# [SOLVED] power source for a row of LEDs



## Steff345 (Sep 14, 2007)

hi
i was thinking of adding a few LEDs as case lights to my PC. actually i have already built the circuit of about 20 white LEDs in total with 47ohm resistors for each of them in a paralell connection and all run fine when supplied with a power source of 5V (which i provided with a few batteries just for testing purposes) 

my problem now is that i was thinking of mounting these up in my case.. and i need to get a power supply for them externally as i am not very happy of powering them from my PC power supply (just to keep the PC on the safe side)

I was thinking of using an old mobile phone charger which i have lying around, which has an output of DC 5.0V and 800mA max current output. when calculated, my circuit uses 500mA (25mA per LED x 20)..

is this possible? i mean using a phone charger to power up a circuit such as mine?? and any possible risks that may arise during use? :4-dontkno

thanks in advance


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## octaneman (May 13, 2009)

*Re: power source for a row of LEDs*

Hi Steff345


To build your array will require you set it up in a parallel configuration using 1 ohm resistors. Each resistor will dissipate 250mW so 1/2 Watt resistors will be the required set up. The 5v output is a little worrisome though, 5v is a little high. To better help you with your array, what was your calculation of the diode forward voltage and current. ?


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## Steff345 (Sep 14, 2007)

*Re: power source for a row of LEDs*

hi
many thanks for your reply - 
actually i have already built the circuit and as you have suggested - the LEDs are connected in paralell with 1 47ohm resistor for each LED.
in my calculation the forward voltage that i took was 3.8V (as i am using high intensity white LEDs) and the current i used was 25mA per LED

the curcuit works perfectly with all LEDs lighting up when i hook up some batteries..


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## octaneman (May 13, 2009)

*Re: power source for a row of LEDs*

Nice job  

If you can take a pic and post your array it would be helpful to others.


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## Steff345 (Sep 14, 2007)

*Re: power source for a row of LEDs*

sure.. will do that

what about the power issue? thats still not resolved.. like i said in my first post, i was thinking of powering up the array of the LEDs using an old mobile phone charger that gives and output of 5V DC and a max current output of 800mA.. theorotically shouldnt that seem perfect for this kind of setup? since the LEDs use only 500mA??

but my problem is will this setup be feasible? i mean powering them up using a mobile phone charger? any experience in this kind of situation anyone?


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## Done_Fishin (Oct 10, 2006)

*Re: power source for a row of LEDs*

your charger was not built to supply a constant load so it may not last very long.

why don't you connect it using isolated wires to the 5V supply on your PC Power supply. It won't or shouldn't begrudge an extra 2.5watts for those LED. Use the Red Black combination from a spare molex connector or even grab a connector from an old floppy drive and use the floppy power connector.


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## octaneman (May 13, 2009)

*Re: power source for a row of LEDs*

In theory you are correct it should be able to work, but like Done_Fishin said it wont last long because the charger was not designed to give a constant feed. But he gave excellent advice on how to power your array. The mobile phone charger will overheat from prolonged use for it does not have anything to cool the circuitry.


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## Done_Fishin (Oct 10, 2006)

*Re: power source for a row of LEDs*

just make sure that you keep *ALL* the connections well isolated .. the 5V supply can deliver a lot more than your 0.5A, perhaps anything up to 40A before complaining and that is a *LOT* of frying power!


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## Steff345 (Sep 14, 2007)

*Re: power source for a row of LEDs*

thanks, both of you for your valuable input..

ok.. so i'll scrap the charger idea then.. and resort to using the 5V connector on the PSU..

btw just curious.. what happens if i get a power adapters of 5V output with suffiecient current - say 700-800mA? will it be the same story as the mobile phone charger?


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## octaneman (May 13, 2009)

*Re: power source for a row of LEDs*

It is the same story because the power adapter is the charger for cell phones.


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## Steff345 (Sep 14, 2007)

*Re: power source for a row of LEDs*

thanks for your reply octaneman, but what i meant by adapter is an AC/DC adapter (like the ones that come for most electronic devices that need DC power)


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## Done_Fishin (Oct 10, 2006)

*Re: power source for a row of LEDs*

that should work .. just remember that, should you get the "adjustable" variety, the off load voltage is higher than stated but should come to an approximate true value whn the load is applied. for a small extra charge you could probably get a stabilised version of the same adapter which means it will have a voltage regulator inside (normally an LM317T) to keep the voltage correct regardless off load. 

Either will do you.


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## Steff345 (Sep 14, 2007)

*Re: power source for a row of LEDs*



Done_Fishin said:


> that should work .. just remember that, should you get the "adjustable" variety, the off load voltage is higher than stated but should come to an approximate true value whn the load is applied. for a small extra charge you could probably get a stabilised version of the same adapter which means it will have a voltage regulator inside (normally an LM317T) to keep the voltage correct regardless off load.
> 
> Either will do you.


yes i got that.. many thanks for all your information


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## Done_Fishin (Oct 10, 2006)

Good Luck and if you have any other queries, you know where to find us :wave:


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