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

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Hi Herman,
Sure the old 2N3055 is in the museum. This entire project is an old design, but it works well when its parts are corrected. 2N3055 transistors are cheap, Mosfets are expensive.

This modified project has a max output current of only 5A, so why use a Mosfet rated at 33A?

The output transistors are linear, not used as switches, so they get hot. The project would  still need 3 Mosfets instead of transistors to dissipate the heat.

Also, the gate voltage of an N-channel Mosfet needs to be much higher that the voltages in this project. 

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As an aside, I've been investigating recharging regular, old alkaline batteries (I know, most think this is not a good thing) with this PS. In any event, I figure that capping the current at ~65mA and setting the voltage to the appropriate value for the cell I wish to charge, it may just work. What do you think?


I think that this is a different subject and should be discussed in it's own thread - not as an add-on to this thread...

MP

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Hi Paulo,
Welcome to our forum. ;D
I forgot about the 723 regulator IC. I haven't seen one for nearly 40 years. We have a power supply project that uses it but the current isn't adjustable, it is fixed at a max of 2.5A.

The circuit you attached is adjustable up to 10A. Its automatic fans are good.

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I have planned to build the modified power supply unit and almost all the parts got collected.But the problem is here i did not find the huge capacitor like 15000uF 63V.I found a 10000uf but its voltage rating is 25v.now i am planning to manage it from a wasted  elecronic devices like tv,microwave oven etc. Do you know inside where i can find these types of capacitor ?I tried getting sample capacitor from some company but the did not respond.please help me... :'(

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I think that smaller-value capacitors in parallel will work exactly the same as a single capacitor with the total of their values, except they will cost more and use more space.
At VHF radio frequencies and higher it will make a difference due to the inductance of a larger capacitor.

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Thnks for uor comment but I heard that it will decrease the quality of filtering


Actually it is better because the ESR will be lower with parallel caps.


At VHF radio frequencies and higher it will make a difference ...


Who would use a capacitor of that value at those frequencies???

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Hi Indulis,
We have had many questions about radio circuits, "Why is there a small-value capacitor in parallel with a larger-value capacitor? Why not use just one large-value capacitor?" Even a moderator wrongly said to use just one capacitor with the total value.

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... "Why is there a small-value capacitor in parallel with a larger-value capacitor? Why not use just one large-value capacitor?" Even a moderator wrongly said to use just one capacitor with the total value.


The moderator is not wrong!! There are cases where you do want to do this, but IT IS NOT nessessary everywhere!!

Guru... how exactly do values in the range of 10,000uF and RF go "together"??

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I was explaining that paralleled capacitors also work well and are needed at RF frequencies.
A low-value capacitor is an excellent low-inductance bypass for high frequencies. A higher-value capacitor is inductive so is a poor high frequency bypass but is good at lower frequencies. Both in parallel bypass a wide range of frequencies not possible with just one value.

This original 3A project used only 3300uF as its main filter capacitor. It was reported to produce plenty of ripple at its output.

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Roomi, as audioguru and indulis already said, there is no problem to exchange a large capacitor to some smaller types in parallel.

ESR and parasitic inductance will be lower with a parallel capacitor assembly, but this will not affect its performance (in fact, if this was a switching power supply, it could even improve the performance).

Of course, price and space will be different, but it's better than not assembly the project due to a impossible to find piece.

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Hi,
this is my first post here, so please dont eat or beat me.

I WAS willing to build either the oiginal or the improved PSU. I even bought some parts for the project today. But after coming home i made the mistake reading the two threats. The one for the original design with incredible 78 pages and this one with 22. A huge input and an outstanding example of endurance. Good work, especially by kain and audioguru. This runs for 2 years and still postings are comming in this proves the interest in this circuit. But after reading all this i am missing a summary. It seems that the double sided layout works out very fine. Maybe the autors can add it to the original projects page to make it easier building it. I find it very confusing to search every bit of information from the posts. Maybe some member could post the recent parts list/schematics/layout?

Bye, keep it going!

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Hi TooExpensive,
Welcome to our forum. ;D
The threads are long because they should be divided: Theory about why the original circuit doesn't work, Max ratings of parts, Substitute parts, Heatsinks, Adding meters, Troubleshooting etc.

Somebody should post a new project about what they have done when making theirs.

