LM78XX & LM79XX questions

[autir]

It is mentioned in the datasheet of the LM78XX series that for the standard operation the pins 1 (input) and 2 (ground) must be connected using a capacitor of 0.33

[ante]

Hi autir,

The cap between input and ground is supposed to protect from self-oscillation in the regulator. Self-oscillation is very destructive and heat generating and should be avoided at any cost. And the output cap will protect against load variations. I do recommend using both of them and always try to have the shortest leads and distance possible between the regulator and the caps.

Yes, I suppose you could use them upside down if you like! ;D

[audioguru]

Hi Autir,
Here is what National Semi says about those caps:

[autir]

What is self-oscillation? ;D
How can a capacitor improve transient response?
How can a capacitor minimise the effects of load variation?

[audioguru]

Hi Autir,
1) Self-oscillation occurs with some circuits like a regulator that have a very high gain and a lot of negative feedback because of phase-shifts in the circuit and load.
2) The output cap improves transient response because it absorbs sharp transients that the fairly slow regulator can't deal with. If a regulator's load is suddenly disconnected, its output voltage will have a increasing-voltage transient until the slow regulator's circuit "catches-up" with it. Likewise, if a regulator's load is suddenly applied, its output voltage will have a decreasing-voltage transient.
An output transient also occurs if the input voltage suddenly changes.
I think that the LM317 regulator's datasheet shows these transients and the smoothing effect of an output cap.
3) An output cap minimises the effects of very high frequency load variation because it has a low impedance (good filtering) at high frequencies. The very high gain and a lot of negative feedback in the regulator minimises low frequency load variations and also slow input voltage fluctuations (ripple).

[ante]

Thanks Audioguru,

Even though I meant the same I think

[autir]

Are the values of the capacitors of significance?
The absorbing of sharp transients in the output line is improved with the capacitance; a bigger capacitor would perform much better. Why just 1 pF?
Similarly for the input capacitor (is it an RF-lowpass filter to filter out AC?).

[audioguru]

Hi Autir,
You should use the capacitors recommended on the datasheet by the IC's expert designers. They know that a ceramic disk cap is much better at very high frequencies than an electrolytic cap, and that larger ceramic disks are also poorer.
They recommend a 0.1uF ceramic disk output cap, not only 1pF. 0.1uF is one hundred-thousand pF.
The input cap is needed when the main filter cap is at a distance because the wiring has inductance which causes a high impedance at high frequencies and the regulator needs a low impedance at its input to remain stable. Big ceramic disk caps have inductance and electrolytic ones have much more.

[trigger]

Those capacitors are used for smoothing the DC voltage from ripple.

If the loading current is high, the ripple of the 78XX regulator will be higher.

[audioguru]

Trigger, a 0.1uF cap has a value that is much too low to smooth mains ripple. The main filter cap will have a value of thousands of microfarads to do that.
As the datasheet says, the 0.1uF cap at the regulator's output improves its transient response. Transients are abrupt, fast and high frequency changes in the output voltage. A 0.1uF cap filters those high frequencies very well.

[trigger]

What I mean is both Ceramic capacitor and Electrolytic capacitors are worked together as a DC smoothing.

You are right audioguru, 0.1uF Ceramic capacitors are as a low pass filter to shunt high frequency to ground while electrolyic capacitors are more like to handle the charge and discharge time of the DC voltages during the rippling.

[Guest Alun]

Also ceramic capacitors have a lower effective series resistance than electrolytics.

[autir]

I went to the store to buy some .33

[audioguru]

Hi Autir,
I have used BC Components (used to be made by Philips and Siemens) 370 series polyester caps for audio coupling for many years. "They work better" than ceramic or electrolytic for audio because they have very low distortion. For a power supply, distortion doesn't matter and since the polyester cap is small it performs as well as a ceramic cap. Either cap will be fine.

You should use the 0.33uF input cap anyway since the regulator will need it when the electrolytic cap dries out.

[autir]

the electrolytic cap dries out.

I beg your pardon ?!? :o ???

[audioguru]

Hi Autir,
He, he. ;D
That's right, the electrolytic will leak or just "dry out" in a few years or many years depending on its quality. Then the regulator will go crazy without an input cap and maybe destroy your load.

[autir]

I always thought that electronics are indestructible by time, and the only way to damage them is improper voltage.
So, for example, a computer motherboard will become unusable when its electrolytic capacitors dry out?
What happens when the cap dries out? Is its capacitance reduced, or its maximum voltage ?

[ante]

Dried out electrolytics is the most common death cause for a computer motherboard.

[trigger]

I got a mobo with electrolytic capacitor dried out......

It can boot up after a few try but will crash in a second.......

[MP]

Don't worry. Your motherboard will become obsolete way before the caps dry out with the way they are changing recently.

MP

[ante]

Not everyone can afford to buy the latest at the same pace as the manufacturer develops new stuff. But if you are careful you can change the caps on your motherboard, they are just about the only thing possible to change yourself.

[autir]

I have read that electrolytic capacitors age within 3 to 30 years, depending on quality. After that they gradually dry out; their capacitance is reduced and their resistance is increased, making them useless. Can't we substitute electrolytic caps with something else if we want a long-lasting circuit?

[ante]

Well that

[autir]

Forgive me for raising this topic from the dead...  :-\

What is the voltage difference between the Ground pin of the LM and the ground of the power socket on my wall?

What is the voltage difference between the ground output of bridge-rectified AC and the ground of the power socket on my wall?

What is the difference between Neutral and Ground? (generally speaking, not related to LMs)

Thank you very much.

[audioguru]

Hi Autir,
This topic is never dead!  ;D
The power transformer isolates your LM78xx and LM79xx circuit from the ground of your mains power socket. Same for a rectifier bridge.

With the isolating transformer, the circuits have their ground terminal grounded if you wire the circuit ground to the ground wire of the mains power plug, and the mains power socket is wired correctly. Or if you wire the circuit ground to an external ground. Or if the circuit is grounded through cables that connect it to something that is grounded. 

The live and neutral wires supply power to the load. The ground wire is for safety so that metal enclosures are grounded and cause a fuse or breaker to blow if the live wire shorts to the enclosure.

The neutral and ground are connected together at the fuse/breaker panel.