DC Motors and digital electronics

[boylesg]

I am confused folks.

For the last 10 years, or whatever, I have been reading that you must include a free wheel diode across any inductive load, including DC motors, to suppress the inductive kick back voltage spike.

But now some one is insisting that you should use a capacitor with a DC motor, rather than a diode. Also to suppress brush noise which I don't understand, unless it is something like the bypass caps you put across the
Vcc and GND pins of an IC?

So which is it?

And also I have observed (in discarded electrical product that I have pulled apart) fairly complex capacitor/resistor networks on DC motors with part of it soldered to the case of the DC motor. What is the purpose of those and how do they work? I have never seen a single cap soldered across the motor terminals like you do with ICs.

 

[KlausST]

Hi,

DC motor just ON/OFF controlled?
Or speed controlled with some kind of PWM?
Or even direction controlled?

****
Basically you talk about different problems:
* a free wheeling diode is good for the back EMF of the stray inductance of the motor... during switch OFF of the power.
* noise caused by the brushes
* and noise can be voltage or current

--> Each application and each problem has it's own solutions.

Klaus

 

[FvM]

DC motor has considerable rotor inductance, respectively an electronic switch controlling it must be protected against kickback voltage, e.g. by freewheeling diode. A capacitor doesn't provide protection.

EMF filter capacitor is an unrelated topic.

 

[KlausST]

Hi,

I generally agree. Just want to add ..
While saying "it needs" does not necessarily mean you need "to add" a freewheeling diode.

Example: MOSFET half_bridge or full_bridge drivers have the "free wheeling diodes" built in in the (opposite) MOSFET.
For many applications this is sufficient.

Half_bridge driven compared to single_transistor driven also has the benefit of better RPM stability vs torque.

Klaus

 

[boylesg]

Hi,

DC motor just ON/OFF controlled?
Or speed controlled with some kind of PWM?
Or even direction controlled?

****
Basically you talk about different problems:
* a free wheeling diode is good for the back EMF of the stray inductance of the motor... during switch OFF of the power.
* noise caused by the brushes
* and noise can be voltage or current

--> Each application and each problem has it's own solutions.

Klaus

Mostly in the context of DC motors in wheeled robots. But one person using some sort of actuator which I am unsure of its nature - solenoid type of thing with on/off functionality or DC motor based like an aileron. This person is using a relay to control the device - OK if it is solenoid like but I can't see how you can control a motor based device with other than a H-bridge.

--- Updated Mar 13, 2023 ---

DC motor has considerable rotor inductance, respectively an electronic switch controlling it must be protected against kickback voltage, e.g. by freewheeling diode. A capacitor doesn't provide protection.

EMF filter capacitor is an unrelated topic.

Yes but all the same that is a gap in my knowledge. How do those capacitor networks suppress brush noise? What are the constructs called so I can google them and understand how they work.

 

[KlausST]

Hi,

basically you did not answer a single question clearly.

But ask new questions:

I can't see how you can control a motor based device with other than a H-bridge.

* single MOSFET / BJT (already mentioned)
* half bridge (already mentioned)
* solid state relay
* mechanical relay
* SCR/thyristor from an AC source
* dedicated DCmotor driver IC
* ....

Klaus

 

[danadakk]

Snubbers and Diodes :

Snubber - Wikipedia

en.wikipedia.org

https://www.ee.bgu.ac.il/~dcdc/notes/Additional_2012/Snubbers.pdf

https://www.cde.com/resources/technical-papers/design.pdf

Regards, Dana.,

 

[boylesg]

Hi,

basically you did not answer a single question clearly.

But ask new questions:

* single MOSFET / BJT (already mentioned)
* half bridge (already mentioned)
* solid state relay
* mechanical relay
* SCR/thyristor from an AC source
* dedicated DCmotor driver IC
* ....

Klaus

The fellow is tying to control his actuator with a mechanical relay.
This type of device: relay
If the device is motor based then I am suggesting to him that he control it with a H-brdige module like this:H-bridge module