# Why capacitance matters on bridge rectifier?

#### voyak

Joined Oct 23, 2020
5
Hi guys I'm working on my homework on PSPICE.

It's about measuring ripples of diode bridge rectifier.

The circuit and its simulation result is attached.

What I wonder is, if I increases the capacitance, ripple starts to appear late.

I mean if I have capacitance of 100nF, the result graphs exactly shows ripple from the first but If I increases it,

It somehow shows delay to be fully charged. Why capacitance matters? I have no idea for it..

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#### ericgibbs

Joined Jan 29, 2010
11,122
hi voyak,
Welcome to AAC.
Consider the time constant of C3 and the R5 load resistor,
E

#### voyak

Joined Oct 23, 2020
5
hi voyak,
Welcome to AAC.
Consider the time constant of C3 and the R5 load resistor,
E
Thank you for reply. What I learned about time constant is for when input source is DC.

Is it same on AC too?

#### MrChips

Joined Oct 2, 2009
22,099
Thank you for reply. What I learned about time constant is for when input source is DC.

Is it same on AC too?
What do you know about time constant?
What is the equation for time constant?

Time constant remains the same independent of the applied waveform.

#### OBW0549

Joined Mar 2, 2015
3,566
I mean if I have capacitance of 100nF, the result graphs exactly shows ripple from the first but If I increases it,

It somehow shows delay to be fully charged. Why capacitance matters? I have no idea for it..
The reason it takes longer for a 10 uF capacitor to charge up and reach steady state than it does for a 100 nF capacitor, is that forward biased diodes do not have zero resistance; they have a finite, non-zero resistance and this resistance, acting together with the capacitor, determines how long it will take the capacitor to become fully charged.

The time constant of R5 and C3 has absolutely nothing to do with this "delay" and only affects the amplitude of the ripple.

#### ericgibbs

Joined Jan 29, 2010
11,122
hi v,
What is the action and purpose of a full wave bridge rectifier when connected to an AC voltage source.??

E

#### ericgibbs

Joined Jan 29, 2010
11,122
I mean if I have capacitance of 100nF, the result graphs exactly shows ripple from the first but If I increases it
It somehow shows delay to be fully charged. Why capacitance matters? I have no idea for it..
hi,
The TS, I believe is talking about the delay in the ripple from min/max ripple , this is due the Time Constant of the C and R

E

#### voyak

Joined Oct 23, 2020
5
hi,
The TS, I believe is talking about the delay in the ripple from min/max ripple , this is due the Time Constant of the C and R

E
Thanks. What I know about the time constant is, the time required to reach 63.2% of final output voltage. But what I can get is, in the case of AC, the source voltage is keep changing.

So I can't understand what meaning the time constant holds on AC circuit. It's weird if it is the time needed to be charged 63.2% of final voltage since the applying voltage is keep changing

#### voyak

Joined Oct 23, 2020
5
The reason it takes longer for a 10 uF capacitor to charge up and reach steady state than it does for a 100 nF capacitor, is that forward biased diodes do not have zero resistance; they have a finite, non-zero resistance and this resistance, acting together with the capacitor, determines how long it will take the capacitor to become fully charged.

The time constant of R5 and C3 has absolutely nothing to do with this "delay" and only affects the amplitude of the ripple.
Can I get any formula that can explain this effect?

#### OBW0549

Joined Mar 2, 2015
3,566
Can I get any formula that can explain this effect?
Probably not; the dynamic resistance of a forward-biased diode is not constant; it depends on several factors, including the current through it.

In any case, focus on understanding the principles involved here, not on getting some "formula."

#### voyak

Joined Oct 23, 2020
5
Probably not; the dynamic resistance of a forward-biased diode is not constant; it depends on several factors, including the current through it.

In any case, focus on understanding the principles involved here, not on getting some "formula."

So may I understand in this way?

Cap charged(but not up to its maximum) -> discharge thr resistor -> recharged but slightly higher than before -> repeat

so that after some period it is charged up to its maximum.

#### OBW0549

Joined Mar 2, 2015
3,566
So may I understand in this way?

Cap charged(but not up to its maximum) -> discharge thr resistor -> recharged but slightly higher than before -> repeat

so that after some period it is charged up to its maximum.
That will probably do for now, until you get further along in your studies.

#### ericgibbs

Joined Jan 29, 2010
11,122
hi v,
Rerun your simulation without the resistor and another run without the capacitor,
that should give an understanding on how the time constant of the circuit effects the output voltage.

E

#### bertus

Joined Apr 5, 2008
20,881
Hello,

You could also try an other type of diode, like a shottky diode.
The SB260 is such diode.
Then you can see the influence of the diode.

Bertus

#### sparky 1

Joined Nov 3, 2018
384
Converting AC to DC ( getting the charges to hold still ). One way to convert AC to DC is rectifying with diodes and filtering with capacitors. The energy stored in a capacitor is electrical potential energy, and it is related to the charge Q and voltage V on the capacitor. AC is a derivative of DC having time variable waveform as a result it is more complex than DC.
An AC source can be made to supply AC current such as a generator. (It includes a transfer of kinetic energy).
Because AC reverses periodically We use a quanitative method that can handle the time varying flow of electrical charge.
The resistor is a good place to establish a quanitative measurement when exploring an AC to DC conversion.