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Filter noise from DC motor

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itmth19:
I know this is not a new problem to a novice like me. I searched some topics in the forum, had some results like this post
--- Quote ---http://www.societyofrobots.com/robotforum/index.php?topic=9978.msg76377#msg76377
--- End quote ---
. However, I have not understood the situation yet, and I could reply on the old topic but the page says that "consider starting a new topic" so.. ;D

My question is:
I saw some schematics use Capacitors and Diode parallel with DC motor to filter noise. I know this is a beginner question, but according to what I learnt from high school:
-Diode is used to limit the current to one direction because when the DC motor stops, it could gives reverse current and could harm the other stuff.
- Capacitor is used to store power, and prevent to current to go through in the schematic.

So, how those things relate to noise filtering for DC motor? Could anyone explain for me, please. I really want to understand throughly in schematic problem before making a real line following robot.

Regards,

mstacho:
Correct about the diode: it will prevent current from flowing backwards.  SUPER important when dealing with more expensive stuff.

Consider the capacitor as a storage of energy.  If the goal is to make the circuit without weird noise spikes from the motor, then you want to be able to ensure it has a constant voltage.  But if the motor is spiking, you might get the actual voltage dipping too high or too low.  Now, if you have a cap in there, if it drops momentarily, the cap will supply the excess, and if it goes too high, and the cap is fully charged, it won't charge much more, and the voltage will remain more or less constant.

MIKE

itmth19:
@mstacho: Thanks a lots for the detail explain. Now I understand the things. Hope you get well with ur current project

jkerns:
A DC motor contains a big coil of wire around an iron core (the armature). This generates the magnetic field necessary to make the motor turn. But, it also acts as an inductor. Inductors resist a change in current - when you turn the power on to the coil, the rise in the current is slowed down. But, when you disconnect the power, the current wants to keep flowing - as the magnetic field collapses it induces a current in the inductor.

Now, where does that current go?

What is the voltage?

Remember Ohm's law?

I = V/R  let's re-arrange that to be V = I * R. If we have an open circuit, what is R? Infinite, correct? So any current at all times an infinite resistance results in how much voltage?

OK, it's not that bad in real life, but you can get large voltage spikes. The diodes allow the current to flow so you don't see the very large resistance and build up the very large voltages.

Also, in a brushed motor, you have the mechanical connection between the brushes and the commutator (which carries the current to the armature). As the brushes bounce, and cross boundaries between sectors on the commutator, you generate a lot of higher frequency electrical noise (pulses). Capacitors provide a place for the higher frequency electrical noise to go instead of having it go back into the rest of the circuitry.

itmth19:
@jkerns: I know that's a little bit jerk but why the diode has to be parallel to the DC motor?
p/s: Thanks for your reply

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