0-30 Vdc Stabilized Power Supply

[audioguru]

Hi Bongo,
Welcome to our forum.
maybe you damaged the project's negative supply when you connected the other supply.
U2 doesn't draw much negative current. If it did then it would be very hot.

The negative supply has only 7 parts. Check them.

[bongo4000]

hi audioguru
thanx for the quick response.
i checked the neg supply, and it seems to be fine.
btw: i always had 25v on the output, since i connected the trafo, although i had the voltage pots on 0v.
(before, with the current limiting lab power supply instead of the trafo, it all worked well)
so i removed the OP U1-3 to check the output stage by itself.
the base of the 3055 is always at vcc, what looks really citical to me.
so i pulled the signal, which is usually driven from U2 pin6, to low, which, i think, burnt up R15.
i.e. the base of Q2 is always at vcc.
is it possible that Q2 has been killed?
how can this be avoided?
would it be better to replace R15 by a resistor with higher impedance, to limit the current through Q2 for not killing it again?
thanx!
bongo

[audioguru]

btw: i always had 25v on the output, since i connected the trafo, although i had the voltage pots on 0v.

Without any voltage control.

the base of the 3055 is always at vcc

That is a severe error.

the base of Q2 is always at vcc.

Now two transistors are defective.

is it possible that Q2 has been killed?

Yes, and since the base of Q4 is also always at Vcc then it is also killed.

how can this be avoided?

I don't know what is wrong to short the transistors. Are their pins connected correctly?
Is the rectifier bridge connected correctly?

would it be better to replace R15 by a resistor with higher impedance, to limit the current through Q2 for not killing it again?

No. The current through R15 is very small. The output transistors are wired as a darlington emitter-follower. So the voltage at R15 should appear at the project's output, minus two base-emitter voltage drops.

Did you make the original or the improved project?
Did you substitute any parts?
Is there a short or overload on the project's output?

[bongo4000]

yes, the 25V have been there with the voltage control to 0V, and current control to 0A.
so i connected a 1E resistor to the output, as i thought the voltage was because of a loaded cap.
so i think, there was some over current for a short time, until the 1E resistor burnt away.
i think this must have resulted in an over current through Q2, which burnt this one.
no idea why the current limiter did not reduce this current.

i think the base of Q4 is at vcc because it is pulled to vcc by Q2 ?

rectifier seems to be ok, as voltages before the regulation (over the big caps) is ok.

my idea to use a higher value for R15 was to limit the C-E current through Q2 to avoid that it burns down, when the output of the power supply is shorted.

attached, you find my schematic for the 6A version. the trafo is 24V/200W

thanx!
bongo

[Sallala]

Dear Bongo!

Why do you place the 1n5402 diode to the top of the 2n3055 transistors?
To save the transistors, when the PSU is off, and there is a voltage connected to the output?
(A charged big capacitor, orr accumlators for eaxmple)

Maybe it isn't enough.

As Audioguru says in an earlyer post, big current can be flown through U2's input
Audioguru suggested a diode in the feedback, like the image below.

I' m waiting for your response.
(Sorry for bad English)

[bongo4000]

yea, this is exactly why the 1n5402 is there.

i think when i add a diode as in your image, i have the problem that the input of U2 can have up to 30v, while this OP is not powered, and i don't think that the U2 is very happy with this situation ;-(

why do you think that the 1n5402 is not enough?

i think the blowing-up of the psu is more a problem of the current limitation which is not quick enough to limit the current on extreme overload.
you don't think so?

[Sallala]

1n5402 protect the 2n3055 transistors, but how to protect U2? In a switched off PSU, current can be flown from the output througt D10->R15 and into U2 output. Isn't it?

What about to use a Shotky diode? It have only 0,3V drop. Or you can limit the maximum voltage with a trimmer potentiometer series to the voltage adjust pot.

I see your diode on various circuits. But I don't understand, how it works. For example, how can it discharge a loaded capacitor on the output?

