replacing led’s

I have a aftermarket 3rd brake light that has some bad LED's in it. It looks like the unit wasn't sealed well and moisture caused a few of them to rust on the inside of the led, taking them out (you can see the black in a few of the ones in the photos below). The light is made up of 40 (5mm) leds with 8 (68R) resistors. My question is how do I know what leds to replace these with? The leds are white in color. Photos below may answer questions if you have any. Thanks for any help.

Going to online LED suppliers with these images will help you find the correct LED required.Jeff

Velocity Custom Home Theater

Mobile Audio/Video Specialist

Morden, Manitoba CANADA

One problem is that it is difficult or impossible to identify LEDs by visual inspection. Other than 5mm white LEDs, it's trial and error. My suggestion is to check track and resistor integrity in case that is the problem, though it's more likely a water-short has blown one (or more) LED(s) in the string(s).    Check how many LEDs in a string (probably 3) and what color they are. Buy supabright LEDs to replace ALL the LEDs in the effected string. If they prove to be too bright, increase the resistor value to dim. Or replace all LEDs, or another string for symmetry. I suspect they are 50mA LEDs but you need to determine their current at a (or various) voltages. Optionally confirm a string's resistor resistance with a DMM. Swap DMM lead polarities and check both reading are the same (they should be). Otherwise assume 68R (Ohms). With +12V or similar applied to the array, measure the voltage across (the measured) resistor, and across one or each LED in that resistor's string. You hence have a voltage for (each) LED. The current is calculated from I = V/R where V is the voltage across the resistor, and R = 68 or whatever you measured. (Your resistor has a gold band which indicates its rated value may have a 5% variance.) If you have a light/Lux meter you may be able to determine their output at that or various voltages & currents and match that against LED specs.

The LEDs they use in that application are CHEAP LEDs. I replace them with Flat top LEDs I buy at Radio Shack. Just don't go over 50mA for an LED, and you can use just about any white LED. They don't even coat the board to prevent water shorts. Make sure the resistors are ok, and you can replace them with a wide variety of white LEDs, or even bright RED ones. The resistor is a current limiting resistor, so an LED that would need or want more current, won't get it anyway. If I see the colors correctly, they're 65ohm resistors, Blue, Green, Black. If blue, grey, black, then 86ohms. You can replace those too, if needed. IF the ones on there are blown, you an jumper a new one in, front or back of the PCB. Desoldering is also not for the beginner. I use a hand-held, spring loaded solder-pult. You don't need anything else really. I only use solder wick when needed. Most small resistors like that are 1/4 watt, which will handle 250mA of current at it's threshold, which would safely run four 55mA LEDs. TO see or take a good guess at how many mA your LEDs are, see how many LEDs are on any one resistor (if you can), and do your math. IF there are FOUR per resistor, then more then likely, they are 55mA. If there are five, they are probably 45-50mA LEDs. Or, count how many resistors there are, then divide that into how many LEDS, then to the math for the mA per LED.

Hmmmm - a few corrections and clarifications. Don't exceed the current rating of the LED. Many are 20mA but supabrights etc can be up to 50mA. (Then there are CREEs etc that can take Amps.) Blue grey black is 68 Ohms (not 86). (See the12volt resistors.) 1/4W resistors can handle 1/4 Watt. That means 250mA at 12V but less than 250mA at higher voltages. (Power = P = VI Watts.) However a 1/4W resistor will handle 50mA provided it's 100 Ohms or less though it's best to leave some overhead.    If white LEDs it's probably 3 per string or maybe 4, but it won't be 5. The minimum resistance in Ohms should be equal to or higher than (V minus n*vL) / iL where: V = 14.4V (or the maximum voltage applied) n = the number of LEDs in series per string/resistor vL = rated voltages per LED at... iL the highest rated current of the LED (eg, 3.3V @ 20mA etc). A lower preferred resistor value may be ok if it's close. Some LED lights have a series diode or more. You could therefore deduct 0.6V per diode from "V" above tho many will ignore that drop - ie, near enough is good enough. There are resistor calculation tools on the web.

BTW - I suggest a conformal coating - eg, Circuit Board Lacquer Spray - for waterproofing.    I fitted a similar strip of internal vehicle stop/tail LEDs to the outside of my vehicle. Rather than try to seal the enclosure I liberally sprayed both sides with PCB lacquer. I've only had it a few months but so far so good - it has been subject to several downpours and my usual rough riding... er, driving.

The fact that you can't go back and edit your posts, keeps TYPOs in. I did mean 68, but I could not go back up and EDIT it, and there is no sense in double posting... If you notice, I said 65 first, if the color was blue GREEN...   Not my fault I can't go back and edit my posts. There is no way it can jump from 65 to 68 with that one color change, it as a TYPO, and I could not go back and change it. I have been doing this for years, I know what works, and I don't have any problems with LED installs, and no one comes back...

See? I did it again, I meant that it can't jump from 65 to 86!! I transposed the numbers, again... Screw this place, and not being able to edit posts, I'm done... Oh, now I get an EDIT button, but only on my post if no one responded yet...

Bingo. You have your required posts and can edit provided it is still the last reply. In someways I prefer that system as it stops "corruption" but I agree it's annoying if you want to correct an error, or add a note to refer to a later reply. However, everyone has the opportunity to Preview their posts. IMO it was obvious to us that you interchanged digits and that isn't a big deal - it can be clarified later. (I too am often keyboard or numerically dyslexic.) But my reply was for the benefit of the OP and readers that may not realise that.   Such simple mistakes can cause protracted confusion. The same applies re a 1/4W resistor taking 1/4A (250mA). That is likely to be VERY confusing especially since you link that 250mA with 55mA LEDs. And a 1/4W resistor at 55mA is only good for a drop of ~4.5V - it'd be no good if there were only 2 white LEDs per string, and it's not far above borderline for 3 white LEDs in a 12V vehicle. And not that I know how you determine from that pic whether it's 1/4W or 1/2W. (The 1/2W resistors I recently purchased look identical to older 1/4W resistors.) Don't worry, I too have been fitting and using LEDs for years. I even explained to one self professed "expert" that 20mA LEDs can take far above 20mA (eg, 200mA at whatever max time and max duty cycle as specified in the spec pdf the HE supplied!), and that he nned NOT add a capacitor to a PWM output that is used specifically for linear LED dimming. In fact after flaming me, he later removed all his embarrassing/incriminating replies (I guess the penny finally dropped LOL!). The latter is one reason I like uneditable replies.