help with resistors?

0 ⤊

help with resistors?

i have a 9 volt Dc input and an led that runs on 1.7 volts and 20 milliamps. i need to know what size my resistor needs to be. please include formulas

2007-01-08 08:43:21 · 2 answers · asked by Anonymous in Consumer Electronics ➔ Other - Electronics

2 answers

450 ohms. the formula is 9volts/.02Amps = 450 ohms. 1/4 watt resistor will do.

2007-01-09 02:02:06 · answer #1 · answered by justme 7 · 0⤊ 0⤋

The first answer will work (not burn your LED out), but if won't be getting 20 mA. Since the LED drops 1.7V, we have:

(9V - 1.7V)/20mA = 7.3V/20mA = 365 ohms

7.3V x 20mA = 146mW

1/4 W will work OK; I'd be tempted to use 1/2 W if this was on 24/7

2007-01-10 03:31:12 · answer #2 · answered by sd_ducksoup 6 · 0⤊ 0⤋