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# Trouble calculating L

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#### dernyarn

##### New Member
In this circuit im trying to calculate the inductor value required to get a frequency cut off of 4khz, how do i go about this? I cant seem to get an equation as i cant use the basic passive equations since R2 is not in its 'normal place'.

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The inductor requirement for a switching power supply not calculated based on a "cutoff frequency"

Im not quite sure what you mean sorry. With this circuit i was given this task:
"9. Change the cut-off frequency of the low pass LC filter to 4 kHz with C1 = 100 µF. State the value of L you used, and describe the effect that reducing the cut-off frequency has upon the output voltage waveform. "

The circuit as drawn is confusing on several levels.

The circuit looks like a fragment of a switching power supply.
The capacitor C2 is effectively out of circuit due to the apparently very high value of R2 (100 M Ohm).
Some context for the question may help to remove the confusion.

JimB

Heres the original question. It has the info for how we had to set up the switching power supply

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Is R2 100 Megohms, or 100 milliohms?

im pretty sure 100 milliohms

looking at a textbook i found; would i have to make a transfer function for the circuit, then equal it to 1/sqrt(2) to get an equation for cutoff frequency, then rearrange that equation to get the inductor value for the desired cut off frequency?

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calculate the reactance of C1 at 4khz, then use reactance and frequency to solve for L.

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