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Motor browning out PIC

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kangyunmei

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Hi guys,

What capacitor would I need to stop a small 12vDC motor (250ma loaded) from browning out a PIC controller?
The motor is controlled with a L293D controller.
PSU is a 12v 5A power supply, the project is not in the situation to be able to have 2 power supplies sadly.

I know a capacitor is probably needed for the motor, but I am unsure as to the value or type of capacitor needed.

Help and advice is, as always, greatly appreciated.
Kind regards,
 

You also need to pay attention to the earth paths.
 

Hi,

Not sure about it being adequate for your circuit but have seen 100nF ceramics across motor terminals. Doubt 1uF would do any harm, either.
 

Not sure if it's really a brown-out (voltage drop) problem. But in any case, you want to filter the processor supply, not the motor supply. Think about a storage capacitor in front of the 5V regulator, reverse current flow to the motor blocked with a series diode.
 

I agree with FvM, 'browning out' means the PIC supply has dropped below operating voltage which is unlikely if it starts from 12V. It is more likely noise generated by the motor is getting back into the PIC supply, reset or IO lines. A capacitor across the motor is rarely the solution, especially if it is driven with PWM or some other intermittent voltage. Usually, it has to do with shared current paths between the PIC and the L293 causing voltage fluctuations as the motor starts and stops. Good wiring and supply decoupling is essential but we need to see the schematic and layout to give better advice.

Brian.
 

Hi,

Read this, it will help:

Dealing with Motor Noise I would assume they know something about this topic...

Besides always using decoupling capacitors for ICs (think of them as an energy reservoir for sudden current draws from other devices or the decoupled IC itself, current draws that pull the supply voltage lower and can affect the IC using the current and/or nearby devices that don't have decoupling caps), motor terminal capacitors are a standard practice to smooth the sudden current draw motors cause on supply lines.
 

Hi,

motor terminal capacitors are a standard practice to smooth the sudden current draw motors cause on supply lines.
Click to expand...
True for AC motors and non PWM'd DC motors.

For PWM'd motors a capacitor is counter-productive....it definitely increases the current peaks.
Only in combination with a series inductance it can give improvement.

Klaus
 
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    d123

    d123

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I added the cap on the output of the [moderator action: removed link] plc and it all works a treat. Thanks for your help again!
 
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