Calculation Tools

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.

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 70ºC). 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.

Rated Power (W)
Limiting Element Voltage (V)
Critical Resistance (Ω)
Below the Critical Resistance the Rated Power is the limiting factor whilst above it the Limiting Element Voltage is the limiting Factor
Resistance Value (Ω)
Maximum Power (W)
Maximum Voltage (V)
Conclusion

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.

Rated Power (W)
Rating Air Temperature (°C)
Derate to 0 Temperature (°C)
Either
Operating Air Temperature(°C)
Max Allowed Power Dissipation (W)
Or leave blank and enter below
Operating Power Dissipation (W)
Max Allowed Air Temperature (°C)

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.

Capacitor Value (μF)
Initial Charge Voltage (V)
Safety Threshold Voltage (V)
Resistance Value (Ω)
Discharge Time (s)
Initial Power (W)
Time Constant (s)
Total Energy Discharged (J)