LED questions : how to measure an unknown one and more
IceFirePro
Posts: 86
Hello,
This is a specific question for LED's.
Let's say I find an unknown LED and want to use it with BS2 and it's 20 mA output pins. How can I measure it in order to determine what's the LED's drain?
And second related question : If a LED is rated 50 mA at 3 Volts, does this mean that I can run it at 1.5 volts and it will drain less "mA" (I am really hard with this stuff still) but want to be able to experiment with different leds... please help!
This is a specific question for LED's.
Let's say I find an unknown LED and want to use it with BS2 and it's 20 mA output pins. How can I measure it in order to determine what's the LED's drain?
And second related question : If a LED is rated 50 mA at 3 Volts, does this mean that I can run it at 1.5 volts and it will drain less "mA" (I am really hard with this stuff still) but want to be able to experiment with different leds... please help!
Comments
For your 2nd question, LEDs are non-linear (look that one up). Look at a datasheet for any LED and you'll see graphs of voltage across the LED vs. current through them. The voltage across an LED is approximately determined by the material and construction of the LED. Read the Wikipedia article on LEDs for more information and examples.
So, there is no "safe" way to, for example, connect a 3V/50mA led directly to P0-P15, using some resistance?
Mike is also a very smart man. Blue LED's and White ones need more power because of what they had to use to get those colors. If you are smart when you look at these forums and you see his name you will be better off if you read his helps than not.
I have better than a junk box, I have a cubboard. Taped to the inside of the doors is Ohm's Law, Resistor values and Transistor variables. I suggest you get one too.
Casey
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I fish, therefore I lie!
You'd wire the LED in series with the resistor. Try it out. If the LED doesn't light, try reversing it (and make note of that fact).
Measure the voltage across the LED and measure the voltage across the resistor. By Kirchoff's Law, the sum of the two voltages should be 5V (or whatever you're using).
Use Ohm's Law to calculate the actual current through the resistor. By Kirchoff's Law, that should be the same as the current through the LED.
You now have one data point. Try reducing the resistance and measuring everything again. That gives you a 2nd data point. When you have 3 or more, you should be able to plot a curve (and compare the data points to known LED datasheets).
Let's say you have an LED that requires 50mA to produce the brightness you want. What does the PIC datasheet say about the absolute maximum current drain on an I/O pin? On a group of I/O pins?
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Chris Savage
Parallax Engineering
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IceFirePro,
You seem to have a lot of different things going on, and that's ok, that's great in fact. But you should sit back and absorb what you have and what people have been telling you before you proceed.
"You can be a jack of all trades master of none" or you can focus on a particular trade or aspect of something and become the best in the field at it. It's your choice, we can help, but be prepared to dig into the books and datasheets. There is no way around that.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
What I do is to use a adj able power supply and start at (0) volts·and (0) mill amps ·and very slowly turn up the voltage until the led get very bright until it just start to turn to another color then you know what the MAX voltage and what the MAX mill amp are
ONE NOTE about this
I have bought a lot surplus LED in the past and lot of time there are no speck when you buy them this way
I can tell you that I·have burn out some LEDs doing it this way but I now know what the speck are
I do this lot of time when·I buy China made LEDs not all LEDs are made the same way
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·Now wanting to learn Spin· Thanks for any·
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Sam
Post Edited (sam_sam_sam) : 6/18/2010 2:00:56 AM GMT
Today I bought the following three LED's and I would like to know a simple way to make them just turn on/off thru P0-P15 in order to learn some basics (I am afraid to try before asking).
So, the first says:
FW current: 20mA
FW supply: 3.5V (typical), 4.0V (max)
Does it mean I can use it directly with the output pins, since they support up to 20mA and probably I need a 470 ohm resistor?
The second is the same, but FW voltage is 3.2 to 3.8 V.
And the third is interesting and more complicated : it says "7-Color Blinking LED" (I was interested what that is)
On the back it reads
Single color, color-changing or flashing modes
FW current: 35mA
FW supply: 3.2V (Typical), 3.5V (max)
Does it mean that if I connect it at less volts, it will consume less mA? Or; can I connect it via a resistor?
