Could use help finding a suitable replacement LED - Page 1
Forum section: electronics / beginners

author: Brad808
author_rank: Contributor
timestamp: November 29, 2022, 01:47:53 pm
content: I have a board with a couple of blown 5mm LEDs on it I'd like to replace. Here are pics of the board:The inner 3 LEDs are white and 2 of them are no good. The rest of the LEDs on the board are fine (they are red and yellow). I swapped all 3 of the white LEDs with some generic ones I got in a kit. I did all 3 so they would be the same colour, but they are too bright compared to the ones I replaced.I'm looking for the best way to find a suitable replacement that won't be so bright. Measuring in diode mode on my multimeter the original LED reads 3.033, the replacement I tried to put in reads 2.587. I then tried to run them on my psu with a 20mA constant current. The original needed 3.612V and the replacement only needed 2.862V.The problem is I don't know what to do with that information. I've never had to try to characterize an LED before. Looking at some specs on Digikey I have no idea how many millicandela, viewing, angle, wavelength, etc. Truthfully I don't need an EXACT replacement, just something within the ballpark that doesn't stand out. Any help is appreciated.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2022, 01:52:17 pm
content: White is the brightest colour.How about increasing the values of the series resistors?

author: Brad808
author_rank: Contributor
timestamp: November 29, 2022, 02:09:32 pm
content: Quote from: Zero999 on November 29, 2022, 01:52:17 pmThat is certainly a possibility as well. I have a small cheap resistor decade like thisC$ 2.44 55%OFF | Programmable Eight Decade Resistor Board 0.1R-9999999.9R 0.1-9.9999999MR (0-10MS) Step Accuracy 0.1R 1/2 W SMD Resistance Modulehttps://a.aliexpress.com/_m0chWKCMaybe I can run that in series with the new LED while testing on my power supply to see if I can find a value that gets me closer to the original.Presumably then I can find the series resistor on the LED board and add the resistance value I find using the decade to the one already on the board and swap in a replacement?

author: Terry Bites
author_rank: Super Contributor
timestamp: November 29, 2022, 03:04:56 pm
content: That looks like a xmas tree star.Choose a viewing angle in the middle of the range, say 40-60'. T-1 3/4 (5 mm)Buy a good high effciency LEDs. Use known brands like Kingbright (WP154 series?), Cree, Broadcom, Rohm &c

author: sokoloff
author_rank: Super Contributor
timestamp: November 29, 2022, 03:21:55 pm
content: If the LEDs are not directly visible in the final product, could you solve this in the optical domain? (Leave the LEDs alone and just insert a diffuser.)

author: Brad808
author_rank: Contributor
timestamp: November 29, 2022, 03:46:01 pm
content: The board is from a pinball machine and sits underneath the playfield. I've attached a photo

author: mariush
author_rank: Super Contributor
timestamp: November 29, 2022, 04:07:59 pm
content: R15 to R22 seem to be limiting the current, and they all seem to be 180 ohmMeasure input voltage, voltage drop on original leds ... V in - (number of leds in series) x Vled = Current x Resistor ... so current is [ Vin - (number x Vled)] / 180Adjust the resistors to lower the current.

author: Swake
author_rank: Frequent Contributor
timestamp: November 29, 2022, 05:23:34 pm
content: Mariush has the correct answer.In practice using the R-decade you have and taking into account the 12-series resistors that are easily available:The 3 values bigger than 180 Ohm are 220 Ohm; 270 Ohm and 300 Ohm. Replace the series resistor of 180 Ohm with your decade set to 270 Ohm and see how that turns out.If it is fine you know you can buy some 270 Ohm SMD resistors.If it is not bright enough try with 220 Ohm.If it is too bright up to 300 Ohm.A touch or two of black permanent marker on the LED's can help too if you don't want to experiment.Or remove 2 of the LED's and scratch the top of the LED with a file to diffuse the light.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2022, 06:13:59 pm
content: Going from personal experience on the difference between brightness of yellow and white LEDs, you'll hardly notice any difference by increasing the value by 20%. You'll probably need to go up by an order of magnitude, so you should try 1k8.

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2022, 07:24:29 pm
content: I would remove the original resistor and tack on some wires to a pot, 5k pot is probably a reasonable choice. Twiddle that until the brightness looks good then unhook it and measure the value, select a standard resistor that is close.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2022, 10:39:28 pm
content: Quote from: james_s on November 29, 2022, 07:24:29 pmIf you add a potentiometer, I wouldn't recommend removing the original resistor, unless you're very careful, because adjusting it the wrong way could blow up the LED.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 29, 2022, 11:26:03 pm
content: Perhaps the reason they original LEDs blew is too much current.Possibly a resistor or other component has failed??And the brightness you are seeing is because of more current than necessary???You can buy frosted LEDs that diffuse light to broaden the viewing angle, You can frost your own LEDs with some fine sandpaper. I do this regularly.