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

[Xenobius1]

Hello first of all I have built the PSU (0-30V 3A PSU) and it's GREAT but I built it becasue the PSU I was building last year which is mentioned below had a faulty circuit acording to my school techer and my work mate.

Last year I wanted to built a little PIGGY (PIGGY = HUGE). SO I had a Transformer CUSTOMLY made for me.

The Transformer is 600VA
Input :
240VAC

Outputs :
1. 30V 5A
2. 30V 5A
3. 12V 12A
4. 5V 5A

I wish someone could help me if possible.

I wish to make a Circuits which can handle the 30V 5A regulation including the current limiting so I can built 2 of it for the 2 outputs.

and I wish someone could tell me HOW to make a 12v 12A into a FIXED 12VDC with 12A (or maybe 10A) and also 5V 5A fixed

THANKS a lot for your assistance

 

[Xenobius1]

I'm going to build the circuit as soon as possible, but i'm very busy bcause of my exams. I think , the diodes will be replaced by 6A types, because i have them at home, and the input capacitor will be a bit bigger, 4700uF or so because i have some in stock

As soon , as it will be complete, i'll do some measurements.

Yes it will work mate because I didn't find a 3300uF Capacitor and used a 4700uF instead. and also the Diodes I used are 6A ( to be safe ) and it works

 

[Xenobius1]

I have built this PSU. It looks nice but I am disappointed with its (or my) performance.
The maximum current (Imax) and voltage (Vmax) outputs vary widely and proportionally to the resistance of the load (Rload).
For example:

Rload (Ohms) Imax (Amps)

4 >4.0
5 3.5
15 1.6
32 0.8
75 0.36
etc. etc.

The Vmax of 30V is gradually approached with increasing Rload but never reached, except under no-load conditions.
As it can be seen the relationship between Rload and Imax (and Vmax) is not linear but parabolic.

I am using a good quality VARIAC rated at 7.5A and set at 24V.
I have replaced the 4 rectifier diodes with an 8A bridge.
C1 is 4700 microF.
Overheating is not a problem (big heatsink with fan).
Apart for an "above board" jumper between pin out #10 and the junction of R4 and R6 - done to avoid the long, winding trace to ground - I have made no other modifications.

Has anyone had a similar experience?
Do you have any idea what the problem might be and how to fix it.
Thank you,
(a rather frustrated) Mastrila.

???

I have a torodial transformer with 2 outputs of 12.6v so I put them in series to get 25.2v and the result is that it gives me 34.8vdc (if you want to see the picture go to

http://www.electronics-lab.com/forum/index.php?board=2;action=display;threadid=128;start=75

 

[Xenobius1]

Hello everyone I need to build this transformer quickly so I couldn't wait for help on this forum so I asked the R&D Engineer were I work and he modified it for me. Now since I recived a lot f info from this forum I will share the modification and even HELP anyone who needs more info.

To make this circuit take up to 5Amps you need to:

1. A transformer with 24VAC 5Amps
2. Change R7 to 0.27Ohms 9W (or else 2x 0.15Ohms 5W In series)
3. Change C1 to 6800uf (or else 4700uF in Parallel with 1000uF)
4. Change D1, D2, D3, D4 from 1N5402 to 6Amps Rectifying Diodes

Also for safety I am going to make 2x 2N3055 in Parallel. (follow diagram attached for help in connecting them.)

View attachment 35416

 

[Xenobius1]

To change this circuit to 5 Amps was already posted in the above thread.... now what if you have a 30v transformer instead of a 24v?

Follow the diagram attached below.

View attachment 35418

 

[Xenobius1]

If by any chance you have a transformer which is 30v and also 5 amps do both the modifications. hope my last 3 posts help all who needed the modification and I would love if anyone would point out any mistake that the engineer has taken ..

 

[MP1]

xenobius,
You should compare these changes with the paragraph in the project of how the supply works. It looks like there will be problems with the limiting circuit with these changes.

