Cannot understand 'ripple rejection' using 78xx ic here-

[Willen]

This is a picture from datasheet of 78xx regulator IC. This is an application of ripple rejection. I am confused little on schematic. What is '120Hz' box? Is it a new device like Xtal? And is it necessary to use a series resistor to reject ripple?

 

[JimB]

What is '120Hz' box? Is it a new device like Xtal?

It is a signal generator.

And is it necessary to use a series resistor to reject ripple?

No.

I am confused little on schematic.

Somewhere in the specification will be a statement of the ripple rejection ratio for this regulator.
The schematic is simply showing you how the manufacturer tests the ripple rejection ratio. If you want to test one of these L78xx devices to confirm the rejection ratio, then you should use this circuit.

It is NOT intended as a typical application. You would not use this circuit for normal operation.

JimB

 

[MrAl]

This is a picture from datasheet of 78xx regulator IC. This is an application of ripple rejection. I am confused little on schematic. What is '120Hz' box? Is it a new device like Xtal? And is it necessary to use a series resistor to reject ripple?

Hi there Willen,

In that circuit they are injecting a 120Hz signal so that the 78xx regulator can be bench tested for it's ability to reject ripple, which is usually 120Hz.

The reason for that 5.1 ohm resistor is so that the 120Hz source (the box) can actually inject some current into the junction on the right hand side of that resistor, and that causes a voltage change there which would be considered to be the ripple input. This ripple can then be measured there and on the output to see how well the device rejects the ripple voltage.
Vi (Vin) is a DC voltage input so that can not change, so that resistor isolates the DC source from the 120Hz AC source a little, just enough to allow a small change of voltage to show up at the input of the 78xx device for testing. Once tested, the 5.1 ohm resistor would be removed but then it would most likely be used with a rectifier and filter and that would already have 120Hz ripple. So the 5.1 ohm resistor is part of the test set up.