• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Value of the capacitor parallel with the DC motor.

bananasiong

New Member
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
 

dknguyen

Well-Known Member
Most Helpful Member
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.
 

bananasiong

New Member
dknguyen said:
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.
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?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
bananasiong said:
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.
 

philba

New Member
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:

dknguyen

Well-Known Member
Most Helpful Member
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.
 

Hero999

Banned
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:

dknguyen

Well-Known Member
Most Helpful Member
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 :)
 

bananasiong

New Member
dknguyen said:
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.


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

dknguyen

Well-Known Member
Most Helpful Member
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).
 

bananasiong

New Member
dknguyen said:
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
 

JayTheLockie

New Member
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
 

MikeMl

Well-Known Member
Most Helpful Member
Capacitors rarely go bad, especially non-polar ones. Why do you think you need to replace it?
 

JayTheLockie

New Member
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??
 

Latest threads

EE World Online Articles

Loading

 
Top