The formulae in these tools are believed to be correct but they are provided for guidance only and no liability is accepted for consequences of errors.
The choice of resistance value can determine the magnitude of voltage, current and power dissipated in a circuit.
The resistance value of a resistor is defined as the ratio of the voltage across it to the current through it. The power dissipated is the product of voltage and current. In this calculator, enter any two known values, and the remaining two values are calculated.
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.
Temperature Derating Calculator
Resistors must be operated at lower dissipations when at high ambient air temperatures.
The power rating given for resistors applies only up to a certain ambient air temperature. This is normally 70°C or 25°C, but 40°C, 85°C and other temperatures are sometimes used. At local air temperatures above this rating air temperature the permitted power dissipation reduces, until at the maximum ambient air temperature only nominal dissipation is allowed. This prevents excessive temperature rise in the resistor body and / or solder joints.
The derating characteristic is normally linear from rated power down to 0W. The calculator performs two functions. Firstly, given the operating air temperature it gives the maximum permitted power dissipation. Secondly, given the operating power dissipation it gives the maximum permitted ambient air temperature.
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.
The pulse performance of resistors is important in inrush, protection and discharge applications.
The way in which this performance is presented depends upon resistor technology. Wirewound resistors have an energy capacity in Joules which depends on ohmic value, but not on short pulse duration. Thick and thin film resistors have a peak power depending on pulse duration. However, the energy capacity is not fixed and there is less dependence on ohmic value.
In order to relate the pulses experienced in an application to the pulse performance stated on the datasheet, it is necessary to calculate peak power for film or energy for wirewound. Common exponential pulse shapes must be converted to rectangular pulses of equivalent energy as rectangular pulses are generally used to characterise resistor performance. Finally, if pulses are continuous, the mean power dissipation must be calculated so as to ensure that the power rating is not exceeded.