0-30V Stabilized Power Supply

[morpheous87]

I have one transformer that confuses me. See the label on it and the ouput voltage it makes.



Isn't the label 24/91 W means 24V 91 W ?

 

[audioguru2]

The voltage from your transformer is far too high for this project.

The transformer is marked "98VA". A transformer does not consume power (Watts), instead it passes power (VA) so its power is rated in VA, not in Watts.

Your transformer does not show its rated voltage. 24/91 are just meaningless numbers that might be when it was made: the 24th week of 1991.

You need a transformer rated at 28V to 30V with a power rating of 119VA to 127VA.

 

[morpheous87]

Thank's for the info. So I can use a transformer with lower output voltage (24V) till i find apropriate one.

 

[audioguru2]

with a 24V/102VA transformer, the max output of this project at 3A is about 24VDC to 25VDC.

 

[morpheous87]

with a 24V/102VA transformer, the max output of this project at 3A is about 24VDC to 25VDC.

Can I get higher volatage than 24VDC with lower current <3A ?

 

[audioguru2]

The max output voltage will increase a little when the current is lower.
But the output suddenly will be full of ripple and will not be regulated near the max output voltage.

 

[morpheous87]

The voltage from your transformer is far too high for this project.

The transformer is marked "98VA". A transformer does not consume power (Watts), instead it passes power (VA) so its power is rated in VA, not in Watts.

Your transformer does not show its rated voltage. 24/91 are just meaningless numbers that might be when it was made: the 24th week of 1991.

You need a transformer rated at 28V to 30V with a power rating of 119VA to 127VA.

Is there a problem if the transformer is 200VA?

 

[audioguru2]

A 28V/119VA transformer will operate at its max allowed temperature when the power supply has a 3A load.
But a 28V/200VA transformer will be pretty warm, not hot when the power supply has a 3A load.
Of course the 200VA transformer is heavier and costs more than a 119VA one.

 

[morpheous87]

A 28V/119VA transformer will operate at its max allowed temperature when the power supply has a 3A load.
But a 28V/200VA transformer will be pretty warm, not hot when the power supply has a 3A load.
Of course the 200VA transformer is heavier and costs more than a 119VA one.

So there isn't problem if 200VA is used. I know that it costs more, but ony that one I found, for now. Thanks!

 

[morpheous87]

In PicMaster's scheme are used 1x12000uF 63V capacitor or 2x6800uF 63V capacitor. Can be used something smaller or on lower voltage? There ones cost much, and hard to find.

 

[audioguru2]

In PicMaster's scheme are used 1x12000uF 63V capacitor or 2x6800uF 63V capacitor. Can be used something smaller or on lower voltage? There ones cost much, and hard to find.

I also recommend 12000uF.
You need a total of 12000uF for low ripple. Use hundreds of small capacitors in parallel if you want and if your enclosure has space for them.

If the voltage rating on the capacitor is too low then it will fail.

 

[morpheous87]

I printed the PCB from PicMaster's file, I think it should be mirrored. So I opened it in Photoshop and mirrored it. Than save it like PDF. Today I printed it, but it's with smaller size. My ask is, should the PCB mirrored, and that size shoud it be. Or just give a ready printable file. Thanks.

 

[PicMaster]

It is sized 1:1 and it does not need to be mirrored.
When you go to print it just make sure the the page scaling is set to none, This is how I did mine and it worked perfectly.

 

[9waste9]

hi guys i am new here , came to this website by searching for a power supply , and i found it in here

http://www.electronics-lab.com/projects/power/001/index.html

But without going deep into the design (the reason was i trusted this site ) , i just bought all the components in the parts list , and only then did i see the flaws in the circuit . I really don't want to do the modified circuit , is there any way i can carry on with the original design ??
  can i not just connect the pin 7 of the tl 081 op-amps to a 7815 regulator ??

 

[audioguru2]

can i not just connect the pin 7 of the tl 081 op-amps to a 7815 regulator ??

Then the max output voltage will be only 8V to 10V and the overloaded transformer and many other parts will still overheat.

 

[9waste9]

Thanks for your reply audioguru , maybe i can do exactly what ivan suggested

"    Received the following email from Ivan on 02/2010:

    Ok. I bulid your project about a day ago. Mounted all the parts on the pcb and then concluded that there is some serious problems in this schematics. First, 2N3055 will overheat, so you have to connect two of them in parallel with emitter resistors 0.1ohm/5w. Second, maximal voltage between '+' and '-' of TL081 is 36VDC.If you connect them as it is shown in this circuit diagram that voltage will be about 45VDC, so they will burn down immediately. To fix this problem you have to reconnect all pins number 7 of U1, U2 and U3, emitter of Q3 and 'upper' end of R19 to out of an 7809 with 18V zener diode between 'common' pin and '-' of 3300uF cap, and input of 7809 connect to '+' of the same cap. Now, on pin number 7 and mentioned parts you'll have 27VDC, and total voltage will be 32.6VDC. Third, instead of using 3300uF, use 4700 or 6800uF/63VDC to reduce the ripple on higher currents (2-3A). The rest of the circuit is perfect. I like it cause it is so inexpensive and easy to make with those simple reconstructions i mentioned.
"

 

[audioguru2]

Ivan's circuit has a max output voltage of only about 20V and the transformer and many other parts still overheat and will fail.

 

[9waste9]

Ok so the only problem that i see now is the supply voltage of the op-amp . suppose i give an external regulated voltage 0f +30 volts and -6 volts to the 7 and 4 pin of the op-amp, will the problem be solved ??

  I am sorry that i am stressing so much on the original design , since i have serious budget constraints , and buying the high voltage op-amp cost's me four times more than tl-81

 

[PicMaster]

You could do what I did went to the Texas instruments web site and ordered some samples then you get them for free.
You need to listen to audioguru the old design has many faults, I have ben there and done that tried everything to get the old design to work but in the end it went in the bin and I designed my own PCB of the upadted schematics and have had no problems.

There is lots of information in this topic.

 

[audioguru2]

Waste, you don't say which country you are in. I usually go to Digikey and buy opamps.  MC34071 opamps cost only $1.02US each and TLE2141 opamps cost only $1.71US each today.
For comparason, the original circuit's TL081 opamps cost $.52US.

So the cost for only 3 of them is almost nothing.
But many other parts also need to be replaced.