# Led Project resistor ?'s

Discussion in 'The Projects Forum' started by joeyespo747, Feb 21, 2012.

1. ### joeyespo747 Thread Starter New Member

Feb 21, 2012
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0
I'm creating a parallel circuit with 2 Leds. I'm using a 5v power source and need to know
what kind of resistor to use.
do I use 1 resistor per led?
If I wanted to use more LED's would I need a different resistor.

Here's some specs on it.

Power Dissipation: 80 mW
Peak Foward Current: 75 mA
Continuous Foward Current: 30 mA
Reverse Voltage: 5-6 V
Forward Voltage: 3.2~3.6 V
Reverse Current: max30 uA

2. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,373
1,355
Hi Joey. We'll need to know exactly how you have it set up, but I believe you will only need one resistor. Let's just say you are using one for now. You connect the LED + to V+, LED - to one side of the resistor, and the other side of the resistor to ground. You'd calculate the necessary resistance using (Vsource-Vled)/Iled. In this case, it would be (5v-3.4)/(.03A), or 53.3 ohms. 56 ohms would be the nearest common resistor value. Now, consider putting two LEDs in parallel. You only need one resistor at the cathodes of the LEDs to limit the total current. Since the two LEDs are in parallel, they will need the same voltage, but twice the current. Therefore, you'd need a resistor with value (5-3.4)/.06, or 26.7 ohms. Notice that that value just cut your original resistor value in half (you need twice the current, so 1/2 the resistance). The closest common resistor value to 26.7 ohms is 27. That is what you would need, if using the configuration I described above.

I hope this helps.
Regards,
Der Strom

Apr 5, 2008
19,549
3,984
Hello,

I would take ONE resistor for EACH led.
Putting leds in paralel, will lead to different current in each led, as the leds will never be the same.
The one with the most current will fail afther some time, when this one is burned, the other gets double the current and will fail too.

A lot of information on leds , you will find in this thread by Bill_Marsden:
LEDs, 555s, Flashers, and Light Chasers

Bertus

Wendy and DerStrom8 like this.
4. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,373
1,355
Thanks Bertus. I somehow keep getting stuck in those "perfect world" situations. I appreciate your correction Regards