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Close BOTH switches SIMULTANEOUSLY to start.
The circuit on the left is typically used as an output ripple filter. It's a classic PI or CLC filter. The circuit on the right is the same filter with a slight modification, which I shall talk about later in the discription.
On closing the left switch, you observe a damped oscillation on the output of the filter lasting almost for 1ms, with a peak of 33V. The peak voltage of this oscillation is 13V higher than the input voltage! This peak can easily damage sensitive components, and must be eliminated. A good power supply's output doesn't feature such peaks.
On closing the right switch, you observe that there is a peak of 20.8V, which means that there is a slight overshoot of around 0.8V, after which the output voltage settles down. This is a more acceptable result.
In the second circuit, the oscillation is HEAVILY DAMPED, due to which the output voltage doesn't swing wildly. The diode (preferably Shottkey) takes care of damping the initial oscillation. When the output of the filter goes 0.3V about the input, the diode starts conducting and prevents the output from going too much higher than the input, thus preventing over-voltage. The diode conducts a surge current with a peak of around 32A only initially for half a millisecond, and then turns off. This helps prevent an output over-voltage due to LC oscillations.
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