0-30 Vdc Stabilized Power Supply

[audioguru2]

Hi Rufinus,
U3 controls the current regulation.
If the two end terminals of the current adjustment pot P2 are reversed, then you will have low current when it is turned up and high current with the LED off when the pot is turned down.

 

[rufinus]

Well, the problem is that the LED is constantly on!

 

[audioguru2]

Hi Rufinus,
Check the values of R17, R18, R19, R20, R21 and P2. Check that C8 and Q3 aren't shorted. Swap U3 with another opamp.

What is the voltage of the negative supply?

 

[egres]

one more question.. ive got twenty high-quality potentiometers, but they are 100K.. how can i modify the power supply to use them?

thx, egres

 

[audioguru2]

Hi Grs,
The opamps have very high resistance FET inputs so yes you can use 100k pots:
1) Their wiring should be shielded so it doesn't pickup mains hum radiated from the transformer or the mains cord.
2) Multiply the values of R17 and R18 by 10.

 

[hana_moh]

i have done my Power....[/b]
my changes is:
R7:1K 10W*2 ON PARALLEL
Q1 & Q2: TIP31 with heatsink
Q3 *2 on parallel with big heatsink and fan
------------------------------------------------------
my problem with:
R7  >>>> to set minimum voltage to 0v
(before change 1 is down but the voltage to maximum then move little bit start from 2.1v to 1.4v)
R11 >>>> to set maximum voltage
U2 temp high
any one need any help...... ;D

 

[audioguru2]

my changes is:
R7:1K 10W*2 ON PARALLEL

Hi Hana,
Welcome to our forum. ;D

R7 must be about 0.47 ohms. Use two 1 ohm resistors in parallel if you want.

Q1 & Q2: TIP31 with heatsink
Q3 *2 on parallel with big heatsink and fan

Good. Q4 should be two power transistors in parallel. Q3 is a small transistor.

my problem with:
R7

 

[rufinus]

OK, found the problem, now everything seems to be working. Q3 was bad somehow and had to be replaced.

Thanx, audioguru!

 

[egres]

well... i have just finished mine.. if i wasnt a bit stupid, it could work on a first attempt :

i have bought two BD241C as Q1 and Q2 and two 7805 stabilizators for voltmeter and ampermeter... i dont know how, but i soldered 7805 in a place of Q1, and i spent 4 hours checking everything  : i replaced it and it works fine..

i must buy a 0,33ohm resistors, because i forgot to buy them yesterday.. with one TIP3055 the heatsink gets quite hot..

so thank you, 

egres

 

[audioguru2]

Hi GRS,
I am glad to hear that you fixed your problem. A regulator makes a lousy transistor, doesn't it?

With your 24VAC transformer, the positive supply will be about 32VDC when the project has a 3A load. If the output voltage is set low or is shorted, then the output transistors must dissipate 32V x 3A= 96W which is way too high for a single 2N3055 so I recommended using two in parallel. 96W is a lot of power for a single heatsink so maybe a fan should be used.
If I make this project then I would use a heatink in each side for the output transistors then maybe a fan won't be needed.

 

[egres]

well, the transistor was hot when i managed to connect 12V bulb on it.. i set 14V on supply, and the current was 2A.. so (BIG 500VA transformer with 4 primary wires.. output can be 24,27 or 30 Volts.. set on 27Volts) -> after diodes it is cca 34Volts.. 34-14=20 -> "unused" voltage  20*2=40 watts on heatsink.. heatsink is not black, and it is from pentium 4, 1ghz CPU, so it is not the best for this app..

i am going to do a two-stage fan controller tomorrow.. (off-20%-100%), using 1n4148 diode on a heatsink, and tl082.. would you be interested in it? should i post a circuit diagram here?

egres

 

[egres]

and by the way... looking at yours wattage : 32*3=96W
and mine: (34-14)*2=40W

is mine wrong? is the heat going to a heatsink determined by U*I ? or (U(source)-U(load))*I?

egres

 

[audioguru2]

Hi Grs,
I am not making this power supply project.
I think there is a temperature controlled fan project already in our projects section. It uses a lousy low voltage zener diode as its voltage reference that has more temperature sensitivity than its sensing diode.

My 96W calculation was with 32VDC at the main filter capacitor, the current at 3A and the output shorted. At 5V/3A output then the transistor will dissipate 81W which is also too high for a single output transistor unless some liquid nitrogen cools it. A fan might be able to cool a single transistor on a huge heatsink until the fan wears out.

Can you imagine the original project smoking? A few guys have seen it. ;D ;D

 

[hana_moh]

thanks to the Boss "audioguru"
  ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D
all is done thanks veryyyyyyyyyyyyyyyyy match
all is ok


U1,2,3 is TL081
Transformer 3A 39V

 

[audioguru2]

Hi Hana,
Does your project work now? Good.
Your opamps shouldn't be TL081. Its absolute max total voltage rating is only 36V. If your transformer is only 24VAC which is too low, then the positive supply would be 35.4V and the negative supply is 5.6V. So the total supply for the opamps is 41V if there is no load.

Yor transformer shouldn't be 39VAC. It is much too high. I recommend a 30VAC transformer for this project and high voltage opamps.

To make 3.0ADC from the output of this project, the transformer should be rated for 4.2A. A project with a 3A transformer can safely produce only 2.1ADC.

 

[Mordechai1]

Hi,
Where i can buy PCB from Power Supply and Ammeter LCD Panel?
Thanks Vy 73/55, DG1JB

 

[hana_moh]

Thanks for interest and amazing continuance "audioguru"
transformer 18 v* 2
-------------------------
R2 is Heating

 

[audioguru2]

Hi Hana,
If you have your 18VAC plus 18VAC transformer wired for 36VAC then the very high DC voltages will damage many parts.

The original project spec'd a tiny 1/4W resistor for R2. Many members complained about it overheating so I recommended using 2W.

 

[hana_moh]

sorry i fault not R2

R1..........

 

[audioguru2]

Hi Hana,
R1 is across the unregulated positive supply. Its power dissipation is easy to calculate:

1) If you have the original 24VAC transformer then it will be about 26VAC without a load on the project. Its peak voltage is 36.8V and is reduced to 35.4V by the rectifier bridge. So the power dissipation of the 2.2k resistor is 35.4V squared divided by 2.2k= 570mW so the original 1W resistor is fine.

2) If you have the recommended 30VAC transformer then it will be about 31.5VAC without a load on the project. Its peak voltage is 44.5V and is reduced to 43.1V by the rectifier bridge. So the power dissipation of the 2.2k resistor is 43.1V squared divided by 2.2k= 844mW which is hot for a 1W resistor so I recommended a 2W size.

3) Your 36VAC transformer will be about 38VAC without a load on the project. Its peak voltage is 53.7V and is reduced to 52.3V by the rectifier bridge. So the power dissipation of the 2.2k resistor is 52.3V squared divided by 2.2k= 1.24W. A 1W resistor might smoke and a 2W resistor will be hot.

Your transformer's voltage is much too high.