0-30V stabilized power supply: construction problem.

[Valve78]

Hi everyone,
at the tenth time of dismantle and rebuild I decided to stop any other attempt, but I must understand what's wrong anyway.
I followed the original scheme and tested every single component (all fine), but the result is always a not working ps.
In the deep:
- led is always on...
- voltage across R5 is always 33-34V (surelly not 11.6V)
- with Q1 inserted, output oscillate between 5.5V and 6.3V, while without it scale up to 15V-34V (oscillation depends on P2 obviously, but it has such a sinusoidal wave, I mean i.e. starting with P2 full left when I turn it output descend to the min. 12V or 5.5V at half of the turns and then gain up again to max while reaching the full right).
I really dunno what's wrong.
Thanks pals.
V78

 

[audioguru2]

The original circuit has many parts overloaded so they fail and it does not meet its spec's for 30VDC at 3A.
Therefore a few threads were made in the forum showing an improved circuit with a modified schematic and parts list. I am attaching them here.

View attachment 42028

View attachment 42029

 

[sanket bhalsing]

Hiii Sir I will purchase all component as per old list and make A PCB etching .........
and this my project ...... what should i do plz reply me sir ..................

 

[audioguru2]

Hiii Sir I will purchase all component as per old list and make A PCB etching .........
and this my project ...... what should i do plz reply me sir ..................

If you made the original project then many of its parts are overloaded. Some of the overloaded parts will fail so the project is not reliable.

The original project will not produce 30VDC at 3A, it might produce 30VDC at 100mA and 25VDC at 3A.

The forum describes a modified circuit that has parts selected so they are not overloaded and it produces 0V to 30VDC at up to 3A very well.

 

[sanket bhalsing]

sir thank you for giving me reply I have another question...
can u send me a new PCB layout of this diagram or either use that old and only change components value are replace...................

plz tell me solution of this problem

 

[audioguru2]

can u send me a new PCB layout of this diagram or either use that old and only change components value are replace...................

I did not save a pcb design but there are some pcb designs for the new version of this project in the forums.

 

[sanket bhalsing]

Sir we used this old components and pcb and final output will reduced upto 0-20v 1.5A its possible............

 

[audioguru2]

Sir we used this old components and pcb and final output will reduced upto 0-20v 1.5A its possible............

The transformer should be 20VAC at 42VA.
The gains of the voltage and current amplifiers must be reduced.
Then it will produce 0-20V at up to 1.5A very well.

 

[sanket bhalsing]

Sir what should i do for psu output of 0-25v with 2Amp

which changes in my old ckt tell me................

 

[audioguru2]

What should I do for psu output of 0-25V with 2Amp? Which changes in my old circuit?

The original project is missing calibration trimpots. If a 5% zener diode and 5% resistors are used then the maximum voltage can be plus or minus 25%.

If the zener diode and resistors are very accurate:
1) Change R11 to 47k, then the maximum voltage output will be +24.5V.
2) Change R18 to 150k, then the maximum current output will be 1.49A.

I think the value of R2 should be reduced from 100 ohms to 82 ohms.
I did not calculate the power ratings of all the resistors, some will get extremely hot like in the original project. 

 

[sanket bhalsing]

Sir and we made  the old ckt first in which there is no change

which problem i have to face........

 

[audioguru2]

Sir and we made  the old ckt first in which there is no change.
Which problem I have to face?

1) The 24V transformer produces a peak voltage of 33.9V at its rated power. The main filter capacitor charges to the peak voltage minus 2V. When the project has an output current of 3A then the transformer must produce 33.9V x 3A= 101.7VA. But it is rated at only 24V x 3A= 72VA so it is overloaded and will get too hot.
I do not know if it will catch on fire.

2) The 24VAC transformer probably produces 26VAC with a very light load. Then its peak voltage is 36.8V and the rectifiers reduce the output to 35.5V. The main filter capacitor charges to +35.5V.
Opamps U2 and U3 also have the -5.6V negative supply so their maximum supply is 41.1VDC. BUT their absolute maximum allowed supply is only 36V so they will probably fail soon.

3) Since the transformer is overloaded then I do not know its maximum output but it might be 3A at 22VAC then its peak voltage is +31.1V and the rectifiers reduce the raw positive supply to 29.1V. Opamp U2 will have a maximum output of +27V and R15 reduces its output to +25V. The maximum Vbe of the driver transistor is 1V and for the output transistor is 1.4V so the output voltage will be +22.6V. At 3A, R7 reduces the maximum output voltage to only +21.2V but your transistors might be better than minimum so your output might be a maximum of +25V.

4) When the output current is 3A and the output voltage is low or is shorted then the output transistor dissipates 25V x 3A= 75W which is too much heat for a single 2N3055 transistor. It will probably fail soon.

5) The maximum heat from the driver transistor is 23.6V x 115mA= 2.7W but its little heatsink will cause it to overheat and probably fail soon.

6) Many resistors are overloaded and will probably fail soon.

That is why I say the original project is not reliable. 

 

[sanket bhalsing]

THANK YOU SIR...

Sir what should i do i will make that old project and i have to run that one and show in practical  exam plz tell me solution ????

 

[audioguru2]

Are you learning how to copy a circuit and learning how to solder its parts together?

If you learned the details of electronics and designed the project yourself then it would probably work perfectly and be very reliable.
Since you simply copied it then show its problems and how you will fix them (that you are simply copying from me) in your exam.

 

[sanket bhalsing]

yes Sir...

I will make it but which problem i have to face it will discuss with you .....

OK na sir

 

[sanket bhalsing]

sir i did  project but my output does not varry final output 31to33v..............
And LED does not glow

what should i do.......................

 

[audioguru2]

sir i did  project but my output does not varry final output 31to33v..............
And LED does not glow

what should i do.......................

Did you build the original circuit without changing anything?
You need to troubleshoot the circuit to see what is wrong.
1) We need to know how close the output voltage is to the unregulated voltage across C1 because the output voltage from opamp U2 cannot go higher than about 1V less than the voltage across C1 and the output transistor and the driver transistor are emitter-followers so their output cannot go higher than about 1V less than the maximum voltage at the output of U2.
What is the voltage across C1? What is the exact output voltage from your project?

2) Turn the voltage pot down to zero and measure the voltages at both inputs and the output of opamp U2.
What are the voltages?

3) The voltage at the output of opamp U1 should be about +11.2V.
What is its voltage?

The LED should glow only when the output current in a load exceeds the setting of the current pot because then the LED warns you that the current regulator is reducing the output voltage. The LED should not glow when the project has no load current.

 

[sanket bhalsing]

Thank you sir i will check and reply you and U2 ic heat very fastly...

 

[audioguru2]

I think your driver or output transistor is shorted or has its pins connected wrong.
U2 will get hot quickly when it tries to hold its output voltage down but is connected to a short to the positive 32V. R15 will also get very hot.