Determining LED to replace on PCB - Page 1
Forum section: electronics / beginners

author: m3jala
author_rank: Newbie
timestamp: May 26, 2023, 10:06:15 pm
content: Hi all - I have a question regarding sourcing an LED I need to replace on an PCB. I know the basics of electronics and have been able to successfully make my way around soldering/desoldering individual PCB components before, however I am by no means an expert, and coming here for advice.Basically, I have a unit that costs $300 on which the LED is missing. I know how the PCB is supposed to work, and I know that the LED (at least per function of the overall unit) has 3 states: OFF, GREEN, RED.Here is a picture of the PCB. On the left is what it's supposed to look like. On the right is what mine looks like.The LED appears to be something like this:https://www.digikey.com/en/products/detail/kingbright/AAA3528ESGCT/7319011Or, something like led id 597-7752-007F in this document:https://www.dialight.com/wp-content/uploads/2021/07/DIA_Selector_Guide_SMD_042321.pdfAssuming the Dialight guide is correct, the cathode mark is to the bottom right and pin 1.I tried to test the voltages coming across while the PCB was turned on, but was getting quite confusing results. First, I saw that various pin combinations carried voltage in both states. And I also was getting voltages of 1.2 and 1.26 volts where I understand that typically the voltage for red is something like 2v.I have a fluke 289 meter.My questions are basically this:1. In absence of the actual LED, how would you go about testing the board to determine what LEDs are required. Like I mentioned, I can turn the board on and off, and I know how to get into the RED and GREEN state.2. If I were to source the board, which costs $300 (what I am trying to prevent spending), how would I go about testing the LED without actually removing it from the board, if that's even at all possible.Appreciate your help everyone.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 27, 2023, 12:27:03 am
content: QuoteIt might be that the LEDs are multiplexed or PWMed. If you have a scope, use that to measure the voltage. If not, set your fluke 289 to measure AC volts. If you get an "AC" reading when the LED is supposed to be ON, and it goes away when the LED is OFF, then it is multiplexed/PWMed.Some questions:

author: Manul
author_rank: Super Contributor
timestamp: May 27, 2023, 09:55:04 am
content: Quote from: m3jala on May 26, 2023, 10:06:15 pmBut it's clearly the same PCB?Anyway, where is the big deal? If this LED light is not somehow functionally important (for example, used to illuminate optical sensor), then the exact LED model, exact power or wavelength does not really matter.From the PCB it looks to me that two joint pins are anodes (positive). So this 2 LED 4 pin package is used in common anode configuration. 597-7752-007F which you found looks like correct pinout. If fliped 180 degrees red and green will switch. But it has InGaN green LED with high forward voltage. In the absence of other information I suggest you use old fashioned LED with the forward voltage of around 2V.Quote from: m3jala on May 26, 2023, 10:06:15 pmWith LED removed you will not see expected LED forward voltage. You are likely to see "compliance voltage". Which is near the voltage of power supply. Given of course that the LED drive is activated. It's not clear how you measured. Also the board might use PWM in that case you will not see meaningful results with a multimeter.Answers:1. I would identify component package. Verify that pinout (cathodes / anodes) matches my board. I would not buy a super bright, high forward voltage LED, unless I know that I need it.2. I would power the board. If that's not possible, I would just assume LED are ok. Indicator LED almost never fail. White high power LED used for lighting applications fail. They are often driven hard and get hot. Low power LEDs inside well designed products don't fail.

author: m3jala
author_rank: Newbie
timestamp: May 27, 2023, 06:11:12 pm
content: Going forward with your recommendation for the 4 pin 2 led low voltage version. Thanks for the response.