# L.E.D. question..... beginner needing help

Discussion in 'Electronic Basics' started by Palmer Hudson, Nov 14, 2004.

1. ### Palmer HudsonGuest

I am wanting to connect 4 LED's together to a 12 volt power source....well
to be specific...my car.
I know that they are 1.7 volt and 20ma per LED and I need to use a 1/4 watt
270 Ohms resistor.
My question(s) is this:
I think that I should be connecting the resistor to the incoming positive
side voltage source but I don't know if I connect all the LED in parallel
(all positives and negatives together) or series (positive to negative and
so on) or if an LED has a negative/ positive side.
I know this must be elementary but I"m just leaning.

2. ### Lord GarthGuest

Connect the LEDs in series, + to - to + to -, etc.
R=(Vs-4(Vled))/Iled
R=((12-(4*1.7))/.02
R=(12-6.8)/.02
R=5.2/.02
R=260 ohms, use 270 ohms standard value.

P=5.2*.02
P=104mA, 1/4 watt is fine

The problem is that the car has a higher voltage than 12 when it is running.
It is also
very noise electrically speaking. This can damage your LEDs. Since this
circuit is in
series, connect your resistor at any convenient point.

3. ### peterkenGuest

connect them in series, including the resistor, sequence doesn't matter,
orientation of leds does
a led usually has a short and a long leg, short one being kathode and long
one being anode
if equal length legs, look "through" the led, the largest (thickest) side is
usually the "table" which is kathode
kathode should be pointing towards gnd
resistor value is ok

4. ### Active8Guest

That's kinda misleading. The *cathode* is *connected* to the more
negative or ground side. The *anode* is sybolized by an *arrow*
which *points* toward ground.

5. ### peterkenGuest

If looking at schematic symbols, then yes it's an arrow pointing towards
ground
For a newbie and looking at the physical led itself, take into account a
newbie needs advice so explanation is done for what he actually sees and not
by interpretation of a schematic symbol 'cause he still won't know the
relation between the symbol and the device

6. ### Palmer HudsonGuest

First of all.....thanks for all the help.
I know to connect them in series and the resistor can go anywhere but now
I"m confused.
Below one person say that the shortest leg = thick side of LED = Cathode
should be ground.
The next person say that The *cathode* is *connected* to the moreMaybe I'm making it hard and not thinking correctly and just reading what
the words are telling me.
So am I correct to say that if the arrow is pointing up towards the LED and
is on the anode side, that it is actually pointing to the Cathode side and
down to ground. Or if the arrow is on the Cathode side it should be pointing
down to the negative side.
Again....thanks for all the help.

7. ### peterkenGuest

view "drawings" in notepad using fixed font

schematic symbol (the "arrow")

|
| anode (more positive side)
---
\ / ->
---
| cathode (more negative side)
|

seen "through" the physical led
(look at the "thick" side IN the led)

___________
anode | \
=======|=====:=== \
| __ |
| __:/ | |
=====|=====:___| /
cathode |___________/

hope this helps

8. ### Palmer HudsonGuest

Helps plenty...makes more sense...... thanks