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Topic: Which resistor to use for blue led (Read 11122 times) previous topic - next topic

Thomas499

I looked up my purchase history on Amazon, these are the two starter kits that I have Starter kit and

Electronic Parts for Getting Started With Arduino book

alnath

seems to be the same kit.

If the led is designed for 12V, you'll have to use a transistor or a fet (included in the kit) to drive it from arduino.
Maybe you should read the book, and find a project in it that uses these "LEDs" (I hope there is one).
BTW, it should work at 9V too (less bright ) 

Paul__B

you get this page :
blue led
and you'll see that there is already an internal resistor, thus it needs to be powered at 12V (not 5V)
Well no, it doesn't.

If you do look at the datasheet, you will see that it is specified for a minimum of 4V, so it just does not shine quite as bright at 5V as it does at 12.

It is presumably - deliberate.  They are "idiot-proofing" the kit, including parts that are difficult to damage by unfortunate mistakes, such as connecting directly across the power supply.  I wonder whether the other colour LEDs are the same?

If the led is designed for 12V, you'll have to use a transistor or a FET (included in the kit) to drive it from Arduino.
Likewise, not really.

alnath

#18
Jan 07, 2015, 07:39 am Last Edit: Jan 07, 2015, 07:40 am by alnath
Well no, it doesn't.

If you do look at the datasheet, you will see that it is specified for a minimum of 4V, so it just does not shine quite as bright at 5V as it does at 12.

right, it was late here and I didn't look at the curves. I .... needed to sleep, I guess ;)

Paul__B

it was late here and I didn't look at the curves. I .... needed to sleep, I guess
That was only 6 hours, 20 minutes ago.  You probably need more sleep than that still!   :D

Thomas499

#20
Jan 27, 2015, 03:49 pm Last Edit: Jan 27, 2015, 03:50 pm by Thomas499
I decided I wanted to upgrade my experience and understanding.

I want to use this blue led and hook it up in series with a relay. I want to do this so that the power must first go through the led, then it will go to relay. Are leds like Christmas lights by the way? The theory is, if the led goes out, the relay will not turn on.

Quote
You need to know 3 things - the rated current for the LED, I, the forward voltage, Vf, and your
supply voltage, Vs

R = (Vs - Vf) / I
I am having a hard time understanding what relay I need. The formula above tells me how to figure out what resistor I need for the led, but after you consider the led with the added resistor factored in, how do you find out how much... What is the energy unit called that is responsible for flipping a relay? Power, current, amps?

Can anyone help me?

raschemmel

#21
Jan 27, 2015, 04:18 pm Last Edit: Jan 27, 2015, 04:25 pm by raschemmel
You seriously need to learn OHM'S LAW.

My guess is that you didn't "blow" any leds and that the "k" in "220 k" was either not there or if it was it was typo and with 9 uA the led was not illuminated leading you to think that it was blown.
If I saw a circuit example showing "220 k" for a current limiting resistor where I have seen "220"
like a thousand times, I would automatically think it was a typo and it is possible my brain would
edit out the "k" and I wouldn't even notice it.

All of your questions are the result of not understanding Ohm' Law. Learn it.

Quote
What is the energy unit called that is responsible for flipping a relay? Power, current, amps?  
AGAIN, read the datasheet for the relay. (or specs on the vendor website, or look it up with Google) It tells you relay current.

Let Vcc = 12V
Let Irelay = 30 mA (0.030 A)
RCLrelay = 12V/0.030 A =400 ohms

The resistor goes in series with the relay coil.
If it is a 5V relay you don't need the resistor.

And BTW,
The word is "ENERGIZE" not "FLIP". At least make an effort to learn the proper nomenclature for
the technology you are "trying" to learn.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Thomas499

Quote
My guess is that you didn't "blow" any leds
I am still learning, but looking back on it, the GreenLed Datasheet on page 5 (or see attachment) says not to bend the wires on the led. The Blue led datasheet doesn't really mention the no-bending warning, but is it possible that maybe the bending is what caused the short? I threw the non-working led away, but I am starting to wish I had kept it laying around.

