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Author Topic: Led's and the proper setup for using them!  (Read 3436 times)

Offline shockwavemikey65

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Led's and the proper setup for using them!
« on: 20/07/2009 05:54:52 »
I would like to know how to wire or solder them in series? also if someone could answer how to properly control them ie( resistors and capacitors to regulate the current to them to provide optimal output vs life span. I will be using both IR and regular led's.


 

lyner

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Led's and the proper setup for using them!
« Reply #1 on: 20/07/2009 12:14:42 »
What is the application? Do you need a lot of power from the LED? If so, you will need to use as much current as possible.
You will need about 20mA through the LED and you'll have about 1.7V across each LED, for a large range of currents.  The supply voltage will be divided between all the components which are in series so, if you have a 6V (DC) supply and you want to have two LEDS, in series. They will take 3.4V, between them. You then need a resistor in series which will let through 20mA with the remaining 2.6V across it.
R = V/I  so
R needs to be 1.3kΩ

If you are after as much light as possible then you will have as many series LEDs and a little series R as poss . The R introduces inefficiency, of course, but the LEDs, left to themselves, can take enough current to cook themselves -so they need something to control the current through them.

This link gives a lot of info.
http://www.theledlight.com/LED101.html

Unless you have a big box of free LEDs, experiment with a large series R, particularly if you haven't all the data about the LEDs you are using and if you can't be sure of your sums.
 

Offline techmind

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Led's and the proper setup for using them!
« Reply #2 on: 20/07/2009 13:06:31 »
For standard 5mm LEDs of a few years ago, yes, 15-25mA was a typical current.
These days however we also get high power LEDs mounted on aluminium heatsinking pieces... which you power with 350mA or more - including white LEDs for lighting purposes.

The original poster needs to give more information on what he is trying to achieve.
 

Offline shockwavemikey65

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Led's and the proper setup for using them!
« Reply #3 on: 21/07/2009 02:15:45 »
For standard 5mm LEDs of a few years ago, yes, 15-25mA was a typical current.
These days however we also get high power LEDs mounted on aluminium heatsinking pieces... which you power with 350mA or more - including white LEDs for lighting purposes.

The original poster needs to give more information on what he is trying to achieve.

The main purpose would be lighting both the normal and IR . I'm trying to construct an IR camera and custom make a harness for night-time use. I mainly would like to have the ability to switch between normal and IR LEDs. also I would like to have the most efficient and a pretty powerful setup without having to use an uber heatsink!
 

Offline techmind

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Led's and the proper setup for using them!
« Reply #4 on: 25/07/2009 12:34:00 »
Typical 5mm IR LEDs operate with 20-30mA of current (maybe up to 50mA - check the manufacturer's data). They'll have a forward-drop of around 2V. If you're running them off a battery you can make series/parallel arrangements where the series-chain forward-drops add up to something less than the worst-case battery voltage.
For maximum efficiency and consistency of performance you'd use a constant-current regulator with a fairly small voltage drop.

For simplicity you'd allow a greater difference between your sum-total forward voltages and your worst case battery voltage, and limit the current using a resistor (and accept that you'll lose more power in the resistor and the brightness will fall as the battery weakens).

Assuming 2V forward voltage and a 30mA design-current...
For a 6V battery and resistor solution, you'd probably use series chains of 2 LEDs and a 68ohm resistor for each chain. For a 12V battery you would use 4 or 5 LEDs in the chain, with eg a 68ohm resistor for 5 LEDs or a 120ohm resistor for 4 LEDs.
The 5 LED varient is more efficient, but the LEDs will get very dim when the battery voltage has dropped to 10V whereas the 4 LED varient will show less deterioration.


These days you can probably get a special-purpose LED driver chip ("just add a capacitor / inductor") which will power quite long series-chains off of a 3-4V supply, and regulate the current for you with very high efficiency. These chips costing c.$1 are designed for driving the LED backlights for LCDs for mobile phones, handheld widgets etc.
 

The Naked Scientists Forum

Led's and the proper setup for using them!
« Reply #4 on: 25/07/2009 12:34:00 »

 

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