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DC Motors and digital electronics

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boylesg

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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.
 
Last edited:

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
 

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.
 

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
 

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 ---

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.
 
Last edited:

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
 

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
 

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