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

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Hi Moutoulous,
With 3 reasonably heatsinked 2N3055 transistors, this modified project can produce 5A output at any voltage from 0V to 30V.
With a fan, about 7A if you use bigger supply parts.

If you use 4 of those powerful but expensive transistors, a 13A/30V transformer, a stronger rectifier bridge, a huge current-measuring resistor R7 and a bigger filter cap this modified project will probably produce 7A to 9A output at any voltage from 0V to 30V.
With a fan, about 12A if you use bigger supply parts.
With liquid nitrogen cooling, maybe 20A if you use bigger supply parts, and you could cook dinner on R7. ;D

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  • 2 months later...

Hi Maksar,
Welcome to our forum. ;D
Of course you can connect two regulated power supplies in series, but only if they have separate transformer windings or transformers. Also, the one with the lowest output current setting determines the load's current.
If you parallel them, they will fight each other unless they each have a resistor in series with their output to the load. The resistors will ruin their voltage regulation. ;D ;D

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  • 2 weeks later...

Hi All!

I'm going to build this PSU (0-30V/5A), but I want to do in addition digital 7-segment volt- and ampermeter, +-5V/1.5A and +-12V/1.5A. Here I thought about 2 ways to do this:

1. to power these additional voltages from the big trafo of 0-30V PSU. As I understand I need two windings (of 15V each) to get negative and positive voltages <35VDC (the requirement of most 7800s), and then I'll have the virtual ground - in the middle of the secondary winding. However, there will be some problems with this way:
  a. the current of 5V and 12V supplys will cause changs in the trafo voltage for the main   
      PSU. Will it be critical for 0-30V PSU? Also this additional load won't be simetrical and
      +5V supply and -5V, for example, will influence on each other.  ???
  b. I'll need bigger and more expensive trafo. I guess something around 11-12A insteade of
  c. I'll have to measure voltage and amperage of the 0-30V PSU driven from the same
      source. It may cause some problems, were said earlyer.
  d. I'll will not be able to use the main PSU and +-5V, for example. in the same circuit,
      because there will be difference between their ground. Am I right?

2. to power these additional voltages from the other smaller trafo, taken from the PC power supply. There are two types of trafo: toroidal and "ragular" for switching power supply (yellow in most cases). Which of them I can use for my purpose, and can I connect it directly to the 220VAC? What current this trafo can give for +-5V and +-12V?

By the way, should I also do +-3.3V for home lab? Or I won't need it?

That's all  ;D  I will be very gratefull to everyone who will answer.
After getting answers on these questions I believe,  I'll be able to begin building this project. I'll put the final schematic on the forum.


P.S. Sorry for my english.  I'm working on it  :P

P.P.S. Thanks for your answer, Audioguru!!!  ;) ;D  You are doing amazing job on this forum!

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Sorry, one more question  ;D

Have I put all this PSU in the metalic box, or plastic box will be ok? As I understand, metalic grounded box will prevent external fields influence on the circuits and so will reduce ripple... ??? How critical this is for home PSU?


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Hi Maksar,
Thanks for the compliment. ;D
Your English is fine. ;D

Each meter will need a power supply that is completely separate from each other and separate from the power supply project.

You can't connect a switching transformer to the mains without a complicated switching circuit driving it. Just use ordinary mains transformers with rectifiers and filter capacitors. I can't remember if the meters need a regulated supply.

The power supply project will become very hot when powering something with a low voltage and a high current, so use a metal box. The heatsink will be pretty big and might need a fan blowing across it. ;D

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Hi Maksar,
Those torroid transformers probably have ferrite for cores, not iron, and they have very few turns in the coil. They look like high frequency switching transformers to me and cannot be used as mains transformers.

I have never used 3.3V except for the 486/100 in my old computer. I have an LM317 adjustable voltage reg project that I use for most things. ;D

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  • 3 weeks later...
  • 2 weeks later...

Hi H198848,
The LM317 and LM338 work very well but only when used within their ratings:
1) They can't dissipate much heat, so they shut-down when they get too hot.
2) The LM338 is rated for an input voltage no higher than 35V, so having a 30V output is difficult.
3) They don't have adjustable current regulation, requiring a second one in series. With two regulators in series, the max output voltage is futher reduced.
4) The LM317 is rated to deliver 1.5A but only with 15V or less across it. With a high input voltage and a low output voltage it is guaranteed to deliver only 150mA.
5) The LM338 is rated to deliver 5A but only with 10V or less across it. With a high input voltage and a low output voltage it is guaranteed to deliver only 1A.
6) Their output voltage cannot be less than 1.3V without a negative supply, futher reducing the max output voltage. ;D

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Hi Kain,
Where is your voltage being lost?
1) D8 is 5.6V. R5 and R6 are equal so the output at pin6 of U1 should be 11.2V.
2) U2 has a high input impedance so there shouldn't be much loss to its input pin3.
3) U2 has 56k for R12 and 27k for R11 so it and the outputs gain should be 3.074.
4) The max output voltage should be 11.2 X 3.074 = 34.43V.

I know! You used the original value of 1.5K for R14, instead of the revised value of 1.2k. Then Q1 is turned on all the time.

No? Then you used the original value of 1k for R15, instead of the revised value of 100 ohms.

No again? Check that the positive power supply pin7 of U1 is at the 43V unregulated voltage. A "Smarter" schematic here had an error with it connected to the output pin6 of U3 instead. ;D

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Hi Kain,
With a quick look, your schematic is fine.
Your U1 stage is sick, its output should be 11.2V.
Check that the values of R5 and R6 are equal.

The gain of U2 and the output stage is (56k/27k) + 1 = 3.074. Your 9.08V X 3.074 = 27.9V.
U2 will have a little more gain by itself because it makes-up for the small loss in the outputs. ;D

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  • 2 weeks later...

Hi Kain!

Good job, your psu looks very good. And you know - ugly planes don't fly.  ;D ;D ;D Can you make some more pictures of your psu? Especially I interested in the bottom side of the board and the case.  :)
I begin to build the same psu as well.  8)


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