The diode served a different purpose here. It is not an extra but essential instead to provide separate paths for the positive and negative feed into voltage clamp.

It has to be connected like that because the LM311 has open-collector output so it cannot source current but can only sink current. Without the diode the path would be +15V into 2x1K2 in series for positive feed whereas the path is -15V via a single 1K2 for the negative feed to the voltage clamp.

I have highlighted the two different paths in the diagram so you can better visualise the process.

CAN U EXPLAIN about DUAL diodes functioning in the above circuit please...

 

Welcome to AAC.

We encourage starting new questions in their own threads to avoid confision. The thread you posted in is 6 months old - http://forum.allaboutcircuits.com/showthread.php?t=17617

I count 9 diodes. Which 2 are you referring t0?

 

Sorry abt that ,, i am new tho this blog.. i am getting confused , i don't understand where to post my Querry..?

Two DUAL DIODS IN THE LOOP (TOTAL 4 DIODES) in upper right hand side.. functioning..

 

Hi ,i am using KA78l05A Voltage regulator (Vin 15V , Vo=5V)in my PCB board, it's showing 5v reading b4 connected in PCB board, and in PCB board it shows output voltage 7V instead of 5V.. why? I am connected one capcitor parllel at input and one capacitor at output in parllel... do u have any idea..?
Interesting point is, I replaced LM340T5 7805(5V output ) it's working fine in that PCB circuit...giving 5v o/p.
KA78L05A regulator giving strange reading when vary the voltage...(O/P voltage proportional to I/P voltage.... regulator o/p must 5V eventhough changes in the input voltage I believe..!)

I know these are very basics Querrys... but theoritically i am ok.. but when comes to this practical sessions .. i don't understand .. ..... Thankq for your relpy
....

 

The circuit you're asking about is a snippet of a larger oner intended to generate sine waves. Follow the link - http://www.national.com/an/AN/AN-263.pdf - and go to page 11 of the PDF. Those diodes are the logarithmic elements in the feedback path.

As far as the regulator goes - a 78L05 is in a T-92 package and rated for 100 ma out. The dissipation caused by the 15 volt input may be overheating the poor thing. It would be much happier with a 7 volt input, but also with current under 100 ma.

 

The circuit you're asking about is a snippet of a larger oner intended to generate sine waves. Follow the link - http://www.national.com/an/AN/AN-263.pdf - and go to page 11 of the PDF. Those diodes are the logarithmic elements in the feedback path.

As far as the regulator goes - a 78L05 is in a T-92 package and rated for 100 ma out. The dissipation caused by the 15 volt input may be overheating the poor thing. It would be much happier with a 7 volt input, but also with current under 100 ma.

Thankyou for your reply with fine solutions.... i will check this today ... as far as, while I building circuit.... MOSFET chip is failing after the continuous running at 60 degrees.. in circuit it's not well designed as per the datasheet.. (with out connecting CAP's in i/p and o/p..Is there any problem with out CAPACITORS... ) http://www.onsemi.com/pub_link/Collateral/MC34151-D.PDF datasheet of MC33151 operating temp upto 85....but particular component not working properly at above 60deg... in my circuit i/p for this mosfet driver comes from multivibrators... o/p indicates (two leds..LED) in my circuit i skipped the resistor and capacitors as per the datasheet.. what is the function of the resistor and CAPACITORS at i/0 and o/p(PAGE 3 in given PDF link) Please clear my doubt in this circuit(page3) about resistor and capacitors..