AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Bryan Martin" <registration@...> 
wrote:
> ... I want to add an additional
> 4 LEDs.  Would it be better to ...
> go w/2 X 4 LED series or move to
> 8 LEDs w/a different resistor?

It depends (it always does).  If your 12 volt source is stable (stays 
close to 12 volts all the time *OR* you're not troubled by variation in 
the LED current, putting them all in series is the way to go.  More of 
the power will go to making light and less to heating up your 
resistor.  But, the lower the voltage you leave across the resistor, 
the more changes in the supply voltage will cause changes to the LED 
current.  If, for example, you have 2.4 volts across the resistor with 
8 LEDs and a 12 volt supply, a 10% drop in the supply to 10.8 volts 
will approximately halve the LED current.  If this is a possibility, 
you can use several smaller series chains, as you have now, and manage 
the heat with a high power resistor (or a network of ordinary resistors 
that gives the same resistance value).  An alternative would be to use 
a constant current drive, which is just a transistor and a few other 
components.  You could look up the circuit on the Web or ask again here 
is this is the way you want to go.

Graham.

Quick question, I am trying to start a simple mini IR LED array but second 
guessing myself.

IR LED Info:
Forward Voltage: 1.2V (max 1.6v)
Forward Current: 100mA (max 1.2A)

I am hooking 4 LEDs direct to 12V DC power in series w/a resistor of 81ohms. 
Thing is the resistor is getting like really really hot.  I dont mean warm I 
mean hot.  Am I ok?  Second question is I want to add an additional 4 LEDs. 
Would it be better to run this like below and go w/2 X 4 LED series or move 
to 8 LEDs w/a different resistor?

I got my figures for the resistor as follows if it matters:

1.2v per LED * 4 LEDs = 4.8v drop
(12v - 4.8v drop) / (100 / 1000) = 72 ohms

Poor planning on your part does not constitute an emergency on my part.