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[SOLVED] Why does the filter capacitor boost the voltage in a full wave bridge rectifier?

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Arunkrrish

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why filter capacitor boost the voltage in full wave bridge rectifier?

hello!
I have design a full wave bridge rectifier,
input is 24 v.a.c
output of bridge rectifier is 23.2 v.dc(r.ms)
but after placing the filter capacitor the voltage across the capacitor is 33 v.dc.(capacitor used is 1000.u.f 50.v.)
i just need to understand why there is increment in the voltage after capacitor.is there any consideration or any other calculation for this point. i just want to understand how there is the increment in this voltage.please clarify this doubt.i will be very grateful.thank you.
 

Re: why filter capacitor boost the voltage in full wave bridge rectifier?

Hi,

Input is 24V RMS sine (I assume).
A 24V RMS sinewave
* starts at 0V
* it rises to it's positive maximum of 24V x sqrt(2) = 33.9V
* then it fall diwn to 0V
* it further falls to it's negative maximum of -33.4V
* then it rises to 0V
.... and starts again

The RMS value is an equivalent value for a DC value. This means a resistor connected to 24V RMS (independent of waveform) will generate the same power (heat) as if it is connected to 24V DC.

For a resistor the (voltage) polarity is not important, thus after the rectifier the resistor will generate the same heat as on the true AC.
A little difference comes from the voltage drop caused by the rectifier diodes.
Thus you see a little less RMS voltage after rectifying.

But when you conne t a capacitor, then it becomes charged as long as the absolute voltage voltage before the rectifier is less than the capacitor voltage (ignoring the voltage drop)
This means the capacitor does not get charged to 24V, but to the peak of the sine wave, which is 33.9V.
You see 33V because the voltage drop of the rectifier diodes.

Klaus

Added:
You will find a lot of documents in the internet explaing this. With scope pictures and more details.

Btw: Even with a half wave rectifier you get a capacitor voltage of about 33.9V (minus voltsge drop).
 
Re: why filter capacitor boost the voltage in full wave bridge rectifier?

@Klaus thank you very much sir.very well explained.one more thing i need to ask,is there any ways to reduce this 33 voltage(i think it cant be reduce ,just need a clarification ) thank you.
 

Re: why filter capacitor boost the voltage in full wave bridge rectifier?

Hi,

There are many was to reduce voltage:
* less AC input voltage (choose another transformer)
* series resistor (only useful with constant current load)
* series (zener) diodes
* linear voltage regulators (ICs or discrete)
... all these solutions will waste a lot of power and thus generate a lot of heat: P = (V_in - V_out) * I_ load.

Thus there are switching regulators: DCDC converters, step down converters, buck converters..
They all don't waste that much power.

Again: search the internet. There are a lot of tutorials on how to design basic power supplies. Even videos.

Klaus
 

Re: why filter capacitor boost the voltage in full wave bridge rectifier?

Basic tutorials often mention a choke-input filter. It seems to be seldom chosen because an inductor is not so adaptable for any and all purposes.
A given Henry value narrows voltage swing to some extent. If Henry value is too large, or if you try to draw overmuch current, it could cause output voltage to drop 50%.

The methods in post #4 are easier to work with and to adjust for various purposes.
 

Re: why filter capacitor boost the voltage in full wave bridge rectifier?

Yes if you want to step down your voltage you can buy an efficienct dcdc converter by eg Traco, Recom, Murata, TDK, etc etc
 

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