hi everyone.

i want to connect 500 led's in parallel and run them of a power source of 5 volts and max load rated as 10ampere

i would like to know how do i limit a led to draw no more that 25ma, the actuall max current rating of the led.

im a bit confused here.. when connected in parallel, will the led's automatically draw only 25ma which is the rated current of led or the 10A output of the powersupply will be pushed into them causing them to burn!!

 

Simply, you use 500 series resistors.

 

But my calculations suggest that at 25mA you can only run 400 LED's from your 10A power supply.
What colour are the LED's? Do you know the voltage drop? Have you studied any of the recent threads about LED's in parallel?

Why do you want 500 LED's anyway?

 

those leds are for my car.
the total led count is 500 however not all 500 led's will be working at the same time. practially around 200 led will be working simultaneously. what i am afraid is that wether the led's will draw the power according to their requirement or the full capacity of the powersupply which is 10A will be forced into the led cluster making the smokeidoes!! :shock:

 

as long as you put a series resistor in line with each LED you will be ok.


(I have another topic on LED's, it might have some info that you can use http://www.electro-tech-online.com/v...ic.php?t=11796)

where are you putting these leds?

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i want to mod my stock tail light and fix a cool led pattren in them so thats why i gotta make a step down circuit and stuff.. but something which keeps confusing me is the logic of how led draw current.. do they draw to the max limit of the power source or they draw only to their max rating so it doesnt matter if the main supply capacity is 10A..??

i base my thinkin on the idea that the main supply at out home might be of 100A however a machine rated for 2A only draws 2ampere from the main and all appliances are connected in parallel!! the 100A max capacity of the main line is not forced into the 2A appliance causing it to smoke out!!

 

Sorry for shouting, but AS LONG AS YOU PUT A RESISTOR IN SERIES WITH EACH LED YOU WILL BE OK!

 

Why Are You Using 5 Volts? The Car is 12 Volts.

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ok and wouldnt those resistors draw extra current makin the 10A power supply over loaded..??

how can i have a god damn 5volts supply in my car to run as many leds i want..without much hassle.. just like u can connect as much as u wish 12 volts blubs in the car electrical circuit!!

resistor with each led would be a painful job to solder..considering that i will be making it for me and many cars of my friends once they start to get jealous!!

geez.. i have forgotten all the high school electronics lessons in physics subject!!

 

bcuz i want LED's to emit light not smoke :lol:

 

It is true that LED's are a current device. But it is also true that they have a forward voltage that varies with the forward current flow thru them.
They can be connected in parallel as long as there is some regulation on the forward voltage, and the forward voltage is very near tha same for all the LED's. For example red, yellow, and green can be connected in parallel if the supplied voltage is near 2.0 volts. Each LED will draw current, the amount will vary slightly between the individual LED's.
White LED's have a forward voltage around 3.0 volts and Blue LED's have a forward voltage of around 3.6 volts.
The idea of needing a current limiting resistor for each LED is not always
true.

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But it's a VERY good idea, and excellent professional practice!.

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LED's don't run on 5 volts.
The average red LED has a forward voltage of about 2.5 volts. This means that it doesn't really start to conduct until the voltage across it reaches about 2.5 volts, a bit like a zener diode. Once the voltage does reach 2.5 volts, the LED draw as much current as supply can handle, or as much current as it takes to destroy the LED, whichever comes first.
So, when you want to use an LED with a 5 volt supply, you need to do the following simple calculation:
First, you take the supply voltage (5 volts in your case)
Next you subtract the LED forward voltage drop(let's call it 2.8 volts)
This gives you a figure of 2.2 volts.
Now you have to work out what value of resistor will drop 2.2 volts at 25mA (if that's the current you want to flow through the LED).
Applying Ohm's law, R = V/I (where R = resistance, V = voltage and I = current) we get R = 2.2 / 0.025
Which means that R = 88 Ohms

Now let's suppose that you decide to wire two LED's in parallel, sharing a common current limiting resistor, commons sense tells you that you need to use a resistor of half the above value, i.e. 44 Ohms, in order to supply twice the current. However, all LED's are not created equal, and in the real world, one will have a slightly lower forward voltage drop than the other one, and will end up hogging most of the current. At some point it will probably burn out, at which time the remaining LED will take all the current (50mA) and it will soon burn out.
You buy get LED's which run from 5 volts (or 12 volts, in fact). They have built-in current limiting resistors, and are usually a bit more expensive.

So, as you've probably worked out by now, there is not much advantage to using your 5 volt 10 amp PSU, you could merely substitute 12 volts in the above calculation. The only difference is that the 500 current limiting resistors would be dissipating about four times as much power as heat. But then again, what is the regulation method used by your 5 volt PSU? If it's based on a linear regulator, it will be dissipating approximately the same amount of power as heat. In the long run, it would be way easier and cheaper to get a big bumper sticker printed saying "Hey everybody, look at me! I'm really cool!".

 

if the led voltage requirements is 5volts and the supply is 5volts , there there is no need for current limiting resistor. IS that correct?? from my search, i found out that current limiting resistor is only required when hooking up a lower voltage led on a higher voltage supply

when led draws 5 volts and supply is 5volts, there is no need of current limiting resistor, the led will only draw current according to its rating.

how ever a professional practice is to still use a resistor in series and limit the led to 4.9 volts only on a 5volts supply hence adding a protection.

 

As has already been pointed out endlessly, you REQUIRE CURRENT LIMITING RESISTORS. As has also been pointed out, it's pointless running them off 5V when you have a 12V supply - it's a total waste dropping the supply down to 5V and achieves nothing!.

Simply group the LED's in fours in series - with a single current limiting resistor for the four, and feed this group off the 12V supply. You can easily calculate the resistor value from ohms law, but for a quick test (assuming 2V per LED, and 25mA) try a 150 ohm resistor.

You then just duplicate this as many times as you need.

There's already a thread about this exact same subject!.

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