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Help with LED's

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PanicMode

New Member
Well im new to the forum, and i hope you guys can help me out since im a noob when it comes to LED's.

I need to connect 5 LED's in parallel to a 12v source.
The LED's specifications are as follows:

yellow forward voltage 2.1
20mAMP current
12 V source.

I bought 100oHm 1/4 watt resistors, but when i connected them in series the resistor got really hot.

I dont know what i did wrong or if this is normal for the resistor.

Any help is greatly appreciated.
 

vne147

Member
12V source - 2.1 V forward voltage = 9.9V

I = 20 mA or .02 A

V = IR or R = V/I

R = 9.9/.02 = 495Ω

Replace the 100 Ω resistors with 500 Ω and they should not get hot.

When you had the 100 Ω resistors the current through them was:

V/R = I = 9.9/100 = .099 A or 99 mA.

P = IV so P = .099*9.9 = .9801 W That's almost 4 times what a 1/4 W resistor is rated for.

The 500 Ω resistor will dissipate only about .196 W
 
Last edited:

MikeMl

Well-Known Member
Most Helpful Member
Well im new to the forum, and i hope you guys can help me out since im a noob when it comes to LED's.

I need to connect 5 LED's in parallel to a 12v source.
The LED's specifications are as follows:

yellow forward voltage 2.1
20mAMP current
12 V source.

You need to put the LEDs in SERIES (not parallel)!!!

you likely used the SERIES calculator on the web link you provided, which comes up with close to a single 100Ω resistor in SERIES with the five LEDs.

The power dissipation in the resistor will drop to (12-(5*2.1))*0.02 = 30mW, which is well within its 250mW rating. It won't even feel warm...
 
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