Resistor needed for: Arduino Micro + 12V Pump + HF3FA/005-ZTF relay
The relay you are using requires a coil voltage of 5 volts. Assuming that your Arduino Micro is able to supply the 5 volts at 72 milliAmps, then no resistor is needed.

Normally a resistor is used when the driving voltage is greater than the coil voltage. The resistor then "drops" the extra voltage across it so that the coil receives the correct voltage. So in the case of your relay, if you were using this relay but wanting to drive the coil with a 12 volt source, you would place a ~100 ohm, 1 watt resistor in series with it (7 volts / 72 mA).







simulate this circuit &ndash; Schematic created using CircuitLab