resistor for 4-20ma sensor

Other Hardware General Electronics
1

Hey!

I would like to use 4-20 ma light sensor with my arduino board.
I already know that I need 220-250 ohm resistor.

Supply for sensor is 24 VDC. Could someone tell me how I can decide how many watts needs to have that resistor or how to calculate this?
I bought 240 ohm 1/4 watts but is so small that I don't know if I can use it..

I want to control lights bulbs depending of sensor reading (one output will consist 5 bulbs(230VAC) 36 Watts)

Thank You for Your reply and help!

2

Hi!

OK, .004A (4ma) into a 250 ohm resistor will give a voltage of 1.0 that is the lowest reading of your sensor
.020A (20ma) will give a voltage of 5.0V that is the max reading.

Max power is 5.0V across 250 ohms = Esquared over R = 25/250 = .1 watt (1/10 watt) so a 1/4 watt resistor would be safe..

Let us know how it works!