# Selecting cap value to convert dc from PWM signall

Discussion in 'General Electronics Chat' started by aamirali, Jan 27, 2013.

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
1
I have to rotate a dc fan with voltgae 10V to 12V dc. Depending upon voltage the PWM freq will change.
Just for example at 12V, the PWM freq will be 1440HZ with duty cycle = 70%. atteched is the circuit

Now I have connected mosfet so that it will switch between high & low.
DC fan need continuous 12V. But due to PWM it will vary so that average dc voltage is produced.
Now I have to select a capacitor value for this.
What is the formula/how to calculate it.
Also should I connect inductor & capacitor or just capacitor will bve sufficient

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2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Is the fan a computer type fan with a brushless motor in it? If not, no filtering is needed for a brushed motor. If so, you need to determine the current drawn by the fan in order to calculate required capacitance to smooth out the ripple, though getting rid of all the ripple will be very difficult, most fans work fine with PWM, try it first.

3. ### aamirali Thread Starter Member

Feb 2, 2012
415
1
Yes I have to use capacitor.
My fan current is 300mA.
Normally I calculate capacitor by Vr = I/fC.
Vr= ripple, I=current, f= freq, C= cap

But in that I take duty cycle as 50%. I use in case for ac-dc converer

But in my case its 70%. Will formula change.

Also what the formual for avg value. I think its Vavg = Vpk*D. Right????? & Vrms = Vpk*sqr(D)

Last edited: Jan 27, 2013
4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Does the fan work?

What value capacitor are you currently using?

The formula is correct, but if you vary the duty cycle and are driving something picky about duty cycle, calculating for worse case is safest.