Bypass capacitor/s to cancel out noise.

[AndreasC]

Hi

I'm designing some circuitry to control lights and headlight motor control of a 1/8 scale car model. I have a small DC motor that draws around 300 mA at approx. 9 V but I also want the possibility to drive a bigger motor if needed, e.g if the motor dies and I need to change it, it's always nice to have the possibility to install a bigger one. I don't need the possibility of driving a motor that draws more than approx. 1 A though. I want to connect this motor to a main control board where all the control electronics sit. The motor is controlled in both directions through some circuitry on the main board and the power supply is connected to the same board.

I want to place a bypass/decoupling net across the incoming power supply for the motor to cancel out noise from the motor and I reckon I would need a big electrolytic capacitor for this.

My question is basically, would I need some ceramic caps in parallel with the electrolytic, what specs etc? How could a functional and effective net look like. Some over engeneering is always ok in my view.

 

[kam1787]

a pi filter would help [a series choke with 2 capacitors on either side] - to greatly reduce noise from the motor
a blocking diode after the electrolytic capacitor [ cap, diode, motor] - it keeps the voltage 'high' for the control board
a reverse diode to inhibit reverse EMF

 

[chuckey]

The biggest (only!) source of noise from a DC motor is the arcing at the brushes, this should be dealt with directly at the motor its self. Connect a pair of capacitors directly from each brush to the motor chassis, they should be disc ceramics , about .01 MF should do. The motor case should then be connected to what ever goes for earth (- volts?) on the car. This is to keep the noisy bits as small and as short as possible else if the capacitors are at the far ends of the motor leads, the leads will act as aerials and radiate the noise. As mentioned RF chokes in the motor leads could help, but again they should be at the motor end.
Frank

 

[AndreasC]

a pi filter would help [a series choke with 2 capacitors on either side] - to greatly reduce noise from the motor
a blocking diode after the electrolytic capacitor [ cap, diode, motor] - it keeps the voltage 'high' for the control board
a reverse diode to inhibit reverse EMF

Is there someting easier that could be used. I have capacitors at the motor and flyback diodes is present in the motor driver. What I want to do is good noise cancelation without too much headache. I don't understand all that much when it comes to electronics. I read somewhere that bypass capacitors should be placed near any electronics that need to be isolated from noise so I figured I need to place an electrolytic across the power supply leads to the motor driver. The motor is connected to an H-bridge on the main board that gets its power from a connection on the board. From that connection, the rest of the board electronics is powered. I guess what I want to do is to protect those electronics from any noise from the motor.

- - - Updated - - -

The biggest (only!) source of noise from a DC motor is the arcing at the brushes, this should be dealt with directly at the motor its self. Connect a pair of capacitors directly from each brush to the motor chassis, they should be disc ceramics , about .01 MF should do. The motor case should then be connected to what ever goes for earth (- volts?) on the car. This is to keep the noisy bits as small and as short as possible else if the capacitors are at the far ends of the motor leads, the leads will act as aerials and radiate the noise. As mentioned RF chokes in the motor leads could help, but again they should be at the motor end.
Frank

Thanks. I have those and also a cap between the brushes. Should I also connect the motor chassis to the minus pole of the battery pack?