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40-LEDs question

@

@nalyzer

n00b
Joined
Apr 5, 2003
Messages
39
1stly, the LEDS are 2.1V @ 20 ma....i think......

my project is.....a cube box.....with 4 layers of perspex....i mean 4 lines from up to down...

then....i just wanna stick or rather stuff LEDS in between the perspex.... so how do u reckon i can do it?

btw the total LEDS is 40 LEDS. so i'm afraid there's not enuff V or the thing might blow.....or etc....so help me out....tell me if i should arrange them in series or parallel...or etc....anywayz...don bother bout how my project looks like....i just
have 40 LEDS... & of coz wires....& what adddaptor i need....
 
Does your LEDs have a specs sheet or page? You should be referencing the forward voltage drop, not the reverse drop or anything else. After knowing this, we can recommend a resistor to drop the voltage and go from there. It's not likely that the LEDs will draw enough current to cause worry. 40 x 20 mA in parallel (which is what you will want to do) is 800 mA, 0.8 A, which is comparable to a single hard drive in normal operation (not spinning up). Check the max current on the +5 volt rail of your power supply, and you will see that 0.8 A is not a big deal.
 
the leds that i'm using are the cheap ones available off the shelf. meaning, no specs included except that it's a 2.1V and 20mA. It's the normal led that you use in cd roms and all, no fancy stuff, not high density....

is the forward voltage drop calculatable from this information? anyway, what i need to know is that how much source voltage and the resistor value for the entire circuit

the ray of leds are in total 40 leds.. with the circuit having 4 rails, 10 leds per rail. this circuit would have its own power supply since a battery would not sustain for long, which is a convertor, 12V or perhaps 9V from a AC supply... i need to know which is a more appropriate voltage and the resistor value for it.

here's how it would look like
40led.bmp


this is an assignment due in a few days time, so i urgently need some input...
 
If this is an assignment shouldn't you be asking your Prof./Teacher for help? If someone just gives you the answer you aren't going to learn anything.

James
 
Where is SarverSystems? He should be in here shortly to show off his led strings. :D I would put 6 leds in serial, then wire 7 of those strings in parallel and hook it up to 12 vdc. That would be 42 leds, but that's pretty close to 40.
 
If that's all the information you have, I'm going to assume that the forward drop is 2.1 volts. Like jpmkm suggested, 7 parallel strings of 6 LEDs would be fine for this, and then you wouldn't need a resistor.
 
There you go ! I painted you a little schematic

so 8 parrallel series of led with a 100 Ohms resistor

You could even use just one 2 watts resistor or 2x 1 watts

circuit.gif


the "K" is the Cathode (negative) leg of the LED
 
Originally posted by Adisharr
The new resistors use 'Ohms' for the resistance unit. :p
Click to expand...

D'oh ! the return of the infamous typo ! you're right ! my bad

only had 5 mins to do it though...
 
I would do the 6 LED's in series and then make parallel strings of 6, like in the diagram biomods made for you but with one more LED and scratch the resistor. Personally I don't think that depriving each LED of .1v is going to do much damage to the LED. It's likely that that would be within the normal operating parameters of the LED's.

Edit: Massive spelling and gramatical errors.
 
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