I'm quite new at electronics so please pardon me for the silly question. To make it really simple, I am building a small board with several features I'd like to monitor at a glance through a few (green or blue) leds on the PCB; some would need to work at 12v, some at 5v and some at 3v.
As you know there are so many brands and models that an idea of a few wide available ones that are successfully used for this goal would be awesome. I don't need them to be super bright, something like the ones usually on the Arduino or ESP dev boards would suffice (well, except for the 12v that is not available on the boards).
You also need to state what you are able to deal with. Not everyone can handle 060tiny smd leds, and of course the resistors to go with them, and they will need resistors to limit current.
iF you are running much current through the LEDs, you need to size the mounting pads because they are the LED heat sinks. Just a tiny light, then small pads are ok.
Hey guys, thanks a lot for the feedbacks! I apologize if it wasn't entirely clear but really, my question was much easier than you think: I simply needed a few LEDs to light up when 12v, 5v and 3v power rails are ok and when a few logic mosfets are switched on (@5v). No need for them to be super-bright or to do anything fancy. Only problem is to find a widely available brand/model that could be soldered by those PCB manufacturing services.
Step 1:
Go to Digi-Key or Mouser and find the cheapest RED LED they have. Make sure they have more
than 1000 in stock. Download the data sheet an post it here.
Beware the brightness!
I had a batch of boards made for a client and used super bright LEDs for a similar purpose; I picked them for low cost and being readily available - I didn't give their selection much attention as other aspects were much more imprtant. Despite only passing about 8mA through them, they were so bright they lit up the lab and it was uncomfortable to look at the board under development. I was teased by the other engineers!
Good day Jim, why RED? I was looking for blue or green ones. Except from this, I will surely do but I am looking at the chinese LCSC catalogue as I'm planning on having them manufacture my PCB. As an example I am considering this:
It is a 3.3v blue led rated 10mA which of course needs the right amount/size of resistors depending on the rail I am monitoring. It is priced US$0.0203 each.
Speaking of resistors, If I am not mistaken:
@ 3.3v should use >= 33ohm resistor
@ 5v should use >= 170ohm resistor (calculated on 5v-3.3v=1.7v)
@ 12v should use >= 870ohm (calculated on 12v-3.3v=8.7v)
Hey Steve, that was exactly what I was scratching my head with. I believe I can do some math about resistors to wire to these little guys but I don't have practical experience of how much mcd is too much to look at for a bunch of simple board LEDs...
The ones I was referring to with Jim are reported to be 180mcd (130 degrees) which should be around 0.6lm... but that still does not give me a sense of what brightness we're talking about.
RED because they all have Vf of less than 3.3V
So now you picked an LED that has a max Vf of 3.3V. That will only be usable at 5V or 12V
So now your question is not as easy as we think.
I designed the keyboards (and display etc) for Xerox office equipment in the early 2000's, we found no way to specify the LED indicators.
The approach taken was based on historical designs and a mock ups were built to show to the 'human factors team'. I demonstrated in bright rooms, dark rooms and with all sorts lighting that the machines may be located. Much discussion, some reiteration and eventually a design considered good by human factors and marketing was built in quantity.
Xerox had teams looking a machine ergonomics and the human interface, this was deemed the best approach.
So, good luck
Do you have any piece of documentation that can help me understand?
In my mind a 3.3v LED connected to a 3.3v line needs a certain resistor to work correctly and that is pretty much it. When I need to connect it to a higher voltage I simply use a higher resistor based on what I know to be the KVL law.
What do you mean when you say that "a LED with Vf of 3.3v is only usable at 5v or 12v"?
To be clear: I need to monitor all of these three circuits on the board.
must have been a fun job Steve! I think I know what you mean with the "human factor team" approach. I see the same needs/behaviors in a slightly different kind of job. Sadly, what I am designing is some "evolved" home project, definitely not Xerox. and I thought it would have been an easy task to find an LED model similar to many others commonly found in small DIY consumer boards.
In the data sheet the Vf is given as min=2.7V and max=3.3V
What that means is that when you buy that LED the Vf could be 2.7, 3.3 or anything inbetween.
So how do you figure out what currenting limiting resistor to use if your supply voltage is 3.3V +/-5%
Now I think I have a glimpse of where you are trying to bring me to!
To be honest I calculated that by simply applying the ohm law as R = V / I meaning 3.3/0,01 but I just noticed it would be 330ohm and not 33ohm.
In this case, at a glance, it sounds quite weird to need a bigger resistor compared to the ones used at higher voltages... so I assume my math is crooked...
Soooo... sending my calculations down the drain, how should I calculate this? Should I grab lower Vf leds to use the same math on all the rails voltages or am I missing something from my equations?
Could not have been too bad as still see former colleagues for beers 