Using LEDs and resistors

Question

I'm about to start playing around with some LEDs, making things and whatnot. When connecting several together it seems from what I've read that it's better to connect several resistors, even one per LED, than just one for the whole lot. Is this right? What difference does it make?

For example, using http://ledcalculator.net/ I put 9, 3.3, 20, 8, into the boxes but I want to make a long string of lights and can't quite see how to do it...

Answer 1

Your best bet is to use 800 to 1k ohm (assuming 12V+) to feed each red LED. That will draw about 20mA per LED.

Answer 2

LEDs give off light based on the current through them.  LEDs have small differences in voltage accross them at the same current.  The voltage vs current curve for LEDs is quite steep.  SO... if you have several LEDs in parallel it is likely that each will want a different voltage to operate at their design current.  Individual resistors will solve the problem.

Answer 3

It is a function of the applied voltage and current used by the LED, and how they are wired, series or parallel, that determine what resistance is needed.

Check the specs of the LEDs you will be using.

In general red and orange consume 20ma and will reach this point with 1.5 to 1.7 volts applied, although some will get to 2 volts. Yellow and green requires a bit more current and will run at around 2.5 volts. Blue and white will use 30ma, sometimes more, and will achieve that at about 3 volts. Infra-red generally have a maximum of 1.5volts at 20ma. Ultra-violet will function like the blue ones although run as high as 3.5v.

Three red will run easily in series from a 5 volt source.

You need to be more specific, how many, what color, high brightness or standard, get the specs, and you may get a more appropriate answer.

There is no standard resistor value, it depends on the supply voltage. Use Ohm's law.

Say you are trying to drive a yellow LED from a 12v source. The LED draws 25ma at 2.5v. So you need to drop 9.5v. 9.5v / .025A = 380 Ohms.

Remember with series circuits current is equal at all points. 2 or more LEDs will draw the same current as a single one.

Also remember if you are doing this with a vehicle that the battery produces 12v while the alternator produces around 14.

You could build a driver circuit with a transistor or two, that's the best method.

Answer 4

As the purpose of the resistor is to limit the current flowing through the LED (so it doesn't kill itself) you can use one resistor if all the LEDs are in series.  If they are in parallel however, you will need a resistor for each.  Putting them in series will use less power.

If you are unsure of the current limit requirement, 20mA is safe but you may not get full brightness out of them.

http://alan-parekh.vstore.ca/product_inf...

These LEDs look the same as you describe and say 30mA continuous forward current.  Try that.

Hope that helps and have fun!

p.s. --Sorry that 30mA is a MAX value - try 25mA instead.