DC & AC Signal Blocking by Capacitor & Inductor

[Anachip]

ac blocking

Hi Guys,

I have some clarification here regarding AC/DC signal blocking using passive components.

AC Blocking
=========

As I know that Capacitors will allow all ac signal but block the DC component. This is described by the formula XC= 1/2*pi*f*C. So, since DC has no frequency, according to the formula, f=0 and XC is finite, thus it blocks DC. But when it comes to AC the allowing of signals also depend on the frequency of the signal, as for low frequency the XC is high, and this situation may block AC as well compared to high frequency where XC is low and pass through AC very efficiently. Please explain.


DC Blocking
==========
As, i know that Inductor will allow all DC signal but block the AC signal component. This is explain by the formula XL=1/2*pi*f*l. So, since DC has no frequency, f=0 and XL will is small, thus it allows DC but as the AC have frequency, the XL will increase with frequency. So, at this case how effecient it can block AC signal, coz at low freqeuncy the XL is low and at this situation it can pass AC rite? please explain

Thanks,
Anachip

 

[Borber]

blocking ac

Those two formulas only define Xc and Xl. If you want to analyze blocking, attenuation or passing signal you must consider attenuator circuit first. Now you can study frequency dependance of attenuation.

 

[Anachip]

capacitor blocks ac or dc

Those two formulas only define Xc and Xl. If you want to analyze blocking, attenuation or passing signal you must consider attenuator circuit first. Now you can study frequency dependance of attenuation.

Hi Borber,

Can you explain me the concept of the both circuits and how it applies to my question. Thanks

 

[safwatonline]

inductors block ac

hello,
actually in using cap.+res. for low pass filter or high pass filter , it pass some freq. band "w/o attenuation" and blocks another band "attenuates it", so the question is which band it pass and and which it stops is completely dependent on the filter it self, so u should get analysis for the filter and find the Transfer function between the i/p and the o/p, finding this tr. function u could get the ratio between o/p and i/p at any freq. , usually we define the cut-off freq. which is the frequecy in which the
T.F=1/sqrt(2)>>> (i.e T.F=-3 dB) "this is for DC-gain of 1", and usually we consider this freq. the separation between the pass and stop band.
hope i was helpful, for more info just google low pass filters and read all about them , u may try this link https://en.wikipedia.org/wiki/Low_pass_filter
then read about high pass filters and other filters configurations , active filters,etc.
regards,
a.safwat

 

[SkyHigh]

blocking ac

Anachip,

The term block is a vague term. To be exact, it is high impedance to force an attenuation of the some signals you wish to "block".

Capacitor connected in series to a signal input is used to block DC and low-frequency signals as defined by Xc = -j/wC. The effective impedance is Zc = R + Xc. Z is the lead or pin resistance of the capacitor.
If connected in parallel, it works the opposite.

Inductor connected in series to a signal input is used to create high impedance, not really a block, to high-frequency signals, as defined by XL = jwL. The effective impedance is ZL = R + XL. R is the pin resistance.

Try working out the TF (transfer function) of a 1st order passive RC and RL LPF and HPF as given by a forumer above. You will get a clearer picture.

 

[IanP]

ac blocking capacitor

AC Blocking
=========

As I know that Capacitors will allow all ac signal but block the DC component. This is described by the formula XC= 1/2*pi*f*C. So, since DC has no frequency, according to the formula, f=0 and XC is finite, thus it blocks DC. But when it comes to AC the allowing of signals also depend on the frequency of the signal, as for low frequency the XC is high, and this situation may block AC as well compared to high frequency where XC is low and pass through AC very efficiently. Please explain.

DC Blocking
==========
As, i know that Inductor will allow all DC signal but block the AC signal component. This is explain by the formula XL=1/2*pi*f*l. So, since DC has no frequency, f=0 and XL will is small, thus it allows DC but as the AC have frequency, the XL will increase with frequency. So, at this case how effecient it can block AC signal, coz at low freqeuncy the XL is low and at this situation it can pass AC rite? please explain
Anachip

It looks you have just answered your questions by yourself ..

Regards,
IanP

 

[Miguel Gaspar]

how to block dc signal

You wrote

DC Blocking
==========
As, i know that Inductor will allow all DC signal but block the AC signal component. This is explain by the formula XL=1/2*pi*f*l. So, since DC has no frequency, f=0 and XL will is small, thus it allows DC but as the AC have frequency, the XL will increase with frequency. So, at this case how effecient it can block AC signal, coz at low frequency the XL is low and at this situation it can pass AC rite? please explain

this kind of inductors are special because DC and AD signal will flow trough them, usually ferrite cores, or special cores.

Another solution are circuits to shift level, used to reduce or eliminate DC bias. You can find them in op amp books in the intermediate stage of the op amp.

 

[ghaly]

dc component of ac signal

Hi
In your case blocking means preventing as you said the impeadance of a capacitor to any sinusodal signal is Xc = 1/2Π f c (provided that the capacitor is in series with the input signal for dc f=0 then xc is infinite ie will be completely attenuated ie has no effect on the output of the capacitor .
as for the inductor its impedance is xl = 2Πfl provided that the inductor is in series with the input signal for dc f=0 then xl is 0 ie the dc signal will pass without attenuation as for ac signal as the frequncy increases its impedance will increase ie the signal will be attenuated proportionaly.
regards

 

[electronics_kumar]

ac dc signal

Those two formulas only define Xc and Xl. If you want to analyze blocking, attenuation or passing signal you must consider attenuator circuit first. Now you can study frequency dependance of attenuation.

Barber
for the diagram provided by you how to find the transfer diagram

 

[Sigma|Six]

inductor block ac

for the capacitor operating concept, you can actually explain using electro-magnetic theory's coulomb's law.

Coulomb's Law provides enough explanation. Charges (electrons) collect on one plate
of a capacitor and repel charges (electrons) from the
other plate. The net effect is a shift of electrons all the way through the circuit. And if the capacitance is large enough, or the frequency high enough, potential change so fast, charge discharge keep repeating so fast, the shift of electrons is effectively indistinguishable from what would happen if there were a conductor connecting the plates.

Please let me know if my thoughts is incorrect.