For a project I need to switch on a simple handheld vacuum cleaner with an Arduino. I am thinking of using a relay or mosfet. I opened the case and a 7.4 V 2200 mAh Li-Ion battery powers the motor, through a regular switch.

On the manufacturer's website, the power of this vacuum cleaner is 120 W, so according to my understanding:

I = P/V

I = 120 W / 7.4 V = 16.2 A

So according Ohm's law, the resistance of the motor has to be 0.46 Ohm right? Isn't this too low?


  • \$\begingroup\$ Isn't this too low? Too low for what? \$\endgroup\$ – Andy aka Apr 30 '19 at 12:12
  • \$\begingroup\$ We need to take into consideration the inductance of the motor also. \$\endgroup\$ – Ak19 Apr 30 '19 at 12:13
  • \$\begingroup\$ A brushed DC motor will have a much lower resistance at rest than you would expect from the current it draws when running. This is due to the back-EMF. That's why they draw a big surge when you first apply power. \$\endgroup\$ – Spehro Pefhany Apr 30 '19 at 12:17
  • \$\begingroup\$ @SpehroPefhany What do you mean by "drawing"? Does the motor lower it's resistance under load, so that a higher current can flow? \$\endgroup\$ – Verjan Apr 30 '19 at 12:22
  • \$\begingroup\$ @Verjan Yes, higher load (higher torque) requires more current (power), so the apparent resistance of a motor (due to back-EMF) changes with varying load. \$\endgroup\$ – JimmyB Apr 30 '19 at 12:26

View the entire system as something like this


simulate this circuit – Schematic created using CircuitLab

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