Limiting Factor Calculator
Resistors have two ratings related to electrical stress. The main one is Rated Power (Pr) in Watts. This is the maximum power the resistor can continuously dissipate at a stated ambient temperature (usually 70°C). If this is exceeded, the resistor will
get too hot. Possible results of this are permanent value change, open circuit failure, and damage to the PCB or surrounding components.
The second rating is Limiting Element Voltage (LEV) (sometimes called Voltage Rating or Operating Voltage) in Volts. This is the maximum voltage that can be continuously applied across the resistor, even if Rated Power is not being reached. Applying
a higher voltage may result in electrical breakdown. Possible results of this are permanent value change and open circuit failure.
In almost every case where a specific type and value is selected, only one of these ratings will be relevant. This rating is the limiting factor, and which it is can be determined using the calculator.
Capacitor Discharge Calculator
Bleed resistors are used to discharge capacitors to safe voltage levels after power is removed.
A bleed resistor may be either switched across the capacitor for rapid discharge without quiescent dissipation, or permanently connected for high reliability and low cost. In the latter case there is a tradeoff between the time to reach safe discharge
and the quiescent power loss.
Selecting a suitable ohmic value is made easier by the calculator. This links the discharge time to the resistor value and calculates the initial power. For a switched bleeder this is the peak power with decay time constant indicated. For a permanently
connected bleeder it is the continuous dissipation, and the resistor chosen must be rated accordingly.