[SOLVED] circuit understanding problem

[twinleuniverse]

Hi,

I am a newbie to the electronics world. could any one help me understanding the function of capacitors and inductors in the attached circuit.

Regards

 

[klystron]

The series inductors and parallel to ground capacitors are low-pass filters

 

[Pheetuz]

As Klystron says, the circuit acts as a filter, any high frequency components in the waveform entering the circuit on the 3V3 line will be able to pass through the capacitors and down to ground, thus keeping them away from the processor.

 

[twinleuniverse]

thanks both of you. Could you explain further the behaviour of the inductors and capacitors in this circuit when a spike/... occurs. I would really appreciate if someone explain me the responses of individual components.

thanks

 

[RetroTechie]

A coil 'resists' changes of the current through it. That is, if circuit on the right draws a constant amount of current & you'd try to input more, a voltage will develop across the coil & energy stored in the magnetic field in it. Same in reverse direction.

A capacitor 'resists' changes in voltage across it. If you raise the voltage, it will draw current (=charge), if you lower the voltage, it will supply current (=discharge). Roughly like a battery, but responding orders of magnitude faster (and much less energy storage).

The combination makes that everything other than constant voltage, constant current will be 'resisted' (filtered, smoothed). Filtering characteristics depend very much on frequency, types and values are chosen to suit a particular application.

Think of this circuit as a 'micro-UPS' that will eat spikes & only lets smoothed DC pass. Or check this Wikipedia page.

 

[twinleuniverse]

Thanks retrotechie!

what if I use a voltage regulator instead of all the capacitors and Inductors. Can a voltage regulator be a direct replacement.

 

[RetroTechie]

Probably the inductors could go but the voltage regulator would need a reasonably clean input voltage. Read: properly designed AC -> DC adapter might be okay, powering by car battery (while car is in use!) not so much.

But you'll probably want those capacitors to stick around, in one form or another. Read more here: Wikipedia - bypass capacitor.

For digital logic, ceramic capacitors in the order of ~100 nF are common (close to IC power supply pins). It would help to know what type of IC it is on the right, what IC package is used (DIP, flat pack, BGA, ...), and on what sort of board it's located (single/double/four-layer board etc).

 

[7@rB]

inductors and capacitors cannot be replaced by a voltage regulators since they have totaly different characteristics.
I=dv/dt , the current that pass through a capacitor equals to the derivative of the voltage across this capacitor with respect to time, the capacitor stores energy as a voltage, when the capacitor is fully charged it acts then as an open circuit. for an AC the capacitor will charge and discharge in a specific time depending on the frequency of the this AC signal. this characteristic of the capacitos is used to design filters for example, frequency filtering, with low pass filters for example, no current will pass through this filter below a given frequency ( high pass filter --> above a given frequency) those filters cn be designed by just a capacitor and a resistor.

 

[RetroTechie]

inductors and capacitors cannot be replaced by a voltage regulators since they have totaly different characteristics.

Agreed in principle, but did you read the Wikipedia article linked above ?

The purpose here is to keep a supply voltage constant. Inductors can help, capacitors can help, voltage regulators can help (and perhaps none of those are needed). What is possible or optimal depends entirely on the application. But there definitely will exist cases where a voltage regulator could do a job previously done by an LC filter.

 

[rahulpsharma]

Wouldnt C3 in the circuit take care of the high freq noise associated with 3V3?? Why are, then, so many other capacitors needed for each pin of the IC to the right....

 

[klystron]

C3 will filter noise on the power line. The other caps and inductors are for noise filtering on the individual lines

 

[twinleuniverse]

It would help to know what type of IC it is on the right, what IC package is used (DIP, flat pack, BGA, ...), and on what sort of board it's located (single/double/four-layer board etc).

Thanks Retro! it is a dsp, ic package is PGA and 4 layer board.