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### Topic: Questions about 1w LED, 0.25w resistors, TIP120 (Read 1 time)previous topic - next topic

#### Energia ##### Jul 15, 2014, 01:34 am
Question 1
I want to power a 1w LED (Vf: 3.2v-3.4v; 300mA) with 5v external power source. But instead of using a higher wattage resistor, I want to use 0.25w resistors in parallel (I'm just curious and didn't order other types of resistors). Is this possible and is the following correct?

The resistor that I will need for this:
V = 5v - 3.2v = 1.8v
I = 150mA (I want to run it at ~50%)
R = 1.8v / 0.15A = 12?
P = 1.8v * 0.15A = 0.27w

So, 0.25w resistors won't be a good idea. But can I use 4 x 47? 0.25w resistors in parallel that will give me 11.75?? So each resistor will only have to deal with ~0.27w / 4 = ~0.0675w?

Question 2
I want to switch this LED on/off with Arduino Uno and TIP120 transistor. I read that TIP120 will introduce a voltage drop (of 1.2v?). Is the following correct?

V = 5v - 3.2v - 1.2v = 0.6v
I = 150mA
R = 0.6v / 0.15A = 4?
P = 0.6v * 0.15A = 0.09w (so 4? 0.25w resistor can handle it)

Question 3
What will happen if I will just give this LED ~3.2 volts with a voltage regulator? How much current will the LED consume at this point? And the same questions for 3.5 volts.

#### Grumpy_Mike #1
##### Jul 15, 2014, 06:56 am
You can not use a simple resistor to control the current on a power LED.
The reasion is that the forward voltage is simply not stable enough for it to work with such a low resistor value.
You need a circuit called a constant current driver.

3) don't even think about it unless you like melted components

#### seanz2003 #2
##### Jul 15, 2014, 09:49 pm
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Question 1
the math looks about right. though actual numbers (Vf and R) may vary a little bit. i have bread boarded 1 watt Leds with a bunch 1/4watt resistors for a quick demo, but I certainly would never consider this route for any practical application. Use a constant current supply for optimal performance and reliability. they cost a dollar or two on ebay.
Quote
Question 2
yea on paper looks like it might work but in real life those slight deviations in the actual values can significantly affect the circuit especially when you only have .6v across the resistor(s). And the voltage drop of the tip120 depends on the current flowing through it. Use a cc supply and a logic level FET.
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Question 3
Smoke will happen.  LED = diode. once it turns(@ 3.2v) it  offers practically no resistance and it will let as much current flow through it as the regulator will source burning  the LED up as well as the regulator. there are inefficient ways to use a vreg as cc supply, but just use a constant current  supply.

#### fungus #3
##### Jul 16, 2014, 10:36 am

Question 1
The resistor that I will need for this:
V = 5v - 3.2v = 1.8v
I = 150mA (I want to run it at ~50%)

Ohm's law says it won't be 3.2V when you run it at 50%.

Question 2
I want to switch this LED on/off with Arduino Uno and TIP120 transistor. I read that TIP120 will introduce a voltage drop (of 1.2v?). Is the following correct?

V = 5v - 3.2v - 1.2v = 0.6v
I = 150mA

Same problem.

Plus 2 more:
a) Whenever you use LED+resistor the resistor should be dropping a decent amount of voltage or it won't really prevent runaway current situations (which is why resistors aren't recommended for power LEDs - they need to be massive wattages).
b) What you read about the TIP120 is wrong. The voltage drop isn't fixed, it depends on the current.

Question 3
What will happen if I will just give this LED ~3.2 volts with a voltage regulator? How much current will the LED consume at this point? And the same questions for 3.5 volts.

Reading between the lines: It seems you're all theory and no practice at the moment.

So I say "Build it and find out!"

Make a variable voltage regulator with an LM317 and potentiometer, connect up a some multimeters to show voltage/current, then.... turn that knob!

PS: Start low.