# RC time

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#### khais

##### New Member
Hi everybody, Does The RC time constant of the capacitor count only in DC circuit or either in AC circuit?
if it's considred in both ,why it can not be changed in RC filter circuits!
and if it's only considered in DC why it counts in the clamper circuit where the circuit in connected to an AC source!
ijust want to understand the behavior of the capacitor and it's time constant when connected to either DC or AC.

any explaination will be appreciated

#### kinjalgp

##### Active Member
Capacitor provides low reactance to AC that means, it is alomst short for AC. Thus there is no charging.
The reactance (Xc) can be calculated as,
Xc = [1/(2 x Pi x f x C) ] Ohms
Where,
Pi = 3.14
f = AC signal frequency in Hz
C = Value of Capacitor in Farads

For DC, frequreny is 0 Hz which results in infinite reactance for any value of capacitor. Thus full applied voltage drop is created across the capacitor.

Even if you consider than capacitor charges with AC, in one half cycle, it charges with one polarity and during the other half cycle the polarity is reversed (or you can say it is discharged). Thus the net charge across the capacitor remains zero.

In clampers, the diode used, works as half wave rectifier which charges the capacitor only in one half of the input cycle which maintans a constant DC level across the capacitor and this is responsible for DC level shift at the output. In the next half cycle, the capacitor is not allowed to discharge.

#### khais

##### New Member
kinjalgp said:
Capacitor provides low reactance to AC that means, it is alomst short for AC. Thus there is no charging.
The reactance (Xc) can be calculated as,
Xc = [1/(2 x Pi x f x C) ] Ohms
Where,
Pi = 3.14
f = AC signal frequency in Hz
C = Value of Capacitor in Farads

For DC, frequreny is 0 Hz which results in infinite reactance for any value of capacitor. Thus full applied voltage drop is created across the capacitor.

Even if you consider than capacitor charges with AC, in one half cycle, it charges with one polarity and during the other half cycle the polarity is reversed (or you can say it is discharged). Thus the net charge across the capacitor remains zero.

In clampers, the diode used, works as half wave rectifier which charges the capacitor only in one half of the input cycle which maintans a constant DC level across the capacitor and this is responsible for DC level shift at the output. In the next half cycle, the capacitor is not allowed to discharge.
i realy know what you have written .My problem is to understand the clamper circuit. why the capacitor keeps it's charge as a DC level.if your answer is becuase of the RC time constant which has to be large enough then why it's not the case in RC filters(both circuits have the same source and components :!: ),in RC filter no matter what the values are, the capacitor will never change it's discharging curve.
ihope you got my point

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