# some power supply & led question

Discussion in 'Power Electronics' started by qingtian28, Mar 1, 2013.

1. ### qingtian28

10
0
Feb 9, 2013
hi there, i'm making an "I" word out of 15 LEDs which connected in series.
i have a power supply of 12v 700mA, it is an adapter which constantly supply power.

the problem:
all the leds wont lit up when i supply the power in, the max led which lit up is 5. if i try lit up 6 leds, the light's dim.

i still got 40 leds to complete the words i desire, i know the voltage is insufficient, wat can i do to make all the leds lit?

i'm quite a newbie in electronic stuffs. your opinion and suggestion is much appreciated. thank you

2. ### GreenGiant

842
6
Feb 9, 2012
your voltage should be fine, as well as the current, for 15 at least

you need to make an array of LED's rather than have them all in series, ideally you would use a resistor per LED and have them all in parallel, though 12V is a little high for the more common LED's

Standard LED's run on 30-50mA so you would probably be able to get a good 20 at full brightness, and if they are lower voltage probably more than that with an array and some resistors, you may be able to get all 40 with the proper setup but you should probably invest in a more powerful supply

Any more details on the LED's would help to really design an array to help you out.

Last edited: Mar 1, 2013
3. ### qingtian28

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Feb 9, 2013
Erm.. The led is a normal led.. Those with red, green, blue..
Lol.. I dont really know how to describe the led..
I will search on the net if u dont understand wat i mean..

Can i solder some step up voltage or amp circuit? Hehehe

4. ### duke37

5,364
772
Jan 9, 2011
If 5 leds in series run all right, then stick with this. 5 leds at 2V will need 10V so there is 2V extra to drop with a resistor. Using 20mA, R=V/I then R=100 ohm

For 15 leds you will need three strings connected in parallel. Total current 60mA.

For 40 leds, you will need 8 strings, giving a total curent of 160mA, well within the capability of your power supply.

Different coloured leds need different voltages. You may need to reduce the number of leds per string to 4 and to recalculate the resistor.

Last edited: Mar 1, 2013
5. ### qingtian28

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Feb 9, 2013
O i c ... Thanks duke. Thanks for the suggestion..

6. ### qingtian28

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Feb 9, 2013
Hey.. I connect red led in a string of 5 in parallel and the bulb burnt.
Then i connect the red led in a string of 6 in parallel it lit up bright.
I not sure how to calculate the resistor needed, if i need to connect 2 leds in a string in parallel. Can help plz?

7. ### davennModerator

13,838
1,952
Sep 5, 2009
did you not use a current limiting resistor as pointed out by duke37 ?
if not, thats why your LED(s) had a very short and bright life

Dave

8. ### qingtian28

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Feb 9, 2013
Erm:: i didnt use bcoz i connect the green colour led without resistor looks fine..
But for red i need 6 in a string so tht i wont need a resistor.
But i got 2 extra red led, therefore i need a resistor, but i not sure how much ohm i need.

9. ### duke37

5,364
772
Jan 9, 2011
You should NOT connect leds in parallel, they should be connected in series with a resistor to make a string.
You can connect strings in parallel.

Different leds will need different voltages. You can measure the voltage across a led with a meter. Use a resistor to limit the current to say 10mA. A 1k resistor will be OK I think.

Put sufficient leds in series to get up to about 80% of the power supply voltage and use a resistor to drop the extra 20% at 20mA. R = V/I so R = V * 50.

10. ### qingtian28

10
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Feb 9, 2013
Sorry my mistake, typo.. I connect them
In series.. For green led, 5 leds will gif bright light.
For red led, 6 leds will gif bright light..
So now i got 2 leds extra.. Need a resistor to prevent led burnt.
Question is i duno how much resistor i need to use..
Mayb i try n error starting from 1k resistor?

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
Duke37 has given you the formula to calculate the resistor value above