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LED help

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MARINE1142

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here is what i m tring to do and have had a complete brain fart. I need 40 superbright led they are 18000 mcd they run off of 4 volts 30ma 120mw what size reistor do i use also what is the fromual for finding out. Also do i need one resistor per led or can i use a pot or something else. Also i m hooking it up to my pc so i only have 5 volts available.
 
You can't connect LEDs in parallel because the voltage for each one is different. Therefore the one with the lowest voltage will hog most of the current and burn out, followed quickly by the rest of the LEDs burning out.

I haven't seen an LED that measures 4.0V, and most have an absolute max current of 30mA. Most white and blue ultra-bright LEDs operate well at 20mA and have a typical voltage of about 3.5V.
A 68 ohm resistor in series with each LED will limit the current to 22mA each. Your 5V supply must provide 880mA for 40 of them.
 
You don't want to operate LEDs at their absolute max current of 30mA when 22mA is safer.
The printed spec says the typical forward voltage is 3.4V at 20mA, but the graph shows 3.5V which is different but nearly the same.
A 68 ohm resistor in series with each LED will be fine.
 
Ok thank you i will get 40 68-100 ohms resistors(will that range work) and hook up each one in series with the 5 vdc supply from my pc thanks again
 
MARINE1142 said:
Here is the spec sheet well since i can not post a pic or attachment here is the link
http://www.superbrightleds.com/specs/W18030.htm

so do those specs above still hold true?

The absolute max of your device is 70ma. 30ma is its continuous rating and it's ok to use this.

By the graph, Vf @ 30mA is about 3.56v. Thus 48 ohms is the required resistance.
 
audioguru said:
You can't connect LEDs in parallel because the voltage for each one is different. Therefore the one with the lowest voltage will hog most of the current and burn out, followed quickly by the rest of the LEDs burning out.

I haven't seen an LED that measures 4.0V, and most have an absolute max current of 30mA. Most white and blue ultra-bright LEDs operate well at 20mA and have a typical voltage of about 3.5V.
A 68 ohm resistor in series with each LED will limit the current to 22mA each. Your 5V supply must provide 880mA for 40 of them.

He could always run groups of series LEDs in parallel with each other. The average forward voltage drop of the series LED's would then be the same, and so the risk of certain LEDs dropping out would be minimised. Of course, the consequences of one LED dropping out would then be much worse because you would be taking out more than one LED at a time ;)

Brian
 
Oznog said:
30ma is its continuous rating and it's ok to use this.
What if the forward voltage is lower than typical, what if the 5V is a little higher, what if the resistor's value is a little low???
You don't want to operate LEDs or anything else at their absolute max continuous rating!
 
MARINE1142 said:
So is 5vdc from the pc too much

5V D.C. is too much if you connect it directly across the LED, because according to Oznog the forward voltage should be 3.56V for the LED you're using - I haven't looked this up myself. But, if you use a 48 Ohm resistor in series with the LED (also as Oznog said) then you'll end up with the correct voltage drop across the LED for a 30mA current. The remaining "unwanted" voltage drop will appear across the resistor.

Brian
 
MARINE1142 said:
I agree so would the 68 to 100 ohms work for 5 volt supply?

Just use Ohm's law. 20mA is the usual supply current for a nice, bright LED working within it's limits but I haven't looked up the datasheet for the device you're using. If you want to use 20mA instead, then use a 72 Ohm resistor (75R preffered value)

Brian
 
So the safe way is to use the 68 ro 100 ohms which ever one i can find the same value for all 40 then i will be ok
 
Well if you use 100 Ohms then you'll be limiting the LED to 15mA, so you'll want to experiment to see if that would be acceptable or not. I probably wouldn't run all of the LEDs with seperate current limiting resistors, but because you're limited to a +5V output from the PC, it looks like you're going to have to. It'll certainly work, yes.

Brian
 
I don't think a PC can spare that much current. You might cause problems with your computer. A 4v LED hmm... could he run 2 X 20 in series off mains (110 VAC)? They are diodes, right?
 
It's interesting with Led's and I agree with audioguru that Led's in parallel need their own series resistor to avoid burning out the Led with the lowest voltage threshold.

I got hold of an Led cluster from a traffic light which got hit by a car,
In here the Led's are in strings of 4 Led's in parallel, then series'd up with the next 4 Led's in parallel and so on. ( no resistors used !) There is a switchmode type powersupply which regulates the current at 80 mA's. pulsed DC to drive the Led's. ( 230 V 19.5 Watts )

That explains the odd Led string faillure in traffic lights. When one Led fails on overcurrent it usually goes not open circuit and will reduce the available voltage for the other 3 Led's in the string hence they go out as well.
The powersupply will keep the current at 80 mA's so the other series Led strings will not get stressed out too much I guess.
 
A company that makes many traffic lights can test and sort the LEDs into groups that have a similar forward voltage. Then they can parallel them.

I bought a bag of 100 Fairchild LEDs and their forward voltage is nearly the same. But I don't want to risk putting them in parallel to see if they fail one-after-the-other.
 
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