Urgently need your help.

I designed a dc power supply shown in figure 1, and implemented on breadboard. But whenever i turn variable resistor to decrease voltage, after decreasing 7V (approx.), the Variable resistor flashes up (i.e burns down). But as shown in second figure 2, when i place diode D5 at input-output of lm317, the variable resistor doesn't burns up. Can someone please explain me how this diode prevents burning up of variable resistor?
R2 (5K) is the place where i am using variable resistor.

And also can you tell me some points that might could be asked by instructor (because i have viva by tomorrow for this project(i.e after 12 hours approx.))

also i am a beginner, so please try to begin from scratch...

 

I don't see a variable resistor on the schematics?

 

I don't see a variable resistor on the schematics?

sorry i forgot to mention that R2 (5k) is being used as variable resistor.

 

I can't see anything on the schematic that would cause the problem you describe. Check your breadboard layout very carefully.

 

I can't see anything on the schematic that would cause the problem you describe. Check your breadboard layout very carefully.

well sorry to disappoint you, but i alone didn't faced this problem, variable resistor of all the students who tried it without D5 burnt up, but after putting D5, it didn't burnt up....

i couldn't understand the reason, that how diode D5 is preventing them from burning...

 

Either the whole class has a bad batch of parts or you all have the chips connected incorrectly. There are six different packages for the LM317 family. Which one are you using?

What is the input AC value in Vrms?

What is the load on the output?

What is the output voltage with R5 turned all the way up?

ak

 

Either the whole class has a bad batch of parts or you all have the chips connected incorrectly. There are six different packages for the LM317 family. Which one are you using?

What is the input AC value in Vrms?

What is the load on the output?

What is the output voltage with R5 turned all the way up?

ak

they were not given to us by the university, we bought them from different shops.
no load was connected.
input ac was 13 Vrms.

where is R5?
i said by using diode D5, the burning of Vr stopped.

 

Typo - R2.

 

Does VR burn out if you connect a load too?

 

Typo - R2.

at 5K it gives 20 v output and at 0k it should give 1.25v, but before i can check it, the resistor burns up.

 

i d

Does VR burn out if you connect a load too?

i didn't tried that.

 

You need 0.1uf caps at input and output to stabilize the regulator too.

 

You need 0.1uf caps at input and output to stabilize the regulator too.

i am using c1 and c2 for that purpose

 

C1 has nothing to do with stability of LM317, it's primary purpose is to smooth out the chopped AC signal into a proper "flat" DC signal.Sure it's going to provide some stability to LM317 but primary stabilizer is low value capacitor such as 0.1uF one.

You need low capacitance caps to remove high frequency noise,high value capacitors remove low frequency noise.


Study this:
http://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf

 

When did R2 burn up? When you were adjusting it, or when you turned the circuit off or on?

 

Also, are you really using an LM317T in the TO-5 power tab package, or the LM317K in the big TO-3 power-transistor can? Make sure you're interpreting VIN, ADJ, and VOUT terminals properly. Note that the ADJ is in the middle on the schematic symbol, but the order in the TO-5 package is completely different.

 

When did R2 burn up? When you were adjusting it, or when you turned the circuit off or on?

R2 burns up whenever voltage for output is less than 7V(without D5), and i am using lm317T, and my wiring is according to pin configuration. there is no problem in wiring.
what i am asking is, what affect does D5 have on circuit?

 

Can you post a photo of your assembly?

ak

 

R2 burns up whenever voltage for output is less than 7V(without D5), and i am using lm317T, and my wiring is according to pin configuration. there is no problem in wiring.
what i am asking is, what affect does D5 have on circuit?

D5 is there to protect the regulator if a large value capacitor is used on the output side. It prevents the potential at the output terminal becoming more than 0.7V higher than the input when the power-supply is switched off.

You must have seen its purpose explained when you read the datasheet... you have read the datasheet?