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Hi there,

I have researched a bit and found this project worth building, it is of usable quality and as a big plus the layout is available so Ill just give it a try.
Since I have a 24V/210VA transformer available this is my starting point. I want to use the layout of the original project of course with the benefits of higher output current. U1-3 will stay TL081, I have them already. 3x 2N3055, each with emitter resistors of 0,1R-0,27R/min. 2W (all will get the same value). Q1/Q2 will be TIP31A or BD241 depending what I can find in my boxes or what is available at the store.
If I got it right the absolute max. current is despite the power dropped over the output transistors depending on the voltage over R7 which is 1,41V (I hope I remember this value right?!) for full current limit with current potentiometer set to max. current? My simple mind says: lets drop R7, add another 2N3055 and get more current safely. I know this will most likely not work out...why...an even better: can it be done and how can it be done?
Anyways for the parts consider the parts list here:

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Since I have a 24V/210VA transformer available this is my starting point.
U1-3 will stay TL081, I have them already.

The 24VAC will be about 25.5VAC without a load. Then the peak voltage will be 36V and the unregulated voltage will be 34.7VDC. Most of the opamps use the positive unregulated voltage plus the negative 5.6V supply for a total of 40.3VDC. The absolute max supply voltage for a TL081 opamp is only 36V. There are a few inexpensive opamps rated at 44V available such as the MC34071AP and the TLE2141CP which would be more reliable for your project.
The losses in the circuit will produce a max regulated voltage output of only about 25VDC at full load because the transformer's voltage is too low for 30VDC. The 25VDC might have high ripple. A 30VAC transformer and high voltage opamps are needed for 30VDC at full load.
The power rating of your transformer is fine for a load of 5A. 

3x 2N3055, each with emitter resistors of 0,1R-0,27R/min. 2W (all will get the same value). Q1/Q2 will be TIP31A or BD241 depending what I can find in my boxes or what is available at the store.

Yes, but use 0.33 ohm/2W emitter resistors. People reported that the 2N3055 transistors have a wide range of current gain and the higher resistance allows them share the current better.
If I got it right the absolute max. current is despite the power dropped over the output transistors depending on the voltage over R7 which is 1,41V (I hope I remember this value right?!) for full current limit with current potentiometer set to max. current?

People found the current too high since the pot has a very wide tolerance. A 0.27 ohm resistor for R7 will produce a max current of up to 7A which will cause the 3 2N3055 transistors, Q2 and your transformer to melt if the output voltage is low. People add a preset trimpot in series with the adjustment pot for calibration to exactly 5.0A for the max current They also add a trimpot in series with the voltage-adjust pot to calibrate the max voltage which is also too high..

One thing i cant figure out about it: C10 10uF/50V. I cant find it anywhere...what did I miss? Is it a good addition to the original layout, whereever it will go ?

There isn't a C10. It was a mistake.

Audioguru, guru now! (as everyone can see, theory is important!)

Some people think so. I hope that I have helped you.

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Thank you for your fast reply,
I have all missing parts together now (mostly high power resistors). Except the OPamps MC34072 was not in stock the other n/a. Ill figure out about this problem later. Tomorrow will be a corrosive day (hopefully). Ill keep you updated (maybe with some pictures from my 640x480 webcam, who knows...)
Good night.

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Except the OPamps MC34072 was not in stock

The MC34072 is a dual opamp. The project needs single opamps like the MC34071 which might be in stock.
Lots of online electronic supply stores have Motorola (now ON Semi) and Texas Instruments opamps that are rated for 44V.

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I have planned to use maxim digital pot instead of the two pots  voltage contrl and current cntrl(10K).But the maximum supply voltage of these maxim's ics is only 5V-6V).The datashhet also indicates, exceeding the voltage at any pin greater than suplly voltage may damage them.Is there any solution and has some body made the ckt with digital pot ?I am eagerly looking for it because it will fly out the mechanical problem and control will be precision. :o :)   

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Hi Roomi,
This project has a voltage reference at the voltage adjust pot of 11.2VDC. Then the output amplifier boosts it up to 30V with its gain determined by the ratio of R12 and R11.

Reduce the 11.2V reference voltage down to 5V with a voltage divider, adjust it with a digital pot then increase the gain of the output amplifier so that it still has a 30VDC output.

The current setting pot has a max of 1.7V across it. A digital pot would be fine without making any changes.

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Hi Audioguru,
A lot of thnks..I have another idea.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..??? ???

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

Also i want to display the max output  current on 7segment display rather than marking on the body i.e.when i change the current pot shaft the max output current will be displayed on the display.Can u  please post a ckt here for this purpose or figure out a point to make it possible..?? ???

Again many many thnks to all u friends specially Audioguru and Ante..... :) :) ;) ;) :D :D ;D ;D

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