The surge condition of passive electronic components is to apply a power level that exceeds the continuous rated power of the component at a specified length of time or pulse width. The pulse width is typically 25% or less of the thermal time constant of passive electronic components. For example, if the thermal time constant of passive electronic components is 20 seconds and reaches 63% of the final temperature, then an application that exceeds the continuous power rating by 5 seconds or a short power pulse would meet your definition. If the same pulse is applied for 60 seconds, the resulting temperature will exceed the continuous rated power temperature. Applications with surges lasting several seconds are rare. Most surges last milliseconds or microseconds.

You need to consider both repetitive and non-repetitive surge conditions. Repeated surges apply power for a given pulse width and then repeat at regular intervals or time cycles. You can usually easily measure the time period from the start of the power pulse to the start of the next power pulse because the leading edge of the pulse is usually well-defined. For repeated surges, the average power consumption during the pulse period must not exceed the continuous power rating of the resistor. To determine the average power, first determine the RMS power within each power pulse. For rectangular pulses, the number is the voltage squared divided by the resistance. For a half sine wave pulse, the power is 0.707V 2/R. For a common exponential capacitor discharge pulse, a conservative estimate of the RMS power at a time constant of the pulse is 0.5V 2/R. For other pulse shapes, the power is usually 0.5 to 1 times the square of the voltage divided by the resistance. To choose the appropriate passive electronic components, be conservative; If anything, overestimate power. The second step is to find the average power for that period. The number is the PULSE power P PULSE multiplied by the ratio of pulse width to period: P AVG=P PULSE (PW/T), where P AVG is the average power, PW is the pulse width, and T is the time.

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