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With DC motors, it is common to put a freewheel diode and/or a capacitor in order to protect the equipment as the motor can induce current into the system.

I plan to use this board to control a 24V DC motor with a Arduino-like microcontroler. In an example in their documentation, they don't put such protection, so I wanted to know if it's unsafe, or is it that the board already protects the system?

The example in question: enter image description here

jonathanGB
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2 Answers2

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The motor driver board is bidirectional: it uses a transistor arrangement called a H-bridge to drive the motor.

H-bridge

The N-type MOSFETs being used as switches in that board inherently contain a "body diode" as a side effect of their construction. That diode is shown as part of the schematic symbol of the transistors. Normally the transistors will take care of the inductive current, but the body diodes will provide a path for the inductive spikes if something goes wrong with the circuitry that controls the transistors.

If you were to add a diode in parallel with the motor that diode would not only be unnecessary, but it would short out the power supply trough the diode when you reverse the motor.

jms
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  • Thanks a lot for the explanation. Where did you see the part about body diodes on this board? – jonathanGB Feb 08 '16 at 23:25
  • They state in the manual you linked "Fully NMOS H-Bridge for better efficiency and no heat sink is required". Practically all powers MOSFETs have a body diode, the reason is a bit complicated but you can read [this section on wikipedia](https://en.wikipedia.org/wiki/Power_MOSFET#Body_diode). – jms Feb 08 '16 at 23:40
  • @jms MOSFET body diodes tend to be too slow for freewheel diode duties. Additional Schottky diodes in parallel with the MOSFET body diodes are called for. (There are specialized MOSFETs with built-in Schottky diodes, but those are relatively uncommon.) – Nick Alexeev Feb 08 '16 at 23:44
  • @NickAlexeev True, but as I stated the body diodes will not conduct under normal operating conditions, and the board in question lacks external diodes. When the motor is pulse-width modulated one side of the bridge is usually held at either the supply rail or ground and the other side quickly alternates between a conducting high side and low side, allowing the current to flow continously without going trough the body diodes. The body diodes only take a role when the dead time is too long or when the motor is abruptly stopped. – jms Feb 08 '16 at 23:53
  • If "pratically all MOSFETs have a body diode", do these have such? http://www.amazon.ca/gp/product/B00L8VC9TK?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00 I use them to control a solenoid. – jonathanGB Feb 09 '16 at 00:14
  • @Jonathan The body diode won't save you when driving a solenoid with a single transistor, since you still need a freewheeling diode in parallel with the coil. If you are unsure about the presence of a body diode within a particular transistor, [check the datasheet](http://www.irf.com/product-info/datasheets/data/irfz44n.pdf) of your part and find out. (spoiler: the IRFZ44N does have it) – jms Feb 09 '16 at 00:23
  • @jms So I still need the extra diide, even if the transistor has one? Why? – jonathanGB Feb 09 '16 at 00:29
  • Because when driving a solenoid with a single transistor the body diode is in the wrong place; it is in series with the coil instead of being in parallel. See http://electronics.stackexchange.com/questions/100134/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil – jms Feb 09 '16 at 00:37
  • @jms Oh ok, so what's the use of a in-series diode for the transistor? (sorry for all these questions) And is there a conclusion for the initial question, aka do I need a diode in parallel? – jonathanGB Feb 09 '16 at 00:50
  • "What's the purpose of the in-series diode for the transistor" I just told you, read http://electronics.stackexchange.com/questions/100134/why-is-there-a-diode-connected-in-parallel-to-a-relay-coil. "is there a conclusion for the initial question" yes, I answered "If you were to add a diode in parallel with the motor that diode would not only be unnecessary, but it would short out the power supply trough the diode when you reverse the motor". Are you trolling? – jms Feb 09 '16 at 08:10
  • Sorry I got confused for a moment and Nick was contradicted you. I believe you. – jonathanGB Feb 09 '16 at 14:24
  • @Jonathan Nick didn't *contradict* me, he misunderstood a part of my answer (because I tried to keep the answer as simple as possible without going into technicalities). Then you bought up the solenoid (a completely different issue) and got confused even more. – jms Feb 09 '16 at 14:48
  • @jms ok everything's good, thanks for your help, and sorry again. – jonathanGB Feb 09 '16 at 14:51
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    @NickAlexeev: When you say "Additional Schottky diodes in parallel with the MOSFET body diodes are called for.", it sounds like you are recommending that the Orginal Poster somehow *additionally* attach *additional* diodes to the board he already has. How do you propose the OP do that? – davidcary Feb 11 '16 at 18:06
  • @davidcary If a motor driver board doesn't have proper freewheel diodes, it's a flaw in the design of the board. Additional diodes can be attached dead-bug style, or however other way one can manage. – Nick Alexeev Feb 11 '16 at 19:00
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summary

You should make sure a 0.1 uF capacitor is connected directly to the motor. A properly-designed motor driver board handles everything else.

details

The only component the user might need to add when using a (properly-designed) motor driver board is a EMI-supression cap attached directly to the motor. Why is this capacitor hooked up to a 9volt electric motor? ; Why connect capacitors to motor body? ; Capacitors and motors ; Dealing with Motor Noise ; etc.

As jms already mentioned, a properly-designed motor driver board already includes the freewheel diode(s).

"For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements." -- Wikipedia: switched-mode power supply.

(Modern "super barrier rectifier" diodes are technically better in many cases).

A properly-designed motor driver board already includes (a) the protection capacitor or other component used to absorb energy from the motor during regenerative breaking, and (b) the capacitor used to supply quick pulses of energy that cannot be supplied through the inductance+resistance of the available battery connection. Often (a) and (b) are the same capacitor.

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