Reducing EMI while driving Small Motors

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Shashank_parab

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Hi !
Rescently When i was constructing the project of automated fishing ,
I encountered the powerful EMI from three Small toy motors (brush type , permanent magnet) , it was so powerfull that , the controller 89C51 was almost unable to work , even after connecting 0.1uf and 1000uf capacitors at all the supply lines. After using two different power supplies , one for controller and other for motors , the circuit worked as required.
Can anybody suggest a solution for this problem so that i can use a single power supply for both.

I didnt tried connecting the Inductor in series of supply yet.

regards,
Shashank Parab
 

Is the power source of your application a series of batteries? If YES and if it's 6V, the following IC might solve your problem. If it's not 6V, you can follow the link provided in DCP020505 website to choose other input spec that suits your application.


DCP020505
Miniature, 2W Isolated Unregulated DC/DC Converters
Texas Instruments


The output of the isolated DC/DC converters should be connected to your 8051 circuit as the circuit usually requires less power compared to motor.
 

Hi !
As i'm making very compact circuit i cant use more parts , and this IC is not available in India, so i've to order it from texas which will take at least four days to deliver , After searching on Internet , i found some h bridge circuits which emply flyback diodes , which eliminates the back EMF produced by motor. This still remain to be tried on my circuit

Do anybody have experiance of similer condition.


regards,
Shashank
 

Shashank_parab said:
Do anybody have experiance of similer condition.
Click to expand...

You can try to filter out noise generated by DC motor.

Connect inductors in series with motor supply lines, every line should have its own inductor. Select inductors that will not saturate even when the motor consumes maximum current. Their inductance is not so important, something like 100 uH should be enough. However, try to use inductors with winding capacitance as small as possible. Ideally you should connect a ferrite bead in series with every inductor.
Alternatively you might use a common mode choke instead of those two inductors.

Connect 47 nF ceramic capacitor across DC motor, as close to the motor as possible.

Connect another 47 nF ceramic capacitor across supply rail after inductors.

If possible, additionally use feed-through capacitors.
 

I think you should consider 2 possibilities,
1- because when you used a separate power supply the problem was solved, maybe your motors draw too much current from your power supply(battery ?), you should check this with an osciloscope. if when motors run, voltage drops then you should use a big cap for your microcontroller.
2- if the problem is EMI, you can put your microcontroller in a faraday cage.
 

Bluechem said:
2- if the problem is EMI, you can put your microcontroller in a faraday cage.
Click to expand...
Interesting... what is Faraday Cage? Thanks
 

picotube said:
Interesting... what is Faraday Cage? Thanks
Click to expand...
a faraday cage is a conductive box which is connected to ground and it protects inside components from EMI.
 

I see. In my opinion, I think, if there's a noise problem in Shashank_parab application, it should be 'conducted EMI', not 'radiated EMI'. If it's conducted, I think 1st solution given by Bluechem would help.

I encountered the similar problem as Shashank_parab when I did line follower. Isolated power supplies solved my problem. I also tried using filter and it did reduce the noise, but there were intermittent errors. Probably the 'strength' of my filter was not enough and needed further enhancement.
 


One thing is to supress/attenuate noise at its source, another thing is to enhance noise immunity of microcontroller. Both approaches should be used together.

Many microcontrollers are most susceptible to noise induced to oscillator input. Typical uC oscillator circuitry is shown below. To make it immune to noise, connect C1 and C2 to microcontroller Vdd pin by a dedicated PCB track; no other components can be connected to that track; then connect uC to common near its Vdd pin. In that case no noise current will flow through C1, thus, no noise will appear on uC osc input.
 

You need to put flyback diodes on your circuit to clamp the back EMF from the motor. I think that is your main problem.
 

information provided by original poster is rather sketchy, what kind of power supply has been used ?
what voltage ?how the voltage has been regulated ?
what kind of motor (voltage, power) ?
it would be very helpful if we could see the circuit. I didn't refer to the back EMF issue, because he refered to the issue and it seems that he is familiar with the concept. so I assume this issue has been taken care of. I hope he has solved the problem by now but if not maybe he can provide a little bit more information so we can be more specific. anyway as I said earlier he can find the origin of noise (if it is the problem) rather easily with an osciliscope.
 

Shashank_parab said:
... it was so powerfull that, the controller 89C51 was almost unable to work , even after connecting 0.1uf and 1000uf capacitors at all the supply lines.
Click to expand...
What do you mean 'almost unable to work'?
1) Do you mean when you power on the system (3 motors and the controllers), the system ran for a while and stopped?
2) Do you mean the system tried to start but failed?

If your problem is like case [1], then it's because of conducted EMI noise problem, which requires filtering, isolated of ground loop, or may be freewheeling diodes across switches.

If your problem is like case [2], then probably Bluechem's solution could help:
 

I did a project similar to your . And I used two GND wire ,one for motor & driver and another for microcontroller . After that I used LC filter close to the LM7805 for microcontroller + filter capasitor at motor terminal . Result = the error about this not happend but I don't sure that I did it right . If you know , tell me about it .Thanks
 

Anyone looking this up now.. optical isolation would be a preffered choice.
 

Your problem may come from the motor suddenly drains huge current to cause the CPU unable to work properly. Please try to verify the attached circuit. Hopefully it helps.

C1 : a big cap. e.g 470uF
D1 : 1N4148 will do because CPU itself drains limited current

When motor switches on, the transient pulls battery voltage down quite a bit and hence affecting cpu functionality. Adding a diode improves this sudden drop because it blocks C1 dischage current that goes backward to motor.

You can add a few decoupling cap. in parallel to C1 as well as close to CPU VDD. Try 10nF, 1nF & 100pF.

Twist the motor wires and directly connect to battery. Do not connect to CPU gnd!!
 

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