I have a project in mind using an RGB LED (here: http://amzn.com/B007RO9X82).
I would like it to be powered by USB. Since the LED is rated at 3.0-3.6V, I understand I'd need a resistor. What resistor would I need and would it throw much/any heat in this situation? The feed wiring and resistor would be enclosed in a hole drilled in a wood plaque…don't want to create a fire hazard.
USB Power specs from WikiPedia: [5.00±0.25 V (pre-3.0); 5.00+0.25-0.55 V (USB 3.0) Max. current 0.5-0.9 A (general)] what resistor would I need? I assume if I sized the resistor for standard USB and it were to be plugged into a charge port (5A), that would cause trouble, correct?
Best Answer
$$LED\;current = \frac{input\;voltage - LED\;forward\;voltage} {LED\;resistor}$$
$$I_{LED} = \frac{V_{USB} - V_{f_{LED}}} {R_{LED}}$$
Let us assume a current of 20mA for the LED.
$$0.02A = \frac{5V - 3.0V} {R_{LED}}$$ $$R_{LED} = 100\Omega$$
$$0.02A = \frac{5V - 3.6V} {R_{LED}}$$ $$R_{LED} = 70\Omega$$
I would use a 100 Ω LED.
At 20mA per LED, you could have \$ \dfrac{0.5A}{0.02A} = 25\$ LEDs in parallel.
The kit is supplied with 200 Ω resistors. Using them you could have 50 LEDs in parallel.