Hi,
Can anyone tell me what's the value of the capacitor which is connected parallel with the DC motor? Is it called bypass capacitor? i know only ceremic capacitor is used, non directional. What's the value? Is it dependent to the usage of the motor or the voltage supplied?
Thanks

 

Yes, and capacitor that goes in parallel across the power supply terminals of anything is called a bypass capacitor. The value....that's really up to you. Try to pick a larger value since it will better smooth *large* motor voltage spikes. Larger capacitor values also filter out low frequency noise better.

The capacitor you use should be rated for at least 2x the voltage the motor is running off on. 4x is preferrable. Otherwise the voltage spikes from the motor will destroy your capacitor very quickly.

Here are some things you should know:
-If you just need the capacitor to filter out motor noise, they can be quite small.
-However, if the capacitor is for a motor controller (to take pressure off the battery and provide power when there are voltage dips), then your capacitor will have to be much larger. For example, the 24V 20A motor controller I have in front of me here uses a 2200uF capacitor. I personally would have used a 4700uF capacitor. I am building a 24V 100+ amp motor controller and plan to use a HUGE capacitor (probably overkill) between 1F and 250mF.

 

why 2x and 4x? the value of the capacitor should be 2 times more than the voltage the motor!?!? how come? they are different unit?
i thought the bypass is to protect the motor?

 

The capacitor is to reduce the amount of interference radiated from the motor, it has no effect on the working at all, and it certainly doesn't protect the motor at all.

As you will find it difficult to find capacitors of such low voltages (for most small DC motors), the voltage of the capacitors is rarely mentioned - as it's there to try and absorb the spikes, being higher than the motor would be obviously a good idea.

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because of Back EMF. A coil, when de-energized, will have a collapsing magnetic field that will induce a voltage (Back EMF). This voltage can be significantly higher than the original voltage feeding the coil. 2X will probably be ok but 4X is good engineering practice.

 

The capacitance of the motor and the maximum voltage it can work at are two different things. You can get a 1 Farad capacitor that can handle a maximum of 2.5V. if you try to run 12V through that capacitor you will destroy it.

The capacitor is not to protect the motor. Its too smooth out voltage spikes from the motor since the battery can't react as quickly as a capacitor can. It is also used to filter out noise.

 

Don't go overboard, large capacitors aren't very good at rejecting very high frequencies at which they become inductive, if your motor is causing interferance on a UHF TV channel then 100nf is probably too big, I recommend 1nF or 470pF, or better still use ferrite beads on the connections.

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Ah, that explains a few things. The big capacitors are used near the motor controller to smooth voltage spikes and assist the battery, and little capacitors are used close to the motor to kill the high frequency noise...well, there is also the practicality of not being able to stick giant caps on motor leads

 

yea, i have seen, usually the bypass capacitor for the DC motor is not big.


how if i drive my 12V DC motor in 9V? any capacitor can do that?

 

I think at those low voltages most things would work. Just dont try and use the 1F supercapacitors- those are rated really low (2.5V).

 

okay, so i will use a small ceramic capacitor to bypass the DC motor..

thanks for helping :P