ledpowerresistorsvoltagewatts
I'm making my electronics project.
I need answer please
I need an resistor
I just need to clarify and validate.
Given
I did:
R= \$\frac{V_s – V_{led} }{I_{led}}\$
= \$\frac{11.5 V}{25mA}\$
= \$\frac{11.5 V}{0.025 A}\$
= \$460 \Omega\$
And a power of 0.2875W, but I bought 470 \$\Omega\$ with 1 W
What will happen to my project?
Fail or burn the LEDs?
Here is the forward conduction characteristic of a typical LED: -
At 2V forward voltage the LED consumes 20mA and is quite bright. At 1.8V forward voltage the current has fallen to 5mA and the LED will be somewhat dimmer than the 20mA scenario. At 1.6V the LED is taking almost zero current and below this voltage it is doubtful that it illuminates at all.
If you have 10 similar LEDs in series, to obtain a forward current of 20mA requires a forward voltage of 20V.
If you are happy with a forward current of 5mA then 18 volts is all that is needed. And, if your power supply is (say) 20V, a series resistor is needed that drops 2 volts at 5mA therefore its resistance will be about \$\dfrac{2V}{5mA}\$ = 400 ohms.
If you have a limited supply voltage, you can wire the LEDs in parallel banks or use a boost converter to lift your 5V to about 20 volts. The CAT4238 device from Motorola can do this: -
The reason the LEDs you're finding are all <5V is simply because that is how LEDs are. Applications using LEDs at higher voltages are either chaining many together in series and/or incorporating special driver circuits. To get more of an answer than that, you're going need to provide more detail about your application.
Best Answer
Your 1W resistor exceeds the power requirement so that's perfectly fine. The only time you run into problems is if you use a resistor with a lower power rating. For example if you used a 0.25W rated resistor it may not be able to dissipate enough heat, get too hot and fail.
On the resistance side 470 ohms will cause a little less current to flow than the theoretical value of 460 ohms you've calculated, so other than delivering a little under 25mA that's fine as well. The approximately 0.5mA lower current won't make a visible difference.