There are so many errors in post #14 that it's hard to know where to start!

First major point: look at the original circuit diagram - it shows a power supply with an LED (which is almost certainly just to show that it is 'on') and a REGULATED 9V output. If you remove the regulator, you will NOT get a regulated output! This is obviously designed, as the OP says in post #11, to power the other examples in the book. Removing the regulator is a pretty dumb thing to do!

Which then makes the second item much easier to determine. If we have 9V regulated output and want, say, 15mA through the LED (though, as others have said, that will probably be more than enough) then subtract the typical forward voltage (2.2V) from the 9V = 6.8V, divide by .015 and get a result of 453 ohms - which is almost exactly the 470 ohms suggested by a couple of other respondents. Job done.

Ignore all the stuff about peak volts and RMS. You have a very basic wall wart, whose output could vary enormously, so we have to deal with it. A large capacitor across the output of the wall wart/input of the regulator is essential, especially if it really does use a half wave rectifier (which I doubt; I would lay odds that any item the OP gets hold of will use a bridge rectifier) since you will otherwise get massive ripple on the regulator input and output. 1000μF, 25V should be fine, but there should also be the 0.1μF parallel capacitor to prevent regulator instability.

A relatively small capacitor can be used on the output of the regulator - say 10μF, 16V - and again, a 0.1μF in parallel will help with high frequency instability. As others have said, these should be located as close as possible to the regulator.

That's all you need to know. Good luck!

 

First - yes, I know what the 78xx series datasheets say about input and output capacitors. But ...

Actually, the schematic in post #1 is correct as it stands. The 7809 is acting as a "regulating rectifier". Whenever the input voltage is above 11 V, the output goes to 9 V regardless of how high the input continues to go. The large cap smooths this so the LED doesn't flicker. The diode inside the wall wart protects the 7809 from reverse voltage out-to-in. Not the greatest circuit in the world, but it will work as shown.

ak

 

There are so many errors in post #14 that it's hard to know where to start!
...

That's all you need to know. Good luck!

Colin, is that you?

 

Are you trying to make a 9V supply with LED indication, or an LED driver? If it's a regulator with indication, that circuit is wide open for "bad things" to happen. It needs an input cap for the regulator, as well as protection diodes around the regulator (especially if you hang 1000µF off the output). Any datasheet on any of the 3-terminal "78xx" will show you the correct way to do this.

If you're looking to drive the LED, the circuit shown will work, but not really the best way to approach driving an LED. Remember, LED's are current devices; voltage is only important from the perspective that you need it high enough to get over the LED barrier potential Vφ. Using the same components shown (more or less) and a bit of rearrangement will give you a constant current regulator (see attached image). LTSpice doesn't have a 7809 so I used an LM7805 for example. The "R1" resistor sets the current through the LED, regardless of input voltage. As shown, there's just a little under 20mA through the LED.

Good luck!
Mike Tripoli

 

Said it before - will say it again: Why use a regulator? All the circuit is doing is lighting an LED. Simple enough to just use a cap and resistor to control flicker and brightness. When it comes to a regulator - FAGHETA-BOUT-IT.

Near as I can tell; its the exercise of doing a project from a book.

 

Only if there's no load.
If there's a load the output capacitor will discharge which will draw current through the regulator to discharge the input capacitor.
That will keep the output voltage below the input.

Another thing not many people bother doing; is include a bleed resistor in the PSU.

 

Except for the LED/resistor...

 

Except for the LED/resistor...

Unless that's via a connector that could get unplugged..........................

There's no shortage of idiots out there - you have to anticipate what they might do.

 

Unless that's via a connector that could get unplugged..........................

There's no shortage of idiots out there - you have to anticipate what they might do.

Gave up on that a long time ago...

 

Another thing not many people bother doing; is include a bleed resistor in the PSU.

Yes.
For an LM317 regulator that's provided by the voltage set resistors.

 

Yes.
For an LM317 regulator that's provided by the voltage set resistors.

The TS isn't using a 317.