I'm interesting this protection, because I use my psu to slow charge my accumlators. I haven't add the diode on my image yet. Just use it in series on the output. But this isn't good for the voltage stability.


And what about D11. This is for protect the psu from a reverse polarity charged capacitor fr example? The original schematic, this is a low current diode. It's enough?

(I'm not an electircal expert, please forgive me if I asked stupid a question, or just forward me to a beginner forum..)  :-X

[audioguru]

Hi Bongo,
Your project has a very powerful transformer and lots of output current capability with four power transistors. You applied its mains voltage with its output nearly shorted.

It is possible that the negative 5.6V supply is slow in reaching 5.6V, so U2 does not have a negative supply in the beginning which causes its input common-mode voltage range to be exceeded when the voltage pot is set to a low voltage. The TL081 has FET inputs that cause the output to suddenly change the output polarity when their common-mode input voltage range is exceeded.
Then you have a very powerful supply that is shorted and it is trying to make its output voltage as high as it can go. MELTDOWN! Do you think so?

Opamps using bipolar transistors at their inputs do not suddenly change the output polarity when their common-mode input voltage range is exceeded. The OPA445AP high supply voltage opamp that I recommend in this project also has FET inputs, but its manufacturer doesn't admit whether it has the output polarity problem.

[bongo4000]

so you think i have to use the ti OPA445AP to solve the problem?

[audioguru]

so you think i have to use the ti OPA445AP to solve the problem?

The OPA445AP also has FET inputs and also might have the same polarity problem as the TL081.
Your supply voltage is very low so the total of its positive and negative supplies is less than the 44V rating of MC34071P opamps, which have bipolar transistors at the inputs and don't have this polarity problem. An old 741A opamp is also rated for 44V and has bipolar transistors at the inputs.

[bongo4000]

so what modifications do you recommend to get the whole thing working reliable?
(so i can go and look for new components)
thanx!
bongo

[audioguru]

Hi Bongo,
For your supply project with the low voltage transformer, I think that MC34071P opamps would make it reliable. Then R10 might need to have its value reduced and connected to the negative supply.
Then you should change the value of R4 to 1k so that the main voltage reference zener diode D8 has enough current to work properly. ;D

[bongo4000]

hi audioguru

i'm a bit puzzled.
why do you call it a low voltage transformer?
there are 2x 12v in series, so i think it's the same as the original project with the 24v.
so why do i have to adjust these resistors? voltages are still the same!

the changes against the original project, i'm aware of, are the additional 3055's to get 6A out of it, and the reduction of the series resistors for current sensing. i think i have not changed anything else.

maybe i don't understand, but i think i have to do some changes to make it 6A proof.

thanx!
bongo

[audioguru]

i'm a bit puzzled.
why do you call it a low voltage transformer?
there are 2x 12v in series, so i think it's the same as the original project with the 24v.

Hi Bongo,
The original project has mistakes that make it operate poorly and causes some of its parts to burn up. We have two very long threads here about it. With a 24VAC transformer its output voltage was a max of about only 23VDC at about 2.5A with a lot of ripple.

A number of people have made the improved version with its 30VAC transformer and get 30VDC at 3A with good regulation. Then the opamps get a total supply voltage of 50V so OPA445AP high supply voltage opamps are used.

so why do i have to adjust these resistors? voltages are still the same!

R10 might need to be changed to match the different opamp.
R4 was originally supplying a current of only 1.2mA to the main voltage reference zener diode D8. A part number for a zener diode was not in the original parts list and some people built it using a zener diode spec'd at 44mA and higher. It barely operates at 1.2mA. So I recommend a BZX79C5V6 zener diode that is spec'd at 5mA and changing R4 to 1k to supply it with 5.6mA.

the changes against the original project, i'm aware of

I am glad you replaced the little 2N2219 for Q2 and the little rectifiers that burned quickly.

