Power rating

All the resistors are given a specification which determines how much power they can dissipate safely. Typical values are 1/4 W, 1/2 W, and 1 W. Units exist with ratings of 1/8 W or 2 W. These dissipation ratings are for the continuous duty.

You can figure out how much current a resistor which is given to us can handle, by using the formula for power (P) in the terms of current (I) and resistance (R): P = I 2R. Work this formula backwards, plugging in power rating for P and resistance of unit for R, and solve for I. Or you can find square root of P/R. Remember to use amperes for current, ohms for the resistance, and watts for power.

The power rating for the given resistor can, in effect, can be increased by using a network of 2 × 2, 3 × 3, 4× 4, etc., units in the series-parallel. You have already learned about this. If you require a 47-ohm, 45-W resistor, but all you have is a 47-ohm, 1-W resistors, you can make a 7 × 7 network in the series-parallel, and this will handle 49 W. It might look terrible, but it will do the job.

Power ratings are specified with the margin for error. A good engineer never take the advantage of this and use, say, a 1/4-W unit in the situation where it will be required to draw 0.27 W. Actually good engineers include their own safety margin first. Allowing 10 %, a 1/4-W resistor should not be called upon to handle more than around 0.225 W. But it is silly, and pointlessly expensive, to use a 2-W resistor here a 1/4-W unit will do, un- less, the 2-W resistor is all that is available.