Not sure to be safe or sorry

[Wiginometry]

Ok, so I am not with the program today maybe it's to much coffee or not enough but I'm having difficulty determining a resistor value.

As I'm a cheap one I like to recycle parts and pieces where I can (makes for a heck of a job soldering sometimes) I have linked 15 recovered 5mm white LEDs in parallel which I have hooked to a push button and 9v battery now I just need to figure the proper resistor value is attach unfortunately I'm having terrible memory on proper math and I also have to assume that they are just basic packages of 20ma/3.3v max. any suggestions before I have fun and watch them all pop?

 

[AnalogKid]

Schematic?
It sounds like the LEDs are in direct parallel. This will not work well, as the individual forward voltages are not matched and do not track, so some will be brighter and hog all the current. A better way is to have a current limiting resistor for each LED A way to save battery power is to have two LEDs in series, with a single current limiting resistor for the string. In this way, 20 mA through an LED string will light 2 LEDs instesd of just one.

E = I x R
R = E / I
E = 9 V battery - Vf for one or more LEDs
I = 20 mA

For one LED per string,
R = (9-3.3) / 0.02 A

For two LEDs per string:
R = 9-6.6) / 0.02 A

ak

 

[kellys_eye]

Paralleling LEDs on one resistor is a bad idea unless they are closely matched in value as any difference in individual devices will cause current to flow unequally, lighting one brighter than the others and quickly causing it to expire.

You can wire a few in series and use a suitable current limiting resistor and then wire multiple lots of the same kind of series connection in parallel - each with their own series resistor.

The resistor calculated by taking the supply voltage, subtracting the LED forward voltage (for one or however many you wire in series) then dividing that by the required current (0.02 in your case).

<edit> what 'he' said above......

 

[Wiginometry]

Lol Yea this was just me messing around with the bits I had in one of my boxes wasn't to worried about it expiring. But here's rough sketch of it. (It's not aligned all that proper either) was just having problems remembering my math

 

[Wiginometry]

Also due to the nature of not actually knowing which ones came from the same device I couldn't put them in series. (Down side to no proper labeling on the parts bins)

 

[Audioguru]

You show strings of 5 LEDs connected in series without a current-limiting resistor. If they are 3.3V white ones then they will begin to glow at 3V x 5= 15V and will burn out at about 17V.
If they are 1.8V red ones then they will begin to glow at 8V and will burn out at about 10V.

 

[Wiginometry]

Today's been fairly s#!t so I actually have them wired in parallel I wasn't thinking when I drew the schematic just kinda threw it on paper with out so much as a look.
They're white leds but they've been scavenged from older broken electronics and various appliances so as I stated previously best guess is they're all standard 3.3v/20ma max bulbs though due to my reckless abandon to sorting after they were desoldered from the old boards they got thrown into the led bits bin.

 

[dorke]

Try sorting them before grouping them, do that In terms of current and brightness.
You can use a 3.3V power supply and measure current.
If you don't have one use a small 3v battery (even a coin one may do the trick for you).

As noted above,don't parallel LEDs use them in series.

 

[AnalogKid]

And even if the supply you are using is set to only 3.3 V, *always* have a resistor in series with the LED you are testing. Some LEDs have current limiting built in internally, but ... "be safe or sorry"

ak

 

[Audioguru]

I have white LEDs with voltages from 2.3V (I couldn't believe it so I tested them twice) for some to 3.6V for others. There is no way they can be connected directly in parallel because the lower voltage leds will take all the current and immediately burn out, then the higher voltage ones will flash as they burn out.
My Chinese flashlight has 24 white LEDs directly in parallel and they all have the same brightness and current because some little girl in China was paid a bowl of rice to test and sort thousands of them into groups having the same voltage and she was good at it.