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

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what does the most recent schematic and part list for the 5A (or more) psu look like?

i myself converted the 24V/3A psu to a 24V/6A version, which seemed to work well as long as i powered it from a lab psu for first test, but then it went up in smoke when connecting a 24V/200VA transformator.

so i would like to see what you have changed when converting to higher current.


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hey every body I think I've entered this discussion a little bit late so I wasn't able to read all the posts so I jumped right to the end and I was wondering did any body considered or did you discuss in early posts making this PSU dual polarity I mean positive and negative since this might be needed in some applications (op amplifiers for example) I think it will make it more useful and it will be like two sources instead of one so what do you guys think ???

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Hi Was,
This project can be a dual-polarity supply and a doubled output voltage supply by simply making two of them with separate transformers and completely isolated from earth. Then they can be wired in series with their junction as "ground" and the positive and negative voltages from each supply. Or used in series for up to 60V output.

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as all of us know, in a linear power supply with such broad spec like 0-30v dc and current demand of 5 amps- lot of power is likely to be wasted-

A) if the spec permits why not the rectified DC be processed thro a switcher and a non- isolated output is taken in a buck regulator form( as the mains transformer provides for the necessary isolation)-- of course taking care of ripple limits.

B) whather thr primary could be controlled using anlectronic fan regulator (thyristor type)  in series to primary, the secomdary rectified and adequate filtering done to suite the needs or

C) procure a transformer with taps at say 9V (for 0 -6 vDC output ), 18V and full-- try switch teh limb to the rectifier-- out put configured to an adj regulator with series pass and the adjustable pot also switches symultaneously-- ganged switch.

with one of these methods perhaps we can device and still try to save on power --

any comments  welcome


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  • 3 weeks later...

ı do it but ı cant take voltaj at output becouse the opam-2 is tu much current why?

Does opamp U2 get hot? Is it an OPA445AP high voltage opamp?
Is U2's output voltage higher than it should be?
Is the project's output voltage too low?
Then something is shorting the project's output to 0V and U2 is straining to make the voltage higher, making a high current in R15.
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  • 2 weeks later...

Ok I have read through all postings in this 3A to 5A list and I don't recall that there were list of components, what you need to change compared modified 3A project. Maybe I have been up too late too many nights, but can anyone help me with this problem. I going to get over first with 3A project, but somehow I know, that one day it will be not eough and then this list will be very handy :-D.

I know that for someone  it might sound a bit silly talk, but I can't help, that my knowledges aint big in electronics....only way to learn anything is through questions...I guess. :-D

PS I've definetly been up too much and too often, because my typing suck real bad.... sorry about mistakes and about my english, which isn't my naitive.

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Hi Muffits,
For a 5A output instead of 3A there are only a few changes to be made to the modified circuit:
1) 30VAC/7.1A transformer (212VA).
2) 15,000uF to 20,000uF for C1.
3) Three 2N3055 output transistors with emitter resistors. Very large heatsink or a fan.
4) 0.27 ohms/10W power resistor for R7.

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  • 1 month later...

I am able to get only 1.5 Amps @23V not at 30 Volts.

Didn't you use parts listed in the original project?
There is a thread in the Projects Q/A Forum that discusses the problems with those parts and recommends changing some to get 3A at 30V reliably with good regulation.
Kain used parts recommended in this thread for 30V at 5A reliably with good regulation.
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Audio Guru , i also have the pic for the heatsink i am planning to use with this powersupply , and i was able to find the power transisitor TIP31A . THis heatsink is as closest i can find on farnell . which has thermal resisitance of 3.3 degrees C/W
is this suitable.

There is also a picture of the transformer



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It is nice but the heatsink is too small for this project.
The 30VAC transformer has a peak voltage of 42V which is reduced to about 40.5VDC by the bridge rectifier. If the output is a low voltage or is shorted and the current is set to 3A then the output transistors will dissipate 40.5V x 3A= 121.5W. If the output transistors are mounted with just thermal grease and not insulators then an additional thermal resistance of about 0.4 degrees/W will be added to make the total thermal resistance 3.7 degrees C/W.
The chips inside the transistorshave a thermal resistance of 1.5 degrees C/W from the chip to the case. Therefore the total thermal resistance is 3.3 + 0.4 + 1.5= 5.2 degrees/W.

So the chips inside the output transistors will rise to a temperature of the ambient of about 25 degrees C plus 121.5W x 5.2= 657 degrees C! Their absolute max rated temp is only (!) 200 degrees C.
Even 3 of these little heatsinks on 3 output transistors would be too hot. 

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BY the way i will try to buy another bigger heatsink , which has four TO-3 Transistors output it is about half the size of the M250 audio Amplfier (250W). It cost's around Rs 100 to 110 . i will take that.

But You were saying , that we can use two Output Transistors paralleled ,( i am talking about one Unit) . can you show me  how  ???

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With a low output voltage at 3A, the output transistors must dissipate 121.5W. With 5A then they must dissipate 202.5W. A single 2N3055 transistor can dissipate 115W and it's chip will be at its absolute max temp of 200 degrees C if you somehow keep its case at 25 degrees C. The thermal resistance of the mounting (thermal grease is about 0.4 degrees C) and the heatsink with fan reduces the amount of power that the transistor can safely dissipate.

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Somebody here asked about the parts that I used to build the PSU. I'm sorry that I respond quite late but I've been really busy lately with FPGAs... About the parts - Audioguru is right. I used the modified list for 5A. The power resistor is actually 15W and it's mounted on the PSU case. Smaller one will do as long as you can keep it cool under load. The large capacitor on the input is 20000uF in my case. I used 2x10000uF since it was way cheaper to get those insead of aiming at 20000uF/63V. The transformers that I used are from Hammond 30V @ 7.5A toroidals. In other words, there is no change in the part list. I bought most of the parts from Mouser, with the exception of the transformers which came from Digikey. Hope this helps.  :)

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