In the attachment, you'll see a picture of a led in a red box. That is what the led looked like when it quit working. Keep in mind, i'm new to this stuff, so i'm not an expert in troubleshooting. What may be common sense to someone that understands this stuff, isn't obvious to someone who is trying to learn.

Quote
All of your questions are the result of not understanding Ohm' Law. Learn it.
I'm trying to. I'm trying to learn Ohm's Law, how to read datasheets, and a few other things. I haven't been able to find any good self testing tutorials to make sure i'm on the right track, so i'm reaching out to ask people to check to see if i'm on the right track.

I'll tell you what's going through my head right now and maybe you can lead me in the right direction so I can figure it out myself. Looking at the blue led datasheet, i see the Forward Voltage is 14 V. Now the arduino only puts out five, but as alnath mentioned, you can use a transistor or a fet (included in the kit) to drive 5V to 12V, so maybe the led works in a similar way. Truth is, I don't know.

So using one of Ohms many laws, R = (Vs - Vf) /I at this point the formula looks like R =(Vs-14)/I. Now I also think that Vs is 5V. So the formula now looks like R = (5 - 14)/I. I am getting the gut feeling that I may be way off, but then again, the datasheet says the led has a Reverse voltage of 5V so maybe I subtract 5 from 14. Maybe that goes somewhere else. Right now I understand Ohms triangle law, (see attachment) but that's about my limit to what i'm confident in saying I truly understand. That's why I am asking for help.

So as the equation in my head goes(as of now) R = -9/I
I know a 220 ohm resistor is safe. so I think the equation looks something like this. -9/220 = 0.0409 A.

So if i understand correctly (which I'm really iffy of) the resistor I purchase needs to be able to "ENERGIZE" using .0409 Amps. But then again, as you mentioned which I understood prior to asking for help, the relay itself adds resistance. So theirs even more complexity of which I don't know how to approach the issue. You said I don't need the resistor now, so I'm wondering do I take out the 220 ohm resistor, and substitute the relays resistance in it's place? If so, that would change the .049Amp requirements, which is a factor in choosing the relay.

Quote
And BTW,
The word is "ENERGIZE" not "FLIP". At least make an effort to learn the proper nomenclature for
the technology you are "trying" to learn.
I will make an effort to try to remember and cite the "proper term" from now on. Between you and me, in my mind, i'm still having to translate Voltage is water level, resistance is the tap, the stream is the current, and power is the rotating turbine. Please bare with me.




Quote
AGAIN, read the datasheet for the relay.
The relay I will use, depends on the output from the led though. I'm trying to figure out which relay to get.

Grumpy_Mike

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So if i understand correctly (which I'm really iffy of) the resistor I purchase needs to be able to "ENERGIZE" using .0409 Amps.
Your right you are really iffy.

The resistor does not energize anything it limits the current flow.

You energize a relay coil by putting a voltage across it.

This is why you need them with LEDs.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

raschemmel

FYI,
If you really do have a led with a forward voltage of 14 V like this one then you need to find
a power supply that supplies 16 to 24V and use a current limiting resistor in series with the led. It says the
forward current is (Typically) 7.5 mA, so for example :

Let Vcc = 18V  (two 9V batteries in series)
Let Vf = 14 V
Let If = 7.5 mA
Then,
Rcl(Vcc - Vf)/If = (18V-14V)/0.0075 A = 533 ohms
R = 520 ohms.
4V/520 ohms = 7.69 mA

Get two 9V batteries and get some 9V battery clips at RadioShack (or wherever you get those where you live)
FORGET about trying to convert 5V to 14 V or higher. If you insist on doing that , get one of these
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Thomas499