Thanks!
Or better yet, just try something for yourself. Worst case, you blow up a 5 cent LED. There is NO substitute for hands on experience. You'll learn far more faster by doing and failing than asking questions.
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·"If you build it, they will come."
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- Stephen
The only way your BS2 is going to last is if you put it into a drawer, lock it and give your wife the keys. The only way she should give them back is if you go to Radio Shack and pick·up Getting Started in Electronics and read it thru (EVERY PAGE) three or four times. Read EVERY PAGE!!! Do not skip thru to the circuits in the back!!! Electricity is fun to learn and it is not rocket science till you get real technical. Learn and understand Ohm's Law.·Understand what resistors, capisitors and coils are used for. Get a better understanding of current and its relationship to volts. There are a lot of sites on the web that will give you·anything you need to know about electronics for free. You must start at the beginning and pay attention. With any self help·you may be a better electronics person than I may ever be, but first you must help yourself.
Please excuse this, I know it should be·in the sandbox but I am an OLD Fart and got carried away.
Casey
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I fish, therefore I lie!
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Leon Heller
Amateur radio callsign: G1HSM
Buy (or build) a bench supply that has voltage and current displays.
When testing unknown components start at 0V and dial it up watching the current display don't just throw the switch to full voltage. many times I avoid smoke. For example wiring an atmel chip on a breadboard and putting the power to something other than the VCC I see the current rise above a few millivolts and that tells me something is wrong. Throwing the switch would let the smoke out.
I like to use a polyswitch on breadboards. If I know my circuit is suppose to stay under 100mA i use a 100mA polyswitch so I don't fry things if I wired it wrong or have a short.
If you can turn the current up slowly on an led you will see the brightness stop getting brighter when you go too high, the heat is about to destroy the led but you can dial it back. That gives you a good idea wht the maximum current is.
For example, the luxeon 3 watt and the luxeon 1 watt leds are 1000 mA and 350 mA. They look the same but you will see the three watt led get brighter all the way to over 700mA. The 1 watt will taper off well below that.
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
I also use these when I·am·playing·with Lithium-ion Battery Packs
http://www.allelectronics.com/make-a-store/item/RXE-065/RESETTABLE-CIRCUIT-PROTECTOR-0.65-1.3-AMP//1.html
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·Now wanting to learn Spin· Thanks for any·
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Sam
Where do you get the· 100mA polyswitch from please post link
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·Now wanting to learn Spin· Thanks for any·
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Sam
MFG NO RXEF010
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
You need to be careful about what you consider to be a 5V 10mA LED. There are some that have a built-in current limiting resistor or a built-in current regulator. That would be ok to connect directly to a Stamp's I/O pin, but these are more expensive than an ordinary LED and a little harder to find.
It's really basic electronics to learn how to use LEDs with something like a Stamp I/O pin. You need to understand Ohm's Law for lots of things and starting with LEDs is a good place.
http://www.mouser.com/Search/ProductDetail.aspx?R=HLMP-3680virtualkey63050000virtualkey630-HLMP-3680
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- Stephen
Newbies tend to treat LED's as if they were a simple light-bulb as it is -- you're not doing them any favors by showing them this LED+resistor device, and CALLING it an "LED".
“[font=Myriad Pro, Myriad Pro, sans-serif]The 5 volt and 12 volt series lamps contain an integral current limiting resistor in series with the LED. This allows the lamp to be driven from a 5 volt/12 volt source without an external current limiter.”[/font]
[font=Times New Roman, serif]This LED will work with out an external limiting resistor because it was design that way. If you do not have the capacity to evaluate a product and adapt then you have a problem. In this case I was not referring to “most” LED'S it was referring to this specific LED and that is way I post the product. I was only trying to show the OP that there are other options, if after he evaluated the information he has more questions I will try to help because this is what the forums are for. [/font]
Jim