# leds in parallel choosing resistor

Discussion in 'The Projects Forum' started by panayiotis, Mar 5, 2014.

1. ### panayiotis Thread Starter New Member

Feb 25, 2014
16
1
dear sirs
i want to connect 20 leds in parallel.
my source is 2x 4.2v batteries parallel
led voltage white 3.4v 20ma
in the following page i can find that i need 20 resistors 47ohms
http://led.linear1.org/led.wiz

now in other page
http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
i find out that i need resistor 2ohms 03w
does it mean that i need 1 resistor for all circuit? or 20 resistors, one to each led as the previous page?

what i want to do, is to connect in the beginning of the positive wire only one resistor to limit the voltage to all leds. instead 20 resistors , one to each led
is it possible? i am little confused!
thanks

Last edited: Mar 5, 2014
2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
You are making a current divider. The best way to accomplish your goal is to have each LED have its own resistor. So. 20 LED, 20 resistors. The reason this is the best way to do it, is because each LED will be slightly different from other LED, this means that in your current divider some LED will get more current, some LED will get less current. LED that will get more current will be brighter. LED that will get less current will be dimmer. To solve this little problem you can select individual resistors for individual LED, this way each LED will get almost the same amount of current and be as bright as other LED.

This is wrong. Resistor here does not limit voltage. The job of the resistor is to limit current.

May 28, 2009
474
31
If you parallel LEDs, each LED will require its own resistor. If you connect LEDs in series, you need only one resistor for the series string. Series strings may be paralleled.

For your circuit, you have a source voltage of 8.4VDC. The LEDs are 3.4V @ .02A. If you put two LEDs in series: 8.4V - 6.8V = 1.6V that the resistor must drop. 1.6 x .02A = 32 Ohms per two LED string. Since 32 Ohms is not a standard value, 33 OVhm resistors will suffice. This way you will use half as many resistors.

I hope this helps.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
Case 1. Two batteries in series.

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Case 2. Two batteries in parallel.

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Last edited: Mar 5, 2014
5. ### pwdixon Member

Oct 11, 2012
488
56
Perhaps a little excessive unless the guy is super sensitive and can manage to kill himself with 8.4V.

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
It is the nature of internetz that you can be anything you want to be.

7. ### pwdixon Member

Oct 11, 2012
488
56
I use the internet a lot, I hope it's not going to make me prone to electrocuting myself on a 9V battery, you've got me worried now.

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
A healthy sense of paranoia is always healthy.
☺7

Apr 5, 2008
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10. ### panayiotis Thread Starter New Member

Feb 25, 2014
16
1
i told i have 2x 4.2v batteries.yes but they are in parallel.
you did not ask you just have big mouth man!
dont be the smart one!

Feb 25, 2014
16
1
12. ### pwdixon Member

Oct 11, 2012
488
56
Now you might have a problem, putting two batteries in parallel can be a problem that might cause them to overheat and that could be a danger. I think anyone would have assumed you were putting the two batteries in series.

13. ### panayiotis Thread Starter New Member

Feb 25, 2014
16
1
hmm ok i want to learn why the cause overheat?

( i am just a learner.i want to learn from members in here.i dont know about electronics thats why i ask.please be patient with me!)

14. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
I apologize. I assumed that you were going to use two batteries in series. A bit later I thought to try two batteries in parallel and I got results that you posted. I have updated my post, please look here: http://forum.allaboutcircuits.com/showpost.php?p=704312&postcount=4

15. ### panayiotis Thread Starter New Member

Feb 25, 2014
16
1
thank you sir