#25
Jan 28, 2015, 06:05 am Last Edit: Jan 28, 2015, 06:09 am by Thomas499
Quote
FYI,
If you really do have a led with a forward voltage of 14 V like this one then you need to find
a power supply that supplies 16 to 24V and use a current limiting resistor in series with the led. It says the
forward current is (Typically) 7.5 mA, so for example :
I'm getting the impression that you think it is unlikely that I have that led, which is really making me wonder if I have the correct information. As alnath mentioned on post #13 That Blue led is suppositively the one that comes in the arduino starter kit Now personally, I'm having a hard time believing it is, solely because I have it hooked up with a brown, black, orange, gold resistor (10k resistor) and it's only being powered by the 5V arduino (and the datasheet says it is suppose to run off of much more power) and that blue led appears to still be much brighter than the red or green ones that are hooked up to a 220ohm resister running off of 5V. Now I get it might be I'm going to quote DraZzy here
Quote
Also, be careful when comparing LED mcd ratings. mcd depends not only on brightness, but how widely or narrowly the light is focused (review definition of mcd, vs the lumen).
that the light may be narrowly focused, but still. This thing is super bright.

Now they didn't give me a datasheet of all the component specs with the starter pack. But the fact that you said If you really do have a led with a forward voltage of 14 V like this one gives me the impression that maybe the led I have is a different model. I'm so confused.

All I want to figure out, is I want a relay to work in series with a led. The bottom line is If the led isn't on (burned out or whatever), I don't want there to be any chance the relay is on. I just don't know what information I need to search for to find the relay that I need. I'd also like to bi-pass the need for multiple batteries, and only use the arduino power supply. I'm only fond of the blue led because that's my favorite color. I'm very well considering using a green one now just to eliminate the confusion in my head regarding the specs of the blue led.

Grumpy_Mike

To have the relay work in seriese with the LEDs is a rearly bad idea. It requires that you know a lot more about your components that you do. I am not sure it is even possible because relay tripping voltages and currents are often totally different from LED ones.

raschemmel

Don,t put them in series You CAN use a relay to turn on the led or put them in parallel with independent
current  limiting  resistors. The question about the led was just that. A led is not suuposed to conduct
BELOW it's forward voltage. I asked the question because you said it was ON at 5V.  The led you threw
away was probably the 14 V led and the one you have running on 5 V is a diffferent one.vThere is more
than one kind of blue led.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Thomas499

Quote
To have the relay work in series with the LEDs is a rearly bad idea. It requires that you know a lot more about your components that you do. I am not sure it is even possible because relay tripping voltages and currents are often totally different from LED ones.
The only components that will be used are a led, and a relay. Possibly a resistor or two if needed. I'm not sure what you mean by voltages and currents being totally different from LED ones. I was under the impression relays are like resistors, you can buy all kinds suited for your needs.

Quote
Don,t put them in series You CAN use a relay to turn on the led or put them in parallel with independent
current  limiting  resistors.
Lets say in theory I had an interest in tinkering with something that very well could hurt me. When the relay is on, the device of interest is hot and tinkering with it at that time is very...... dangerous. I would like to have a way to know for certain if that relay is on. If the led is on, I know the relay is on. If I put it in parallel then in this scenario, the led could burn out, yet the relay would still be on. I unknowingly assuming its safe to tinker when it isn't, and I meet the bad news bears. I don't want this.

So by putting the relay in series with the led, the power must first go through the led, which would insure the led is on. If the led is burnt out, power will not go through the led, similar to old Christmas tree lights, and in turn that relay can not be on.





raschemmel

#29
Jan 28, 2015, 04:10 pm Last Edit: Jan 28, 2015, 04:15 pm by raschemmel
You are STILL not understanding Ohm's Law. If you put two different devices ( a  led and a relay,
EACH with DIFFERENT current requirements ONE of them will NOT get the correct current because the current is the SAME through BOTH devices ! That should be OBVIOUS. There is no way the
led (which , by the way, CANNOT be the one with a 14V forward voltage if it is lit at 5V) and the relay have the same current requirements. That's why they should NOT be put in series. They can be put in parallel but you have to calculate the current limiting resistor for each device because they are not the same. You will need two resistors, one for the led, one for the relay.
Are we clear now ? As I stated, you CAN use the relay to turn on the led or have two relays, one
just for the led. Either way they should NOT be in series. If you put the current limiting resistor
for the led with one end connected to GND, you can use an analog input to measure the voltage
across the resistor. If it is not what it should be, you can shut down the relay.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

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