Grumpy_Mike:
You want to measure the voltage across the LED. You can use the probes any way round as if they are wrong you just get the same number only it shows up as minus.
Yes, I figured that was the case. The important part I'm forgetting is what setting to use on the multimeter.
Safe case is use 5V and 1K resistor. Vf will generally vary from around 2V for Red and 2.7-3.2V for green, blue, yellow, white.
(5V-2V)/1000 = 3mA
(5V-3V)/1000 = 2mA for the others.
Enough current flow usually to turn on the LED and let you measure its Vf.
Go down to as low as 220, 270 if you want it brighter while measuring.
Your mistake was doing experimentation with no understanding of the physics. You're supposed to research a subject first, then design your experiment. Experimentation without prior research will always get you into trouble. You should never operate a led without a resistor unless the power source is a current limited source that cannot exceed the max forward current of the led.
If you were going to experiment , you should have started with at least a 47 ohm resistor in series with each led. That being said, it is a wonder that any of your leds still work.In case you didn't get it, all your talk about ohms law is not relevant because leds are nonlinear.
The resistor calculation only works if you already know the rated forward current at the rated forward voltage. It would appear it has not even occurred to you to try to find a datasheet for the leds you are using to obtain that information.
Wow. So much for the nice welcome I'd been experiencing. Did you intend to be so rude and condescending?
I don't see anything rude in my statement. If may not be what you want to hear, I agree, but I think it is something you need to hear. It is not going to do you any good if you are praised for wiring up leds without resistors.
There is absolutely no reason you could not have waited until you posted with the question :
"I have some leds. I have no experience. I want to wire them up. I don't know what to do . Can you help me please ? "
is there ?
We are not here to make you "feel good". If you were not welcome here we wouldn't be answering your post. I'm just more direct than others. I feel there is no excuse for what you did other than pure inexperience. We could have told you exactly what to do if you had asked. I'm not going to apologize for the way I phrased it. Let's look at the facts. You had some parts but no experience. Instead of posting for help wiring them up, you just started making assumptions and wiring them up whichever way came to mind until you had exhausted all the possible combinations and then and only then did you post here for help. Now you are offended because someone said something that might suggest you might have made some mistakes. There is only one thing that matters here. The truth , the whole truth and nothing but the truth. The truth is you were reckless and hasty. That being said, I wouldn't loose any sleep over iit because it is probably not going to be the last time it happens, and every other noob does the same thing in one way or another. If making you feel like an idiot inspires you to ask first and act later, then I have accomplished something. You're better off checking your ego at the door and focus on the technical details.
How exactly does one go about finding a datasheet on a simple one-off part sold in a tiny baggie with a simple label stuck on?
FYI,
Try googling common blue led datasheet
Next time, if you don't know what you are doing, don't do anything.
raschemmel:
I don't see anything rude in my statement.
"You're supposed to...", "In case you didn't get it...", "It would appear it has not even occurred to you...", "Next time, if you don't know what you are doing, don't do anything.", "If making you feel like an idiot... then I have accomplished something." Don't pretend you don't see the rudeness.
Let me put it this way: Up until your post, I felt welcome and like this would be a great place to learn. You, single-handedly, have made me feel unwelcome and want to leave this forum permanently. Does that make you proud?
raschemmel:
There is absolutely no reason you could not have waited until you posted with the question :"I have some leds. I have no experience. I want to wire them up. I don't know what to do . Can you help me please ? "
is there ?
Sure there is. That reason being, I'm new but I'm not completely obtuse, had done a bunch of research, saw information from someone who really knows what they're doing which led me to an experiment based on what they said, and didn't have any reason to ask the question until the experiment failed and I didn't know why.
Before I did it, I had no reason to expect it wouldn't work. I think from the rest of my posts here it should be clear that I know about wiring LEDs with resistors, I know at least the beginnings of the reason why, I know to apply Ohm's Law to figure out the minimum resistor recommended, I knew enough to do some troubleshooting on the circuit... I am a beginner, but I'm not down at the level of "I have some leds. I have no experience. I want to wire them up. I don't know what to do . Can you help me please ?" And if I was, and I asked that, someone like you would just tell me to go Google it or get a book and at least make a start, then come back when I have a more specific question--exactly what I did.
raschemmel:
You're better off checking your ego at the door and focus on the technical details.
To paraphrase: Engineer, heal thyself.
Somebody, give me a reason not to leave and not come back.
@zenheathen,
You just gotta grin and bear it sometimes. Folks have different ways of communicating, some more blunt than others despite having been asked to tone down the bluntness more than once. Take what you can from the reply, don't let him get under your skin. Others will post more helpful replies with methods to help you figure stuff out.
raschemmel is not an engineer, I am tho and I think more comments should address newbies who obviously unfamiliar with technology, either from them saying so or by the context of the question posed, as if they students - a little help, some suggestions, but no need to stand on their neck while telling them to go read the textbook.
A quick lesson on meters.
measuring across something in a live circuit should only be done
on DC or AC voltage scales.
Always start at a voltage higher than what your supply is and work down
to a good reading that won't full scale.
ohms or diode scales should only be used on powered down circuits.
Placing the leads in a live circuit can damage many cheap meters.
Some have fuses but they may not protect the meter.
Current scales should normally only be measures in series with a
power source ( except for some special cases when measuring circuit
parameters. That right now is beyond the normal uses ).
Placing the leads across a live circuit ( in parallel ) can damage the
meter or if lucky blow a fuse. Always start at a higher scale than expected
and work down.
Print this out and stick it up someplace.
zenheathen:
Somebody, give me a reason not to leave and not come back.
