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

[audioguru]

If i use maxim's 2.5V shunt voltage reference instead of D8 then 5V should be obtained on the outout of U1 which doesn't require any voltage divider.What do u think..

I thought about using a 78L05 to replace the zener diode and the opamp, then also power the digital pots from it. You could add a trimpot in the gain-determining parts for calibration.

Is there any need of finer adjustment pot(1K) if digital pot is used..

Maybe there are enough steps and the resolution is good enough for you. You won't be able to smoothly sweep the voltage up or down. As I mentioned above, a trimpot is needed for calibration.

Also i want to display the max output

[TooExpensive]

Hi,
as promised I want to post the fruit(s) of my efforts. Thanks again to everybody in the front lines for developing the circuit to this state.
The image quality is very low but its the best I can offer, therefore I have not tried to make an image of the scope line when testing the circuit.
Please keep in mind that its an experimental arrangement for testing purposes.
While building the board I noticed that the board layout is in some places a little unsuitable for this circuit, its very tight and the main conductor pathes are a bit small, but nothing serious. There is always a soltion.
The main facts: The values are always checked by oscilloscope/AC/0,1V-div/2ms-div to get no measurable disortion at the output clamps while driving my selfmade restistor network (pic.5)
Smoothened by 22000uF extern plus 4700uF on board.
Transformer ratings: 630VA 2x29,4V running free (original 1x26,3V off two paralleled windings to which I added 3 turns each)
The smaller transformer on pic.4 is 250VA 1x24V for the first tests but I want to have two outputs.
I exchanged the opamps to LM741 which runs at its supply voltage limits but nothing gets warmer than it shoulds an everything is stable.
What do I get for my money: max Volt out without load: 32,4V
Shortened output current at max. Volts and max. current: 8,3A
Maximum stable output Voltage with 4,3 Ohms attached while turning the Volt. Pot. up and the Curr. Pot. set to max. 27,6V pure flatness on the scope line at given settings while current rises to 6,2A.
Voltage set to 30V without load and turning up the current under load gives 6,25A and the clamp voltage drops to 27,8V again flatline on the scope.
R7 is two paralleled 0,47Ohms/5W but will be exchanges against 0,22Ohms/20W, they get very hot.
Of course bigger heatsink for the output transistors are needed, but its just the testsetup.
Alltogether I hope you will agree to the rating 25V/6A which was my target. It could deliver more by lifting the transformer voltage and using better rated opamps, maybe worth a thought.
Bye for now, and dont fry much power resistors.

PS: on the first test R16 smoked a bit because I exchanged Base with Emitter on one 2N3055 but nothing happened to the rest of the circuit. Man, I hate mounting TO-3 in the late evening hours .

[audioguru]

Hi Too Expensive,
You probably can't measure it because the waveform will be distorted but what do you think is the modified transformer's voltage rating when your project has a 5ADC load?

How many milli-volts DC does the output voltsge drop from no load to 5A load at 25VDC?

[TooExpensive]

Hi,
I can measure the output disortion, what I did was after my mind to determin the point when the circuit becomes instable or the supply voltage drops too much. When I set the potentiometers beyond this point when driving low resistance loads it shows in form of fast growing spikes on the screen which also shows in dc setting. Of course I cant get this resolution in dc. I cant say how much the output voltage drops at 25V/5A because i didnt measure it yet, but I can say for 30V preset at 4,3Ohms. I measured 6,25A and the clamp voltage dropped to 27,8V. I try to keep that in mind to measure it. Sure it is an important stability point.
Can you repeat the question about the transformer please in other words, my english is not good enough to give you the right answer.
Bye for now.

[audioguru]

Your temporary wires are too long and have too much resistance to properly measure the regulation of your circuit. A wire with only 0.002 ohms will create a voltage drop of 10mV at 5A.

I think your "spikes" are ripple since the U2 opamp is saturated up at the positive supply.

Transformer voltage ratings are at full rated current. We don't know the voltage rating of your transformer because you modified it.

[TooExpensive]

Hi again,
now I understand the question about the transformer:
Without mod.: free: 26,33V
loaded circuit with 25V preset/4,3Ohm: 25,7V
modded: free: 28,96V
loaded: 28,3V
I got these values from my records and I could also read the Voltage after the rectifier/smoothing sinks on the big transformer from 37,8V to 34,4V. Should I think about shottky rectifier? Or will thicker wires do the trick...Ill see.
In the original thread the BD249 for output transistor is mentioned...is this usable...are modifications needed to do this?
Bye again.

[MAXPAYNE]

Is there any advantage using  darlington power transistor like DTS4075A(30Amp) instead of 2N3055 ?

[audioguru]

Hi Roomi,
Datasheetarchive.com doesn't list the darlington. It would overheat by itself so at least two are needed. The driver transistor and a couple of resistors might not be needed.

[mvs sarma]

Your temporary wires are too long and have too much resistance to properly measure the regulation of your circuit. A wire with only 0.002 ohms will create a voltage drop of 10mV at 5A...............
 

Hi audioguru

you may recollect the type of power supplies that had two more wires(4 wire method) and the lead drop was taken care of and the voltage is corrected at the load-- i don't mean that we can then use thin wires and take the reisk of fire due to I^2*R loss . in specific case where voltage drop is to be minimised, in addition to proper guage wirs, this remote votage monitoring and correction maybe adopte for this supply also.

regards

Sarma

[audioguru]

Hi Sarma,
I have seen too many circuits posted that are built on a breadboard with thin long wires all over the place. Some are clipped together with intermittent alligator clips. They cannot work properly.

[mvs sarma]

Hi Audioguru

i agree with you completely-- power projects , for that purpose any project soud be built with proper discipline-- i was only teeling about lead loss compensation from the power supply to the load point--( say 50 to 100 cm for each lead). this itself drops the voltage and the resultant voltage at the actual load would be ( expected - lead drop). this could be compensated by a 4 wire system. that all.

regards

sarma

[audioguru]

Hi Sarma,
I know about remote voltage sensing at the load so that the feedback causes a power supply to produce an accurate voltage at the load without the voltage drop of the connecting wires.

This project is smart because it has the current sensing resistor R7 outside the voltage sensing circuit. However, some people ruin the excellent voltage regulation of this and other power supply projects by connecting a current meter with its internal resistance in series with the load. I think that current should be measured by the voltage drop across R7 in this project, then there will not be a voltage drop.

[mvs sarma]

Hi Audioguru,

Current sensing outside cct Voltage sense-- yes. I shall come back  after a little study

Fine sir

regards

Sarma

[boco]

Hi guys. I am new on this forum. First of all I am sory for bad english but I hope you can understand mee. I read this posts about power suply modification to 5A. It is too much to read.I read 20 pages but after all I dont hav answer what should I change to get 5A. I know that you will said .....read. I am very busy (family, busines ....) and will be very greatfull if somebody write for me vhich part I must change.
Thank you for your answers.
Best regards
Boco