12 LED resistance circuit help

[Brilla]

Hi, I want to make a simple LED circuit with 12 LEDs running off a
nine volt battery. I've managed to dig up enough information about
most things, so I know I'll have to wire them in parallel. But the
resistance I should be using still confuses me.
Should I have one (or more) resistors at the beginning of the circuit?
Or one before each LED in the circuit?

The LEDs I'm using have a 3.6v voltage drop, and they are supposed to
get 20mA I beleive.
So that's 12 LEDs off a 9v battery.

I have a bunch of 27 Ohm resistors because that's what I (probably
mistakenly) calculated I should have before each LED. But I have no
idea what i'm doing. So if anyone can help me out on what resistors I
should use and where I should put them that would be greatly
appreciated!

Thanks,
Steve

 

[Ban]

Anthony Fremont wrote:

"John Bokma" wrote:

snipped exactly what I was going to post

Note that I did this calculation, later checked it with a voltage
meter, and yet already 2 exotic LEDs died on me. I think it's a bad
badge. (Unless someone can point out my errors).

I came up with the same calculation as you. If LEDs are dieing, then
20mA may be a bit too much current. Since there are two LEDs in
series, one may hog more current than the other resulting in its
demise. You might consider not driving them so hard. There is
probably a relatively insignificant brightness difference between
10mA and 20mA anyway.

--
ciao Ban
Bordighera, Italy

 

[dB]

"Anthony Fremont" <spam@anywhere.com> wrote

Since there are two LEDs in series, one may hog more current than the other

In a series circuit the current is equal through each component.

 

[dB]

John Bokma <postmaster@castleamber.com> wrote

and due to the resistor, one LED can only have a higher voltage
if another LED has less.

"Due to the resistor"? What a strange thing to say.

The resistor limits the current, it has no direct effect on the
voltage developed across each l.e.d. The actual value across each
l.e.d. varies from device to device at any current. The data sheets
give a "typical" Vf and sometimes a max figure.

 

[dB]

"Anthony Fremont" <spam@anywhere.com> wrote

Vf _is_ dependant upon current.

I neither said nor implied that it wasn't.

At extremely low currents, Vf will be
significantly lower than the nominal value. As current increases, so
will Vf. The curve is steep, but it is not vertical.

Yes, the curve is in an article on my site. (I can suck eggs.)

You seem to have misundersood my post which was regarding the
ambiguous sentence "However the current should
stay 20 mA, and due to the resistor, one LED can only have a higher
voltage
if another LED has less."