Choosing the correct wattage for Converting Analog Input 4-20mA to 1-5V is very important. With higher DC voltages you can quickly reach the upper limits of your resistors and possibly blow them up.
For understanding how to convert 4-20mA to 1-5V Analog Inputs see our blog post converting analog input 4-20ma to 1-5v.
Calculating Wattage of Resistors
It is generally safest to simply use a 1 watt resistor when converting 4-20mA to 1-5V, however, if you have less than 1 watt or dealing with higher DC voltages it is important to calculate the max wattage of the resistor.
The formula is simple
MI = Max Analog Input Current * 150%
V = Voltage of System
So on a 24V system with a max analog input current of 20mA * 1.5 = 30mA
V * MI = 24V * .030A = .72W
In this case if you were to use a commonly used 1/4W resistor it would most definitely blow at it’s highest levels.
Notes:
Just note that depending on the resistor it may take 1 second or 5 hours for the resistor to burn up when it is over it’s maximum wattage. To be safe always size your resistor to a higher wattage to ensure accurate readings.
