Electronics LED voltage and Amps?

Dragonite · 7 Apr 2011

I have a number of different LED:s in my storage box. The problem is that I for some unexplainable reason that I can't remember have throw away the original bag where the voltage and amp are posted. So now I have some small difficulties to know what resistance I should use so that I don't burn the LED:s So is there anyway to determine what volt and amp they are on a voltmeter or something? Amps would be the easiest one but it would be hard to determine voltage I could be wrong though. So anyone that's better on electronics that could answer this?

mvagusta · 7 Apr 2011

I dunno if there is a way to reliably do that

You can give them 1.6v, and slowly turn up the voltage if neccessary until the led turns just begins to turn on. An led's recommended voltage is usually ~.15v above the minimum.

The current however is the hard part... you can give a low brightness led 20ma more than it's supposed to, and it will seem to be very happy, shining brightly... but then it dies a hour or so later, as it wasn't rated for that much current

D-Mon Taurus · 7 Apr 2011

I also think i highly depends are the kind of LEDs using.
If i read correctly and the knowledgeable may correct me if i am wrong. Cause like you i am also new on this and trying to learn as much as possible.
Most LEDs have a voltage around ~3V depending on color, the amps go from 20mA up to 30mA depending on the kind of LED using.
That is forward voltage and forward current. And if i am not wrong i also read youo wanted to use dual color LEDs making it slightly more difficult i guess.

Using the idea Mwagusta gave by starting with 1.5V looking at the minimum and add the ~1.5V to find the forward voltage seems to be a good way to start.

D-Mon Taurus · 7 Apr 2011

One issues is that the voltage of an LED is relatively unimportant; LEDs are current-driven devices. Most LEDs operate at their typical brightness with a current of about 20mA, and almost all will operate from 4.5V or less. So if you use a 9V battery with a 470 ohm resistor, the current will be somewhat less than 20mA if you had a 0V LED, and about 10mA if you had a 4V LED, and the LED should light pretty well in either case. Once you have the circuit set up (9V battery, 470ohm resistor, LED) and working, you can MEASURE the LED voltage with a voltmeter by putting one probe on each side of the LED, and this will give you some indication of how few batteries you can get away with, and how to get a more exact resistor value for maximum output. But in general, one uses a voltage significantly higher than the LED voltage and a current limiting resistor, rather than trying to set up a power supply at the right voltage. And as UziMonkey says, there are a fairly limited set of values for "typical" LED voltages. About 1.7V for read, 2.2V for yellow and yellow/green, and about 3.6 for white, blue, and modern "exotic" colors...
Click to expand...

Found this solutions right after i wrote the other comment hope it helps you in some way.

And to correct myself a little on the above comment. The voltage is depending on the LEDs as well. Old LEDs use about 1.5V-2,2V approx and the new ones can handle a bigger Voltage

Tealc · 7 Apr 2011

This is what I have written down and is good as an indicator when calculating resistance required:-

Red 1.8-1.9v
Yellow 1.8-2.1v
Orange 1.8-2.2v
Green 2.8-3.2v
Purple 3.2-3.6v
Blue 3.2-3.8v
White 3.2-3.8v

These are based on Ultra bright 20mA LEDs. Standard or Indicator LEDs will probably draw less current.

mvagusta · 7 Apr 2011

D-Mon Taurus said: ↑

Found this solutions right after i wrote the other comment hope it helps you in some way.

Most LEDs operate at their typical brightness with a current of about 20mA, and almost all will operate from 4.5V or less. So if you use a 9V battery with a 470 ohm resistor, the current will be somewhat less than 20mA if you had a 0V LED, and about 10mA if you had a 4V LED, and the LED should light pretty well in either case. Once you have the circuit set up (9V battery, 470ohm resistor, LED) and working, you can MEASURE the LED voltage with a voltmeter by putting one probe on each side of the LED, and this will give you some indication of how few batteries you can get away with, and how to get a more exact resistor value for maximum output.
But in general, one uses a voltage significantly higher than the LED voltage and a current limiting resistor, rather than trying to set up a power supply at the right voltage. And as UziMonkey says, there are a fairly limited set of values for "typical" LED voltages. About 1.7V for read, 2.2V for yellow and yellow/green, and about 3.6 for white, blue, and modern "exotic" colors..

And to correct myself a little on the above comment. The voltage is depending on the LEDs as well. Old LEDs use about 1.5V-2,2V approx and the new ones can handle a bigger Voltage
Click to expand...

+1

Much easier to find the correct voltage this way, no variable regulated psu necessary

The only note i'd make to this is that many new leds need 50ma or more to deliver full brightness, which is wear it becomes risky, as low power leds won't last long at 50ma
I guess if you don't go past 20ma for small leds, and don't go over 30ma for 5mm leds, then they should be fine.

Resistor calculator clicky: http://led.linear1.org/led.wiz