using atm have a 30 degree view angle, they are more commonly used in those battery powered temporary traffic lights used at roadworks according to the firm i bought them from.i'm going to re-make them using double the number of led's to have 2 rows instead of a single row.. and go from 3 sets to 4 sets that come on sequentially... not sure if to go for larger view angle led's, or angle the 30 degree ones up and down on their upper and lower rows along the pcb.But i still have (and will have) the original 12 volt 16 watt filament bulb indicators in place,these led sequential indicators are added in the reversing lamp housing (with some white led's on the pcb for reverse lamps.. plus the 16 watt reversing lamp bulb still in place that shines around the pcb's nicely)The reverse lamps are next to the indicator lamps but on the boot/trunk lamp housing, where as the indicator lamp is on the bodywork mounted lamp housing (and the reverse lamp is about 6 inches long, indicator is only 2 inches long)And as for battery voltage, the alternator is ECU controlled and can put out upto 16 volts if the battery is very low after a start, hence why the underhood electrical centre/fuse box has a regulator in it to feed the more sensitive modules in the car...So i figured 14.8 volts would the the most normal upper voltage the indicator led's will see, dropping a bit when the battery is fully charged,Mind, the voltage will vary a bit as the car has electric power steering that pulls around 60 amps in use, this causes the voltage to drop to 12.6 if it's held on full lock.. then bounce back upto 14.8 when the steering is released from full lock.

author: Gazz292
author_rank: Newbie
timestamp: May 17, 2018, 04:55:59 pm
content: So i'm thinking,the sequencer boards seem to just pass the voltage through to the led's it's powered on.... within reason.. i think the led's started lighting and sequencing on about 7 to 8 volts going into the board (this was with the supplied flexible led strips)So if i put a buck regulator on the input, set to say 10 volts? this will handle the varying input voltage of 12 to 14.8 or so volts, and if i calculate the resistors for 10 volts, and use 4 led's in series i should be ok?