Hi, on searching circuit in Google, i got this schematic in which capacitor & resistance are connected in series with transformer input.

how it cancel noise??

 

If there is current flowing through the transformer primary when the switch is turned off, a large voltage "spike" will result. The resistor and capacitor in series across the switch act as a "snubber" to limit the amplitude of the initial spike, and to cause subsequent oscillations to quickly decay.

 

If there is current flowing through the transformer primary when the switch is turned off, a large voltage "spike" will result. The resistor and capacitor in series across the switch act as a "snubber" to limit the amplitude of the initial spike, and to cause subsequent oscillations to quickly decay.

But, how to find there values??

 

Have a look at this PDF: http://www.cde.com/tech/design.pdf

Lots more info by using Google to search for "Snubber design"

 

Provided the switch is a standard SPST mechanical switch as shown, then this snubber application has nothing to do with noise prevention. However, the snubber does help to prevent the switch from internally arcing whenever it is opened or closed which helps keep the contacts in good condition.

When snubbers are used across dynamic switches, such as transistors, then they help to reduce repetitive switching noise.

 

Noise can be audible or electrical (Radio interference). Snubbers work on the electrical noise.

 

Sir if i am not wrong the audible sound/NOISE produce if the winding is not good (having AIR GAPS) or there is vibration in bobbin frame ie.- I,E's of the transformers

Noise can be audible or electrical (Radio interference). Snubbers work on the electrical noise.
written by bill he is right
the electrical noise can be reduce by good combination of L,C,R CKT with diode