Protection diodes for 7809 circuit???

[Lizard King]

All the 317 circuits show protection diodes for the regulator.  Do you need those same protection diodes when using a 7809?  I never see them in the 7809 circuit layouts.

Thanks.

[R.G.]

They are there in the application notes the manufacturers turn out. Yes, you need them.

This is the reason: a sudden INPUT short can leave any output filter capacitors fully charged. The interior circuits are designed to resist voltage pressure from the input side, not from the output side, for reasons having to do with how the integrated circuit processing works. An input short amounts to instantly reversing input and output. If the energy in output filters is of any significant size, it can damage the insides. This can kill the regulator instantly if the caps are big, or slowly, damage accumulating with each on/off cycle.

If the output is loaded so that the output voltage goes down fast enough so the output is never more than X volts bigger than the input voltage, then there's not enough voltage on the output to really break things down. A slowly dropping input avoids the danger. This is why I said "input short" above. That's the worst case.

A $0.02 diode is VERY cheap insurance. There are protection diodes on every linear regulator circuit I do these days, although I didn't start this until I found out about the damage process the hard way.

[Lizard King]

So you put a 400n between input & output & another between output & common?

[R.G.]

Yes.

But more importantly, you need to understand that the diodes go so they are reverse biased normally, but when the input is shorted (or reversed!), the diodes conduct and clamp the voltage across the regulator chip to only one diode-drop.

Once you understand that, you will always get the diodes the right way round, and also be able to use diodes in similar situations in other circuits.

[Lizard King]

Thanks....if I could figure out how to attach a pic I'd post the circuit I came up with....

[R.G.]

This forum does not host pictures. To post a picture, you put it somewhere, like one of the free image storage sites, then insert a link to it with the "insert image" button above the box you're typing your post into.

[DIMstompboxes]

Yes.

But more importantly, you need to understand that the diodes go so they are reverse biased normally, but when the input is shorted (or reversed!), the diodes conduct and clamp the voltage across the regulator chip to only one diode-drop.

Once you understand that, you will always get the diodes the right way round, and also be able to use diodes in similar situations in other circuits.

R.G.,
Two question
1) is it okey to put diode just the input & output only and no need for output & common?
2) I done this to my wallmart adapter using 7809, add one cap to output and common for more output filtering, add that diode anode from positive output and cathode to input. Is this coorect?
Thanks

[R.G.]

1) is it okey to put diode just the input & output only and no need for output & common?

They do two different things. The one with anode on output and cathode on input (for positive regulators) is to protect the regulator against suddenly-shorted inputs. If the input polarity is reversed, this may not help much. The one with cathode on (-) and anode on output is to protect against reversed polarity input voltages if the input-output diode is there. Both are protection devices that will never be needed - until one day, something goes wrong. How many times have you used the fire extinguisher in your kitchen?

Uh - you DO have a fire extinguisher in your kitchen, right? $10 and you can quell a grease fire before the house/apartment burns. It doesn't happen often, but, that will be thin comfort if it does.

Meanwhile, back to your regularly scheduled questions:

2) I done this to my wallmart adapter using 7809, add one cap to output and common for more output filtering, add that diode anode from positive output and cathode to input. Is this coorect?
Yes.

You can do either one or both of the diodes, and the supply will work fine, and may never be needed. They are protection devices, like fire extinguishers and life preservers - useless trivia, until the day they're needed, then you're glad you had them.

[DIMstompboxes]

1) is it okey to put diode just the input & output only and no need for output & common?

They do two different things. The one with anode on output and cathode on input (for positive regulators) is to protect the regulator against suddenly-shorted inputs. If the input polarity is reversed, this may not help much. The one with cathode on (-) and anode on output is to protect against reversed polarity input voltages if the input-output diode is there. Both are protection devices that will never be needed - until one day, something goes wrong. How many times have you used the fire extinguisher in your kitchen?

Uh - you DO have a fire extinguisher in your kitchen, right? $10 and you can quell a grease fire before the house/apartment burns. It doesn't happen often, but, that will be thin comfort if it does.

Meanwhile, back to your regularly scheduled questions:

2) I done this to my wallmart adapter using 7809, add one cap to output and common for more output filtering, add that diode anode from positive output and cathode to input. Is this coorect?
Yes.

You can do either one or both of the diodes, and the supply will work fine, and may never be needed. They are protection devices, like fire extinguishers and life preservers - useless trivia, until the day they're needed, then you're glad you had them.

Thank you so much  R.G., so what I only did was just to protect that Regulator's IC.  You see the wallmart pcb is cramped that I could only sneaked in that three pieces part, I may add one more reverse diode protection see if I can make point to point connection OR I could just remove that polarity tab maybe to prevent that. Thanks for enlightened me on this R.G.