Hobby Issues

[Vikesrock]

Originally Posted by Don Rotolo

In addition to what was said before, some other considerations:

1. Be sure to consider the power dropped by the resistor. As an example, because P=I*V, 40 mA at 18 volts = 0.72 watts - use a 1 watt or larger resistor or it'll overheat quickly.

Shouldn't power calculations be done with RMS voltage and current?

P=IV=V^2/R

For 270 ohms:
P = 6.5^2/270 = .156 W

For 390 ohms:
P = 6.5^2/390 = .108 W

1/4 W resistors should be fine as far as I can tell. I am still learning this stuff though (Sophomore EE major) so please feel free to correct anything here not quite right.