My question may sound very basic, but I am very confused by the difference between the voltage and power ratings of a resistor.
Vishay's document says:
Rated Power
The maximum amount of power that can be continuously loaded to a resistor at a rated ambient temperature. Network and array products have both rated power per package as well as per element.
Rated Voltage
The maximum value of DC voltage or AC voltage (rms) capable of being applied continuously to resistors at the rated ambient temperature.
I read this datasheet for a 27Ω, 0.2W resistor. Page 3 of the datasheet shows this formula:
\$ RCWV = \sqrt{P\times R}\$
Where RCWV = Rated DC or RMS AC continous working voltage at commercial-line frequency and waveform(volt)
P = power rating (watt)
R = nominal resistance (ohm)
The above 27Ω resistor on the link has 50V voltage rating and 0.2W power rating, then I place the values into the provided formula
\$ RCWV = \sqrt{0.2\text{W} \times 27\Omega}=2.32V\$
Could anyone explain me why the voltage rating is 50V, not 2.32V?
When I want to calculate the maximum current that the resistor can stand using the power rating of the resistor (0.2W):
\$ P=I^2 \times R \$
\$ I = \sqrt{\dfrac{P}{R}} = \sqrt{\dfrac{0.2\text{W}}{27\Omega}} = 86\text{mA}\$
If I use the voltage rating:
\$ I = \dfrac{V}{R} = \dfrac{50\text{V}}{27\Omega} = 1.85\text{A}\$
By looking at these results, I should use the power rating, right?
Answer
The voltage rating is for the resistor series typically and specifies the maximum peak voltage you can apply without danger of damaging the resistor due to corona, breakdown, arcing, etc.
The power rating is completely independent of the voltage rating. It specifies the maximum steady state power the package is able to dissipate under given conditions.
You have to conform to both specs. If placing the maximum voltage across the resistor results in more power than the spec allows you have to reduce the voltage until you meet the spec. Likewise you can't increase the voltage above the rating just because you're not hitting the maximum power limit.
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