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Choosing a capacitor...

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2camjohn

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I am running a battery powered pic based circuit that drives a coil.

I already have decoupling capacitors near the pic and the voltage regulator.

I want to improve reliabiltiy when the battery is running low by adding a capacitor that stores some charge for the coil.



How do I work out what value to choose for the capacitor?

I know the voltage into the coil and the resistance of the coil, aswel as the 'on time' for the coil.

Thanks
John
 
2camjohn said:
I am running a battery powered pic based circuit that drives a coil.

I already have decoupling capacitors near the pic and the voltage regulator.

I want to improve reliabiltiy when the battery is running low by adding a capacitor that stores some charge for the coil.



How do I work out what value to choose for the capacitor?

I know the voltage into the coil and the resistance of the coil, aswel as the 'on time' for the coil.

It's probably as easy to just find it by experiment, you should also isolate it from the PIC circuit via a blocking diode - otherwise it could crash the PIC.
 
Use the formula: deltaV = I*deltaT/C
Where deltaV is the voltage sag of the capacitor
I is the current to the coil
deltaT is the time the coil is drawing current
C is the capacitor value.

This is approximate but will get in the ballpark.
 
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