Most of my 'karma points' came from one post suggesting a gentler tone. I wish it weren't such a silent 'amen' system though.
CrossRoads:
...I think more comments should address newbies who obviously unfamiliar with technology, either from them saying so or by the context of the question posed, as if they students - a little help, some suggestions, but no need to stand on their neck while telling them to go read the textbook.
Thank you, Crossroads. I appreciate the note, more than I can say.
dwightthinker:
2. ohms or diode scales should only be used on powered down circuits.
Placing the leads in a live circuit can damage many cheap meters.
Some have fuses but they may not protect the meter.Print this out and stick it up someplace.
Thank you, dwightthinker. That is extremely helpful! I am indeed going to stick that up somewhere so I can keep it handy.
Question about (2): You mentioned "start high and work down" for (1) and (3), but not for (2). I guess for diodes, you just use the setting with the diode symbol, but for measuring ohms (checking the rating of a resistor), it's "start low and go up", is that right?
zenheathen:
Question about (2): You mentioned "start high and work down" for (1) and (3), but not for (2). I guess for diodes, you just use the setting with the diode symbol, but for measuring ohms (checking the rating of a resistor), it's "start low and go up", is that right?
Ohms, it makes little difference. It will do one way or the other
On a volt scale, many cheap meters will smoke if connected to 100V
while on the 200mV scale.
Some high quality meters are protected on voltage scales and
are designed to blow fuses first on current and ohms scales.
Typically the diode drives about 2 volts through a resistor to limit the current
A signal or rectifier diode will measure over scale or around 650 someplace,
depending on the direction. Most DVMs have the correct lead
polarity but many cheap analog meters reverse the drive for diode
and resistance test. It is a test, something like an ohm meter test
but looks at the voltage on the diode instead.
The diode scale may not be useful for LED didoes as the meter's driving voltage
may not be enough.
When measuring diodes in a circuit, do realize that there can be other components
in parallel with it that can significantly change expected reading. The diode
test is only truly valid for diodes with at least one lead pulled.
Dwight
but for measuring ohms (checking the rating of a resistor), it's "start low and go up", is that right?
With ohms it is not so important you can start either way. If the range is too high everything looks like a short circuit and if too low it looks like an open circuit. A lot depends on the meter. If it were a mechanical meter I would start on the high range just so that the needle is not banging against the end stop, but with digital meters it really doesn't matter.
Don't pretend you don't see the rudeness.
There's "frank" and then there's "rude". I think I was "frank".
Let me put it this way: Up until your post, I felt welcome and like this would be a great place to learn.
It is a great place to learn, but it is an forum of experts. Our job is to tell you what you are doing wrong, in no uncertain terms. We don't have to sugar coat it. We are volunteers. We devote our own time to help others. Is there a more polite way to tell you that you that what you did was wrong ? probably, but it is what it is. If you post here you have to be prepared to handle the truth, and that's really all there is to it. Once you accept that, you will learn very fast here. There is no better place to learn (if you can handle the truth).
Enough said about your (hurt) feelings. Trust me, you'll get over it. Like I said, you'll learn more if you just focus on the technical details and don't take it personally. If you leave the forum you are just shooting yourself in the foot. You have it backwards. We are here to help YOU. You aren't here to help US. You want it free but you want it YOUR way ? Really ? This isn't a social media site. It's a technical forum. I made some observations based on your post. You don't have to argee with them but you have to agree I have a right to make them.. If you don't like what you read , you are free to ignore it. If you really had researched led wiring like you say you did you would not have found anything on the net that told you to wire them the way you did. What you did was simply electronics Russian Roullette. You're here because that's where the experts are. You're here because you want to learn, and you just did. Let's just move on.
Somebody, give me a reason not to leave and not come back.
Because it would be dumb...?
Most of my 'karma points' came from one post suggesting a gentler tone. I wish it weren't such a silent 'amen' system though.
I find that hard to believe but if it were true it sort of defeats the whole point of karma points , which are supposed to reflect how many posts you have helped "solve" or how many OPs you have helped with their projects/questions. I earned all my karma points. Sometimes you have to sacrifice your karma points to help the OP change their ways and start implementing a more professional , technical approach. Telling them what they want to hear is not always in their best interest. The objective of a post should be to cause the OP to "think" about what they have said or done and ask themselves " what could I have done differently that might have led to a better outcome ? " As Crossroads pointed out, I'm not an engineer. I'm a technician. We follow procedures and guidelines that have proven to be reliable, one of which is, "if you don't know what you are doing, don't do anything..." That's just common sense and pretty much an unwritten rule in any electronics company. (especially in a wafer fab where an E-chuck costs more than a Ferrari) Without it, there would be chaos. Any technician who goes around trying things without knowing the outcome wouldn't keep their job very long. If I am asked to do something and I don't know how to do it, I start researching the subject and seeking out people who can help me get on track. I simply approach them and say "I need to get this done and I don't know where to start".
FYI,
If you want to monitor your led forward current, you can just connect the cathodes to GND and connect analog inputs to the anodes. Start with a high value resistor (from anode to 5V) like 1 k to be safe, and then reduce it incrementally but not to less than 47 ohms. Read the analog inputs and multiply the analog counts by 0.00488 (V) to obtain the Vf . Subtract the analog voltage obtained (the led voltage) from 5V to obtain the voltage drop across the resistor. Devide that voltage drop by the resistor value to obtain the led current. You can print the led voltage and led current to the serial port and monitor it with the serial monitor.
ie:
Vf= 3.2V
Rcurrent limiting =100 ohms
Iled= (5V-3.2V)/100 = 0.018V = 18 mA