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Topic: capacitor stopping motor?? (Read 927 times) previous topic - next topic

vek11

Hi, I am using a sn754410 to control a motor of 1.5-3v(a standard dc motor). I managed to get it to work with a power supply of 3V, but once I put my 0.1 capacitor, the motor does not do anything. Any suggestions to get it working ?

zoomkat

It depends on why you added a capacitor and where you placed it.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

vek11

I added it to reduce noise and I attached it across the two terminals on the motor.

zoomkat

I haven't come across an instance of capacitors actually fixing a motor issue during reading this forum. In your other thread you said "So I hear that I should attach a capacitor of 0.1u". As you have apparently learned, the capacitor has reduced the noise by apparently keeping your motor from running.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

dc42

I'm always skeptical of advice to put a capacitor in parallel with a motor that is being controlled using PWM. The capacitor causes high peak currents to flow through the switching transistor, mosfet or h-bridge, which may damage it. It may reduce noise coming from the motor, but it will certainly increase switching noise. IMO a capacitor across the motor terminals should only be used in conjunction with an inductor between the motor and whatever is driving it.
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zoomkat

Quote
I hear that I should attach a capacitor of 0.1u


That sounds like a typical  electrolitic capacitor value, which if used across the motor leads, would probably short out when the motor direction is changed.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

vek11

So I should take out the capacitor because it will damage the H-bridge? Is that what is being said?

Drew Davis


vek11


MarkT

Well its a balance really - capacitance at the motor terminals really will reduce interference - so long as this
isn't too large its not going to overload the driver circuit (but you have to do the maths and/or measurement
to get the best value).  Remember the leads to the motor have some intrinsic inductance and that a little capacitance
in the right places can moderate the high speed transients. 

Perhaps a simple rule of thumb is try capacitance at  the motor terminals thats about the same as the drain
capacitance of the driver MOSFETs - that way you are not loading them drastically but you are shorting out
RFI at source and reducing the radiation from the motor leads.

Monitoring the amount of interference with an AM radio tuned between stations can be very educational, BTW.
[ I won't respond to messages, use the forum please ]

Grumpy_Mike


Quote
I hear that I should attach a capacitor of 0.1u


That sounds like a typical  electrolitic capacitor value, which if used across the motor leads, would probably short out when the motor direction is changed.

Now to my mind 0.1uF does not sound like a typical electrolytic capacitor. It sounds like it is non polarised capacitor which should be fine and not stop anything from working.
The fact that you report it does stop the motor from working tends to suggest you have got something else in the description of your system wrong.

zoomkat

Quote
Now to my mind 0.1uF does not sound like a typical electrolytic capacitor. It sounds like it is non polarised capacitor which should be fine and not stop anything from working.


The main problem may be that there was no actual issue, but the op acted upon "So I hear that I should attach a capacitor of 0.1u", based on somewhat of an electronic "old wives tale".
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

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