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Value of the capacitor parallel with the DC motor.

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, May 7, 2006.

  1. bananasiong

    bananasiong New Member

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    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
     
  2. dknguyen

    dknguyen Well-Known Member

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    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.
     
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  3. bananasiong

    bananasiong New Member

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    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?
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    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.
     
  6. philba

    philba New Member

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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.
     
    Last edited: May 7, 2006
  7. dknguyen

    dknguyen Well-Known Member

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    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.
     
  8. Hero999

    Hero999 Banned

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    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.
     
    Last edited: May 9, 2006
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  9. dknguyen

    dknguyen Well-Known Member

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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 :)
     
  10. bananasiong

    bananasiong New Member

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    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?
     
  11. dknguyen

    dknguyen Well-Known Member

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    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).
     
  12. bananasiong

    bananasiong New Member

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

    thanks for helping :p
     
  13. JayTheLockie

    JayTheLockie New Member

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    Sorry to hi-jack an old post but I'm trying to fix an old 12v motor that pressurizes the ABS system on my old range rover, it has what I guess is a capacitor (flat square blue thing) marked "1uom§ 2f463" I have no idea how to test it (there is no ohm reading from it) so just want to replace it before I put it all back together. Having read the other posts in this thread I'm assuming its to help protect from spikes and help with the HF noise.

    So my question is, what do I ask for when I go to maplin as their not that clued up in my local one and I have no idea what I need!

    Thanks in advance if anyone replies.
    Jay
     
  14. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Capacitors rarely go bad, especially non-polar ones. Why do you think you need to replace it?
     
  15. JayTheLockie

    JayTheLockie New Member

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    well the motors not easy to strip and rebuild so I only want to do it once, plus the motor wasn't running great and was struggling to pressurize the system so just want to replace anything I can to try and get it running 100%. Not knowing much about motors I thought it best to replace it??
     

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