Hi guys. I am new here and new to LED tech, so please pardon my ignorance 
I need to renew some LEDs on a PCB, but need to find out which type to buy.
One of the LEDs is gone, probably over aged (see image url attached). I measured 200mV/6.75mA at the open terminals. Not sure how to tell which type I need, but they are about 1mm wide and maybe 2mm long.
Can you help me with this? Can someone explain to me how to measure these leds properly?
Regards
Dirk
You asked about measuring them. SMD (Surface Mount Devices) with only two terminals (i.e., resistors, diodes, etc.) are measured “length by width”. You’ll need something like a caliper to measure them accurately. (I’m rather fond of digital calipers, as they can be switched between inches and metric easily.)
I suggest that you go to www.digikey.com and search for “LED”, select “Indication - discrete”, and start narrowing down using things like “color”, and “surface mount” (there are a huge number of “filters” available because there is a huge variety of them these days). Once you get down to, say, fewer than 100 results, start glancing through them. (You might also want to check the “In stock” box – sometimes that can eliminate a lot.) Be careful as some “packages” 3,000 or 4,000 of the things. They’ll gladly sell to hobbyists, but they also do a lot of business selling to huge factories.
You mentioned that you measured 200mV “at the open terminals”. That’s not anywhere near enough to light up an LED – so there may be other problems with the circuit.
BTW, I’ve done business with DigiKey since about 1973, and always gotten very good service, both as a hobbyist and in my professional life.
I’d consider replacing all of the LEDs. The new one might not exactly match the color & apparent brightness of the others.
ArizonaClark:
BTW, I’ve done business with DigiKey since about 1973, and always gotten very good service, both as a hobbyist and in my professional life.
You got in early; they were founded in 1972 as a distributor to amateur radio operators. They [employ a whopping 4000 employees in a county that has a population of about 14000.](DigiKey - Wikipedia)
I recall the delight when I first saw their catalog (“scandal-sheet” proportions) that had important details on the parts they were selling (like the dimensions of electrolytic caps), and reasonable prices on first-quality goods. It was a time when most hobbyists were stuck buying parts from places like PolyPaks or Radio Shack, and getting ahold of actual datasheets was on par with getting Donald Trump’s tax returns. You could order parts from the likes of Allied or Lafayette, but their catalog only listed values and max ratings and not the physical size or temperature ranges. (I was a freshman in college at the time.)