The power dissipation rating (P), measured in watts (W), for a resistor indicates how much power can be converted to heat without damage to the resistor caused by its rise in temperature. The rating is closely linked to the physical size of the resistor, so that ¼Wresistors are much smaller than 1 W resistors of the same resistance value. These ratings assume ‘normal’ surrounding (ambient) temperatures, often 70◦C, and for use at higher ambient temperatures derating must be applied according to the manufacturer’s specification. For example, a ½ W resistor may need to be used in place of a ¼Wwhen the ambient temperature is above 70◦C. In Figure 1.7 is shown the graph of temperature rise plotted against dissipated power for average ½ W and 1 W composition resistors. Note that these figures are of temperature rise above the ambient level. If such a temperature rise takes the resistor temperature above its maximum rated temperature permitted for its type, a higher wattage rating of resistor must be used. Resistors with high ohmic values may need to be derated (run at a lower dissipation) when they are used in hot surroundings.
The power dissipation in watts is given by P = VI, with V the voltage across a conductor in volts and I the current through the conductor in amps. When current is measured in mA and V in volts, VI gives power dissipation in milliwatts, often more useful for electronics components. This expression for dissipated power can be combined with Ohm’s law when the resistance R of the conductor is constant, giving:
The result will be in watts for V in volts and R in ohms, or I in amps and R in ohms. When R is given in kW, V2/R gives P in milliwatts; when I is in mA and R in kW then P is also in milliwatts.
Note that power is defined as the amount of energy (also called work, W) transformed (from one form to another) per second. The unit of energy is the joule ( J), and the number of joules dissipated is found by multiplying the power in watts by the time in seconds for which the power has been dissipated, so
Be careful not to confuse the abbreviations W (work or energy) and W (watts of power). The abbreviation p is used for pressure and P for power.