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Convert 0-30V 3A PSU to 5A or more

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Hi Kain,
Be careful with your transformer because it is rated for only (!) 210VA. Its peak current is normally about 50A into the rectifier bridge to replenish the charge lost in the main filter cap during high loading but its average power rating is still 210W. The rectifier conducts only near the transformer's voltage peaks of 42.4V, so the max average DC current from your supply is limited by the transformer's power rating to 210/42.4= 4.95A.

I think you are over here with a 60Hz mains. Your 20,000uF for C1 will discharge about 2.8V each half-cycle with a 5A load. The 2.8V ripple should be reduced by the opamps to only about 0.5mV of ripple at the project's output but yours is 800mV when the output transistors aren't saturated. I think that using a proper "star grounding" technique will solve your ripple problem:
1) The 0V wire of R4, R6, U1, the voltage adjust pot P1 and R11 must be connected directly to the output without any of the current that goes into R7. Therefore R7 should have a separate wire connecting it to the output 0V terminal. Voltage regulation will be improved.
2) C1 and the rectifier bridge should be connected together with very heavy short wires to carry 50A current peaks.
3) The positive voltage feed to the circuit must connect directly to C1, not to a terminal strip.
None of the 50A current peaks must reach the circuit.

With an output voltage less than 2.5V, I aren't sure why it doesn't give max current. Check that its negative 5.6V supply is OK.
I couldn't find a schematic of an OPA445 opamp, but its datasheet shows a different connection to its offset voltage adjustment from the one on U2 in our circuit. Try connecting R10 to negative 5.6V instead of the existing output 0V. Then try removing R10 and connect the slider of RV1 to the negative 5.6V. ;D

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Hi everybody!

Audioguru, your're doing an excellent job! Thank you very, very much for your commitment!
I'd also like to built this PSU and understand how it works. As far as I can see there were lots
of changes by the component values but the circuit's topology stayed the same except the
additional resistor Rl. I've read the function description, but I still don't understand half
of this circuit I think.Now here are my questions:

1.This additional resistor Rl forms a load that is needed for correct function of the regulation even if no external load is connected?

2.What is R1 good for? Why is the smoothening-capacitor C1 not enough?

3.Why 15W for R7? With P=R*I^2 and a max. current of 5 amps follows a max. dissipation of 6,75W. Is this because a 15W resistor doesnt get as hot as a 9W resistor? Does this resistor need an extra heat-sink? The lower the value of R7 the better, because less power is dissipated in R7?

4.U1 provides a constant voltage which is amplified by U2. The output voltage is fed back to U2 by R12 (C6 is for stability?). C9 is for stability, but what are R15 (base-resistor for R13?)
and D10 good for?

5. I don't know what D11 and C7 are doing. Same with D10

I hope somebody (I think it will be audioguru ;-)) can help.

Bye
Benno

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Hi Benno,
Welcome to our forum. ;D


1.This additional resistor R1 forms a load that is needed for correct function of the regulation even if no external load is connected?

2.What is R1 good for? Why is the smoothening-capacitor C1 not enough?

R1 isn't additional, the project has had it from its beginning. It simply discharges C1 slowly when the mains is turned off.

3.Why 15W for R7? With P=R*I^2 and a max. current of 5 amps follows a max. dissipation of 6,75W. Is this because a 15W resistor doesnt get as hot as a 9W resistor? Does this resistor need an extra heat-sink? The lower the value of R7 the better, because less power is dissipated in R7?

The circuit is calculated to have about 1.41V across R7 at its max current, 3A uses 0.47 ohms and 5A uses the nearest standard value of 0.27 ohms. People have complained that its high temperature caused their pcb to char. Power resistors operate extremely hot (250 degrees C?) at their rating. Resistors with a higher power rating are larger and are better able to dissipate heat. Therefore at power dissipation levels below their max rating they operate cooler. I've used little power resistors in an aluminum case that are easily cooled by bolting them to an aluminum project box.

4.U1 provides a constant voltage which is amplified by U2. The output voltage is fed back to U2 by R12 (C6 is for stability?). C9 is for stability, but what are R15 (base-resistor for R13?)
and D10 good for?

Correct. C6 and C9 provide lead and lag compensation respectively since the old 2N3055 output transistor is much slower than the opamp.
R15 might not be needed. Its original function was possibly to isolate the output of Q2 from the capacitive load of Q2, to ensure that U2 doesn't oscillate. Its original value of 1k wasted a lot of voltage so I reduced it to 100 ohms that still effectively isolates the capacitance of Q2.
D10 discharges C7 and any other capacitor on the output by Q1 when the mains is turned off.

5. I don't know what D11 and C7 are doing.

D11 conducts to discharge a reverse-charged capacitor that might be connected across the output, I guess. Maybe a bigger one would be better.
C7 is also for compensation and high frequency noise noise reduction. It is also a high frequency low impedance bypass.

I hope somebody (I think it will be audioguru ;-)) can help.

How did you guess? I've been waiting for this site to come back online all day! ;D
"Server not found" :'(

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typing error:I meant Rl at the output.


Where do you know this from? Designers determination?


But without R15 there would be no current-limiting, when the load is discharged through D10 and Q1?

- Does C5 shorten high-frequency disturbances of the mains?

- Q2 is a emitter-follower, correct? Is this the reason why R16 is needed? And why emitter-follower instead of a common emitter configuration?

- The 3 parallel output-resistors: Why do they have resistors at their emitters?


I know, had to wait to ask my questions.

Bye
Benno

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typing error:I meant Rl at the output.

