What Is the Role of the Small Capacitor in a Full Wave Bridge Rectifier Circuit?

[Schnitzerxx]

Sometimes in Full Wave Bridge Rectifier circuits a small value capacitor is placed in parallel to the RC block at the output, such as the circuit diagram in the attached jpg. C1 has a much greater value than C2, and the result value of the parallel capacitors is almost the same.
Can anybody please help me out with this, what is the purpose of the of the small capacitor, I have been asked this question and I can’t find the proper answer, I have tried simulating the circuit with & without the extra capacitor on Multisim, but couldn’t find any difference.

Thanks a lot for your time.
Schnitzer

 

[dlgoff]

R with C1 will have a larger time constant and R with C2. You can think of it as two low pass filters. With C1 you can remove the ripple (120Hz) and with C2 you can remove higher frequency noise. Check out http://en.wikipedia.org/wiki/Decoupling_capacitor" .

Welcome to PF

 

[zeitghost]

The impedance of the electrolytic capacitor increases with frequency... so while it's great at 120Hz, it's not so good at higher frequencies.

The small capacitor compensates for this at the higher frequencies.

The property of electrolytic capacitors involved is the ESR, equivalent series resistance, which is important when you select capacitors for things like switch mode power supplies which typically run at a lot more than 120Hz.