Thank you for pointing out a possible problem when FET input opamps are used, the output is overloaded at startup and the voltage and current pots are turned down.

[bongo4000]

thanx!
so where can i find the modified schematic and partlist for the redesigned psu?
bongo

[audioguru]

where can i find the modified schematic and partlist for the redesigned psu?

The thread for modifying the project to a 30V 5A output is here:
http://www.electronics-lab.com/forum/index.php?topic=196.0
A few people have attached their schematics. The final parts list isn't posted.

In this thread I have this recommended parts list:

[Tristable]

Community, Hi!  ;)

I would need your help.
I'm not realy total newbie, rather a electronics student [newbie]  ;D.
Ok, lets get serious now.
Of course, I built this power supply. (modified 3A version, 30V trafo, OPA445's, 10000 uF cap. ...)
And also there is a 'little' problem... Voltage regulation works very nice, smooth, from 0.0 V to 33.3 V. But, temperature of U1 and U2 rises and after 30 sec i turn off power supply. (I realy care for my OPA's  :P). Also, R1 gets warm althrough it's 5W. Btw. current pot. at testing is set somewhere in middle. I dont know if current setting and limiter are working, have not tested PS with load.

I hope that someone can help me. And I know that my english is far from perfect... Sorry...

Modify:
That's when you're posting in hurry...
I forgot one, maybe important thing; when PS is turned of U2 temp. quickly raises a lot... Thats it.


Triple

[audioguru]

Hi Triple,
Welcome to our forum. ;D
The opamps have an idle power dissipation of only about 200mW so might be a little warm but cetainly not hot.

Without a load on the project, U2 still doesn't have much of a load. If it is hot then it is oscillating or something is loading it down, maybe a transistor with its emitter and collector pins reversed.

U1 drives only a few milliamps into the zener diode so it should also only be warm, not hot.

R1 dissipates only about 0.8W. The unregulated positive supply is about 43V with a 30VAC transformer and without a load on the project. A 2W resistor would be hot but your 5W resistor is normal to be warm.

[Tristable]

Audioguru, hi!
Thanks for your answer.

I don't know about opamps, but they seem to warm to me.
I ckecked some things once again, and I've done some measurments.
So far, here are the results:

All transistors are OK - not burnt and correctly connected
I was checking voltage refernece and found that I changed R14, but I didn't had BZX79C so I used BZX85C (45mA) - I readed about this modification - something serious?

Some numbers:

At pin 6 of U1 there is 10,9 V
At pin 3 of U2 voltage varies from 0 - 10,6 V as changing P1
And on pin 6 from 0 - 34,1 V

Also, I did some tests with loaded PS. Well, voltage regulation is realy nice, but current regulation... I was testing with LED and small DC motor. They do not react on rotating P2. Only when I turn it to max, then current limiter LED comes up and voltage drops a lot.

Can you said something from this data, or give some more measurments to preform?

Thanks, Triple

[audioguru]

I don't know about opamps, but they seem to warm to me.

Just warm is fine, hot is bad.

All transistors are OK - not burnt and correctly connected

Good.

I changed R14

Good.

I didn't had BZX79C so I used BZX85C (45mA) - I readed about this modification - something serious?

With R4 at 4.7k its current is only 1.2mA so it is barely regulating and its voltage is a little low. Use 1k for R4 and it will be better.

Some numbers:

At pin 6 of U1 there is 10,9 V

A little low, see above.

At pin 3 of U2 voltage varies from 0 - 10,6 V as changing P1

A little low because of above.

And on pin 6 from 0 - 34,1 V

Good.

Also, I did some tests with loaded PS. Well, voltage regulation is really nice, but current regulation... I was testing with LED and small DC motor. They do not react on rotating P2. Only when I turn it to max, then current limiter LED comes up and voltage drops a lot.

The current setting pot has a wide range. Your LED and motor's current was near the bottom of the pot's range. I think the pot should have its wiring reversed so that it gives max current when it is turned to max.

