SuperBright LEDs

[Outlaws]

I got these and my receipt has the maximum voltages they can handle.  The guy said they are a lot more suseptible to damage from getting the wrong voltage.

So I was testing what resistors would give what voltage (by puting my DMM on 20v, connecteding the battery black to ground, the red to my breadboard and putting a resistor inbetween the DMM and the red pointer.

Both 10k and 2.7k don't limit the voltage at all.


!?


Whats the deal?

[Outlaws]

ouch....a 1M dropped it to ~8v, and two of tthem in series dropped it to ~7v.

The biggest ones I have are 4.7M.  Guess I might have to look for some bigger ones.   :lol:

[bwanasonic]

What you want to limit is the current. I use around 8k7 for superbright blue LEDs, but you could use a much lower value (1k, but wear shades :wink:) . The resistor is called a *current limiting resistor*, not a *voltage reducing resistor*.  There is a proper way to calculate this value from the specs of the LED, but I'm not the one for that info. I can assure you that 4M7 is well beyond overkill though. Try around 8k -10k.

Kerry M

[zachary vex]

if you're working with 9 volt batteries, just use Ohm's law to figure out what current you'll be sending through your LEDs.  current is all that matters in this case.  you probably never want to send more than 20 mA through any LED, and with bright ones you can limit that to 10 mA for sure, or run it way down at 1 mA for longer battery life.  the formula is E=IR, or voltage=current X resistance.  with a 10k in series with your LED, using 9 volts for your power supply...

9 volts= ? current X 10,000 ohms
9/10,000= .0009 Amps of current, or .9 mA, which is a good starting point.  if the LED is not bright enough, cut the resistance about in half and try again.

[Outlaws]

if you're working with 9 volt batteries, just use Ohm's law to figure out what current you'll be sending through your LEDs.  current is all that matters in this case.  you probably never want to send more than 20 mA through any LED, and with bright ones you can limit that to 10 mA for sure, or run it way down at 1 mA for longer battery life.  the formula is E=IR, or voltage=current X resistance.  with a 10k in series with your LED, using 9 volts for your power supply...

9 volts= ? current X 10,000 ohms
9/10,000= .0009 Amps of current, or .9 mA, which is a good starting point.  if the LED is not bright enough, cut the resistance about in half and try again.

:?:
I don't quit get exactly what all that meant, but let me state better what the guy at the shop said...

They are 20mA LEDs.  My reciept says the max Volatage they can take.

He said that the Yellow ones be killed for sure if the voltage is exceeded.  They are listed at 2v for yellow and green.  Blue is 3.5v.

So explain, if you will, one more time how any resistor that is in the 4k-10k range will cut the voltage to those levels.
I need a "current limiting resistor"?  I will look for those, but my other LEDs  I used before (I forget if they were superbright or not) worked fine with my normal 4k7 resistors that I used for my circuits.  I didn't check their voltage they were getting though... :?

[amz-fx]

I don't quit get exactly what all that meant, but let me state better what the guy at the shop said...

They are 20mA LEDs.  My reciept says the max Volatage they can take.

He said that the Yellow ones be killed for sure if the voltage is exceeded.  They are listed at 2v for yellow and green.  Blue is 3.5v.

All you need to do is limit the current to less than 20ma.

If you don't want to get involved with the math, use my online calculator: http://www.muzique.com/schem/led.htm    

Just enter the power supply voltage you are using, the voltage rating for the LEDs and the amount of current that you want (any value less than 20)  -- the calculator will figure out the series resistor to use to make it all work.

regards, Jack

[birt]

i think you should measure the voltage when there is a led in the circuit.
if you only connect 1 resitor you'll measure the total voltage over that one. if you put in something else in series (like a led) you can measure the voltage over the led and over the resistor. both togheter will be the total voltage again.

[amz-fx]

With the proper dropping resistor, you can power an LED from just about any voltage source...  even the AC power line as shown here:



This is a typical circuit as could be used in a wall outlet tester.

Note: do not play around with the wall outlets unless you know what you are doing as this circuit is not isolated nor current limited except for the resistor.

regards, Jack

[Outlaws]

I don't quit get exactly what all that meant, but let me state better what the guy at the shop said...

They are 20mA LEDs.  My reciept says the max Volatage they can take.

He said that the Yellow ones be killed for sure if the voltage is exceeded.  They are listed at 2v for yellow and green.  Blue is 3.5v.

All you need to do is limit the current to less than 20ma.

If you don't want to get involved with the math, use my online calculator: http://www.muzique.com/schem/led.htm    

Just enter the power supply voltage you are using, the voltage rating for the LEDs and the amount of current that you want (any value less than 20)  -- the calculator will figure out the series resistor to use to make it all work.

regards, Jack

Very cool.

Thanks everyone.

I only keed about a 600 Ohm resistor lol.

[Hal]

you need to be VERY careful about the voltage if you're not using a resistor.  Theoretically, if you put 2v across the yellow one, it _should_ only draw 20ma.  However, if you put 2.1v, it might draw 50ma, and maybe 150ma at 2.2v.  150 ma is enough to kill it in less than a second, if 50 ma didn't already kill it.

SO use a current limiting resistor :-D

[bwanasonic]

I only keed about a 600 Ohm resistor lol.

Keep in mind that the larger the resistor you use, the less current the LED will use, minimizing battery drain. I think you will find the LED will be plenty bright with a larger value (5-10k).

Kerry M

PS. <SMACKS HEAD WITH GREAT FORCE AND EMITS A PAINFUL *DOH!>

I totally forgot this thread:

http://www.diystompboxes.com/sboxforum/viewtopic.php?t=30168&highlight=led+current+limiting

[Outlaws]

I only keed about a 600 Ohm resistor lol.

Keep in mind that the larger the resistor you use, the less current the LED will use, minimizing battery drain. I think you will find the LED will be plenty bright with a larger value (5-10k).

Kerry M

Current limiting right?  I can not use the normal ones correct?

[Jason Stout]

I only keed about a 600 Ohm resistor lol.

Keep in mind that the larger the resistor you use, the less current the LED will use, minimizing battery drain. I think you will find the LED will be plenty bright with a larger value (5-10k).

Kerry M

Current limiting right?  I can not use the normal ones correct?

ANY resistor will try to limit the current it is offered. The "normal ones" are the only type you need for this.

[Outlaws]

Thanks everyone they work great.  And wow ... blue is worth every penny!