Hi all,

I have searched and found some info but still need clarification on how I should wire this circuit.
I have a 9v battery and i want to power 5 leds (2 blue 3.2v 20ma 1green 3.2v 20ma and two red 2.1v 20ma)
is this the correct way to do it?

 

Also would this led circuit work?

3v power 3 blue 3.2v 20ma and 1 red 2.1v 20ma with a 47 Ohm resistor just on the red led all wired in parallel

 

Your R values seem useable, I came up with 290 = 300 ohm, & 135 = 150 ohm, but who really needs 20 mA.
Second pt. Will work but not suggested. A CR2032 Li., 3V, lights a white LED quite well. Used often in white or blue as throw away key hole lights.

 

Most LED's do not need the 20mA to make them visible. Reduce the current and extend the life of the battery. here is forward voltage for LED's with 5 and 10 mA of current.
COLOR vf 5Ma vf 10mA
yellow 1.9--------- 2.0
green 1.95-------- 2.0
red 1.95-------- 2.0
orange 1.8 --------1.9
white 3.0---------3.2
blue 3.6---------- 3.8
I use as low a 1mA on blue and white and they are very bright.

 

Your 3 volt circuit can work but driving LEDs directly from a voltage source should be limited to cases in which you have the particular Current-Voltage curve for the LED you plan to drive.

Problem statement: If you don't know the Current-Voltage curve for your LED, how can you know how much current is going through the LED (unless you measure it, and then it might be too late)?

 

First off - Thanks you for your responses
!
Ok I think I am getting it now. Ill have to do some experimenting.

let me know if I have it.

If I connect a 3.2v LED directly to the 3v battery it takes the 20ma straight out of the battery but it dose not really need it. I could use a small resistor to limit the ma entering the led and still light it. This would reduce the risk of damaging the led with any ma spikes from the battery.

What would you sat the risk of destroying my blue and green LEDs is if I don't use the resistors and connect then straight to the battery keeping in mind that I will only be powered for short periods of time max of 1 min a couple times a day.

 

here is the graph for the blue LED I'm using

 

What would you sat the risk of destroying my blue and green LEDs is if I don't use the resistors and connect then straight to the battery keeping in mind that I will only be powered for short periods of time max of 1 min a couple times a day.

That would depend on the nature of the battery. If it had low internal resistance and was capable of providing enough current to destroy the LED, that would be bad no matter how little the LEDs were used. If the LEDs were driven hard enough to shorten the amount of time that they were capable of delivering full brightness, that could also be bad.

Do yourself a favor and just use resistors. It will avoid having people look at it in the future and scratch their head wondering why resistors weren't used.

On the other hand, there are many commercial LED flashlight products that don't use current limit resistors. The money they save adds up, and they don't care if they never have a repeat customer. Similarly, they don't care if it burns out and you end up buying another from them.

 

Thanks for the advice. all makes sense now.

 

You say the LEDs have a forward voltage of 3.2V and show a graph of "your LED".
But you cannot buy a 3.2V LED, its datasheet shows its range of forward voltage that might be 2.8V to 3.6V and you get whatever they have.

If your LED actually has a forward voltage of 2.6V then it will blow up if you connect it to 3V and if it is 3.6V then it will be a dark emitting diode.

 

Re. Post # 6, With the Li. 3V battery & white LED, current is 10 mA, battery V 2.8 V. With a blue & green LED connected to 3V battery, they will still be lit long after I'm gone but with many battery changes. If the LES are too bright, add a resistor. Take measurements of all active components to see what is going on.

 

Those who buy production quantities can specify a narrow range of voltages for a specific current. Some datasheets show the binning values, in which case you can go by the detailed part number to narrow the range.

This is a lot like transistors that are binned according to the range of their beta under specified conditions.