[sdiy] Stupid resistor tricks?

[sbernardi at attbi.com]

Just compute the power in each resistor individually:  P = I*V = V^2/R = I^2+R.
In the case of two equal parallel resistors, they have the same voltage but 
split the current in half. So the power I*V will be one half in each, so two 
parallel 1/4W resistors of twice the value will do the same as a 1/2W.
For series resistors, they share the same current but will have different 
voltages. So compute the maximum current (Imax = Vmax/(R1 + R2)), and then 
compute power for each resistor:
P1max = Imax^2*R1
P2max = Imax^2*R2.

Say Vmax = 24v. Imax = 24/1.2K = 20mA
P1 = (20mA^2)*1000 = .4W  so 1Watt OK
P2 = (20mA^2)*200 = .08W so 1/4W OK
> Mail order is my only real option for parts. I often find myself wanting to 
> experiment, but not having the right stuff on hand. I'm wondering if I need, 
> for example, a 1.2k 1watt resistor could I get by with a 1k 1watt and a 200 
> ohm 1/4watt resistor in series? Also would two 1/4 watt resistors in parallel 
>  be an appropriate substitue for a 1/2 watt resistor? Works in my brain, real 
> life is probably a different story! :(