Capacitors

[Steve_hi]

I read that a capacitor should have a value of at least twice the input voltage does this mean that only the capacitance is important and the voltage could be any value as long as it is twice the input voltage?
and with resistors does it matter if it's .25 watt or 5 watt as long as the resistance is the same i need to know because im trying to use used parts i salvaged from an old monitor and photo copier

 

[audioguru2]

Hi Steve,
I make many projects that are powered by a 9V battery and they have capacitors. I can't find 18V electrolytics so I use 16V and 25V ones. My metalized poly capacitors are 63V and 100V and my ceramic disc capacitors are 50V and 100V.

A resistor must dissipate power as heat. A 1/4W resistor is small and can dissipate a max power of 1/4W when it will be extremely hot. A 1/2W resistor is extremely hot when it dissipates 1/2W. A 1/4W resostpr will overheat and might fail if it dissipates 1/2W.
So calculate the power in resistors when you design a circuit and select their size (power rating) so that they don't overheat.