I don't know Rl.


Where do you know this from? Designers determination?

Ohm's Law.

Does C5 shorten high-frequency disturbances of the mains?

C5 is the circuit's high frequency bypass cap for stability from oscillating. C1 is big and has inductance which reduces its effectiveness at high frequencies.

Q2 is a emitter-follower, correct? Is this the reason why R16 is needed? And why emitter-follower instead of a common emitter configuration?

No. Q4 gets hot which causes it to leak current. R16 bypasses its leakage current.

The 3 parallel output-resistors: Why do they have resistors at their emitters?

The current gain of transistors having the same part number and manufacturer are different. Without the emitter resistors then the transistor with the most gain will carry the entire load until it fails. The emitter resistors reduce the difference between individual transistors.
Good point! ;D Somebody reported that even with 0.1 ohm emitter resistors one transistor got much hotter than the others. Therefore the emitter resistors should be 0.33 ohm, 2W instead.

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But at the moment i dont have the money to buy the heatsink, its gonna be a real fat one, and it is very expensiive.

//Staigen


Im no expert in anything, but would an old CPU heatsink and fan be up to the task of cooling without too much expense?  :-\

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I know that this is a bit out of topic since it is not directly related to the PSU, but can anybody here suggest a good CAD to draw a cover label for the face pannel of the PSU? I finally put it together - now it is going all the way to 30V/6.9A with fan cooling. The ripple problem is solved - audioguru was right about the proper star grounding technique. I shortened all wires as much as possible, and avoided crossing them too to avoid inductive coupling. After I make the label I will post the picture of the final result which I think looks quite nice even now (and works quite nice too

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Ok, I finally managed to blow this PSU up. I burned Q2 and it was quite obvious since R15 begun to smoke. The output current was 7A at 2V when this happened. Any idea why it burned? I haven't replaced Q2 yet, and I don't wan to if it is going to burn on me again since this is not a fix obviously. I still coldn't figure out how i burned Q2 neither... The datasheet says that the max collector current is 3A. I think I haven't reached this value.

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Ok, I finally managed to blow this PSU up. I burned Q2 and it was quite obvious since R15 begun to smoke. The output current was 7A at 2V when this happened. Any idea why it burned?

Oh, oh. If Q2 has a pretty good heatsink then it should be fine.
If the three 2N3055 transistors have minimum gain of 32 at 7A/3, their total base current is only 219mA. Q2 will have about 39.2V across it when the output voltage is 2V so dissipates only 8.6W. With a "perfect" heatsink a TIP31A is allowed to dissipate 40W.

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I actually found the problem. As I said I was assembling the PSU and what happened was that one of the 2N3055 was not hooked up correctly. I exchanged the base and the emmiter so when I loaded the PSU on 7A I blew all of the 2N3055 transistos and the TIP31A one (Q2). I think I might have blown something else too. I haven't placed the parts back yet but I am getting maximum output on pin 6 of U2 - no regulation possible. I replaced the chip too and result is still the same. The output on pin 6 U1 is 11.2V so I think this is acceptable. U3 is working fine too. U2 is acting like this even without Q2. Any idea what else might be blown? I was thinking Q1 maybe.

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I actually found the problem. As I said I was assembling the PSU and what happened was that one of the 2N3055 was not hooked up correctly. I exchanged the base and the emitter

A good way to blowup stuff.

I am getting maximum output on pin 6 of U2 - no regulation possible. I replaced the chip too and result is still the same.

Without the transistors then the feedback will normally force the output high.

The output on pin 6 U1 is 11.2V so I think this is acceptable.

That is normal.

Any idea what else might be blown? I was thinking Q1 maybe.

It is probably OK. It is used only for a moment when the power is turned off.

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Ok, I thought so about the feedback but wasn't sure - no enough time to analyze this circuit anyways, but this is why we have Audioguru here  ;D! I measured the negative input of U2 and it was quite enough to force the output to max so it does make sence. I am about to replace the 3 power BJTs now. Hopefully i won't see the smoke again... ::)

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Alrighty, I assembled the machine and it is fully operational now. The unit was tested again 3 days ago in my university and the results were more than satisfying - ripple under 6A load was merely 2mV and I am not sure if that wasn't partually scope noise. On any other working modes the ripple is less than 1mV. The maximum current I got out of it with dead short output was 6.97A. I will take few pics soon and post them too since I put the effort not to only get it to work well but to look well too. People in school nearly freaked out when I got to 6A without problem, and it is not surprise given the fact that the lab PSU's that we use are limited to 1.5A

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Hello
      I have been reading this thread for awhile and I just have to ask jassam where can you find the ic CD-1846P. I did a web search and couldn't even find a data sheet on that ic let alone a source to buy one. There are a few sites that show the circuit you posted buts thats about it. Maybe it goes by another name.
                                              Let Me Know
                                              Thanks gogo

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Ok, here are pictures of the completed PSU. It is now set at 12V on both sides as one can see on the picture. The fan on the top cover is pushing air in the case causing circulation. The air gets out from both sides - left and right as it cools down both h/s. The fan is actually temperature controlled - it senses the temperature of each of the h/s... The PSU also starts up with a delay - I am passing the start up inrush current through a resistor in series to limit it down otherwise the fuses blow up. That is pretty much all. The info about ripple and so on I already posted. I'm happy the project is finally finished

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Hi Kain!

Holy Cow!  :o
An outstanding job, you must have put quite a few hours in this one!
I am sure most of our members would like to have one just like it! :D

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