[Tristable]

Hello, Audioguru!
Thanks for answering.

OK, maybee I'm just care to much about opamps. I have to recalibrate my heat sense.  ;D

... I changed R14, ...

My mistake. I was thinking on R4. It's 1k. I built PS regarding to modified list (just used a 100 VA trafo, BD243C as Q1 and Q2, and BZX85C instead BZX79C), so all other values are like on mod. list, including R4.
And referent voltage is 10,9V. I will try to get BZX79C, but they would give it to me if they had, so I doubt. To drop R4 more?

I think the pot should have its wiring reversed so that it gives max current when it is turned to max.

I reversed it and then output transistor burned. I know, I should use two of them.
Now I can buy other one (two) in Monday, not before.  :(

Before I reversed it, I runed some tests with bulbs. But they aren't reacting on current pot no mather how I turn it. Except on max setting, when current limiter drops voltage on 0.something volts and bulb shuts down. So, I still think that current regulation is not working.

But, however, now I can't do anything without power transistor before Monday (or can I?), so I have another question (idea)? Of course, if You can and want help me... Hope i'm not to annoying.
What about nice digital controls instead unprecise pot's? Or just as addition for precise setting.
I searched this forum and google, but I didn't found anything to satisfy me.  ;D
Cause I want dirty, cheap solution without microcontrollers, damn expensive IC's... Just simple plug&play  8) . I see that You are real master here, so maybe you have a solution? 

Thanks, Triple

[audioguru]

R4. It's 1k. And referent voltage is 10,9V. I will try to get BZX79C, but they would give it to me if they had, so I doubt. To drop R4 more?

With R4 as 1k then the BZX85C will be fine. If R4 is reduced more then the zener will heat up and change its voltage, and U1 will get hot.

I reversed the current setting pot and then output transistor burned.

Maybe the load drew too much current and the current regulation doesn't work.
When the voltage from the current sensing resistor R7 exceeds the voltage from the current setting pot, then the output of U3 will go low enough to keep the project's output voltage low to regulate the current in the load.

Your 30VAC 100VA transformer will be at its max rating with about 2.4ADC output from this project.
A single 2N3055 will dissipate about 85W if the output voltage is low at 2.4A. Its heatsink must be enormous and have fan cooling for such a high amount of heat. An insulator on the transistor would cause it to overheat. The project is best with two 2N3055 transistors to share the heat.

[Tristable]

Hi!

With R4 as 1k then the BZX85C will be fine.

Well, then allright.

The project is best with two 2N3055 transistors to share the heat.

No problem, I will fix this as soon as I get.

Thank you!

[MonSSter]

Hi!

I just builded your power supply but i doesn't work ;) ... at output it always shows ~34V and i can't regulate it with P1 ... U2 is heating like mad... i checked and rechecked the PCB and will do it again :) ...

voltage on C1 is 34V, also on R1 ..

the voltage on U1, 6pin is 11.2V and it doesn't change if i move P1 ..

voltage on U3 is always 34V it doesn't change ...

but if i move P1 the voltage on R9 changes from 0.5V to .. 11.11


If you need any more measurmeants u ask ;)


PS: and of course ... sorry for my bad english

[audioguru]

I just builded your power supply but i doesn't work ;) ... at output it always shows ~34V and i can't regulate it with P1 ... U2 is heating like mad...

If U2 is faulty then the output voltage would be 34V minus two base-emitter drops. Measure the voltage at the output pin 6 of U2 to make sure. If the voltage is higher at Q2 then the transistors or one of them is shorted. Also, maybe U2 is destroyed by overheating.
Check that the pins on the transistors are connected correctly.

voltage on C1 is 34V, also on R1 ..

It is low, maybe you used a 24V transformer?

the voltage on U1, 6pin is 11.2V and it doesn't change if i move P1 ..

Perfect. P1 controls how much of the 11.2V goes to U2.