CIRC-03 why the diode?

Hi all,

I am a complete novice when it comes to electronics (but not programming). I am working through the Arduino exercises and am having no trouble building the circuits and coding them, but this is very ‘parrot fashion’, I would like to understand what is happening within the circuits.

In CIRC-03 a diode is used, but I don’t understand why it is placed and connected the way that it is. I understand that it is there to protect the circuit and that it only allows current to flow in one direction. However, when I look at the schematic and board layout, it just seems to provide a one way connection between the + and - connections of the motor, it does not seem to provide a bridge or break in the circuit. For example I can see that the resistor provides a bridge that the current must cross to get from pin 9 to the base of the diode. I cannot see this bridge in the case of the diode - how is it protecting the circuit?

Sorry for the long question, I hope it makes sense.


Please post the schematic or a link to what you are referring to

I presume you are referring to the Oomlout starter kit rather than the official Arduino one?

Wow, thanks for the quick responses. Yes this is the Oomlout Circ-03. Schematic attached.


From the link I provided:

The 1N4001 diode is acting as a flyback diode for details on why its there visit: 4001 Redirect

To be a little more elaborate: A motor is a inductive circuit meaning it stores electrical energy in a magnetic field. An inductive circuit acts in such a way that it uses its stored energy to counteract changes in current by generating a voltage. When the current is disrupted this voltage can reach tens or even hundreds of volts even for a small motor causing malfunction or damage in other components. The freewhelling or flyback diode provides a path for the stored energy to discharge safely

Thanks nilton61, that makes a lot more sense to me. Is it called freewheeling because the motor will generate electricity whilst it is spinning down (freewheeling)?


Almost but not quite. A electrical machine with a magnetic (motors, generators and transformers) circuit works this way:

  • The current builds a magnetic field
  • Part of the energy stored in that magnetic field is converted (to mechanical energy in case of a motor, to electric energy of another voltage in a transformer)
  • The current drawn from the source is exactly so much as to replenish the converted energy. That's why current increases if you increase the load.

It is the energy stored in the magnetic field that must be dissipated by the diode (or more excact, all resistance in the current path). The same is true for relays. solenoids and all other inductive loads. So it has nothing directly to do with the motor acting as a generator although these issues are related through the magnetic field involved.
The term freewheeling is related to the metaphor of an inductance being equivalent to a flywheel with the current being the speed of the wheel and the inductance being the inertia. You cannot stop a flywheel immediately because the stored energy has to dissipate somewhere, presumably friction. Nor can you disrupt the current through an inductor also because the stored energy has to dissipate, presumably through resistance which is equivalent to friction.