MP

 

[audioguru2]

Xenobius,
Your 24V regulator plus 5.6V zener is limiting the positive supply to U2 to only 29.6V. U2's output cannot swing to the rail, but instead to about 28.1V. Under load, the bias current for Q2 through R15 could be 3mA, causing a voltage drop across R15 of 3V, therefore the maximum voltage that can be applied to the base of Q2 is 25.1V. The base -emitter voltages of Q2 and Q4 could be as high as 2V, an additional voltage drop.
Therefore your power supply will have a maximum output voltage of only 23.1V, under load.
Your 25.2V or even a 30V transformer won't make any difference except to cause more heat.

Why are you limiting the positive supply voltage of an upgraded (aren't you?) U2 to only 29.6V?

Your suggested 24V/5A transformer is rated for only 120VA.
If the circuit could be capable to supply 30V at 5A, then that transformer must supply 150VA, overloading the transformer.

Did you upgrade the little Q2 to a TO220-case power transistor that can be bolted to a real heatsink?

 

[Xenobius1]

Well as may know I was crying for help on some previous posts... I don't know a thing.. I just passed on what the engineer told me !

he said that if the circuit wants a 24vac transformer with 3 A , than I should change those components to make it take up to 5A and if the transformer is 30VAC instead of the 24VAC you should make a voltage regulator so that you would not have to change the IC's

Audioguru.. you told me that the 24v regulator is limiting supply to U2 to 29.6V ... but if you have a transformer of 24VAC would you have the same voltage on U2? (because 24VAC will go up a little after smoothing)

About R7, (the current limiting resistor) he told me that it needs to limit 5A now instead of 3A .. so he changed the value to make it limit 5A instead of 3A (somthing like that)

About the Capacitor he worked out a sum so that with 5A, there will be the SAME ripple as in the original design and the value of the capacitor was 5500uF but since it's not a standard value he told me to make a 6800uF or else a 4700uF in parallel with 1000uF.

About the transformer I did not suggest it mate ... but it was written in red in the project/power section. (PASTE below)

Input Voltage: ................ 24 VAC
Input Current: ................ 3 A (max)
Output Voltage: ............. 0-30 V adjustable
Output Current: ............. 2 mA-3 A adjustable
Output Voltage Ripple: . 0.01 % maximum

About Q2 I did not change anything (I am still waiting for someone on this forum to confirm the changes so that I wouldn't etch a pcb for nothing) but anyways.. should I Upgrade it? ?

Hope you reply soon because im on HOlidays and soon will end and wouldn't have time to build it.

Thanks for the help both of you

Oh and by the way MP ... I don't have a clue how to compare :-\

 

[Xenobius1]

I was thinking a little... So you need a 24VAC 3A transformer for the original circuit.. and you need a 30VAC 5A transformer to make it go up to 5A??

When you rectify the 30VAC they would go up to 44vdc !! Isn't 44vdc a lot for the circuit? That is why I reduced the voltage with a regulator!!

What are your opinions!

 

[MP1]

...and I would love if anyone would point out any mistake that the engineer has taken ... I doubt it thou ! he's the company pedaments lol

I did not mean to sound critical. I was responding to your above request. It is my opinion that the higher transformer will not work with this circuit because of the current limiting devices used. One example: Q3 and U3 have a critical role in the current limiting circuit (why I pointed you to the description of how it works), but your company engineer has isolated them from the raw power source and has regulated this buss. He has also caused this circuit to have the negative side grounded, which limits the uses of the power supply. It cannot be used as a floating supply or in conjunction with another supply in certain configurations. I have not looked closely, but I am sure there are other considerations, too.

There are many designs for power supplies. I am wondering why not choose one which has the capability you need instead of trying to tweak on a design that is not intended for your specific use. Perhaps I walked into the conversation a little late, and for that I apologize.

MP

 

[ante1]

MP,

What will bee the result if you change the 7824 for a 7924?

Ante :

 

[MP1]

ante, 7924 is a negative voltage regulator. Is this what you mean?

MP

 

[ante1]

MP,

Yes, if

 

[audioguru2]

Xenobius,
1) The original circuit doesn't work with only a 24VAC, 3A transformer, similar to why I explained with your added regulator and zener. That's why there are so many complaints here and on the other post. Your engineer assumed that the original transformer is correct and simply scaled-up to a 5A transformer.
2) The author of this project did not realize that U2 and U3 are operating with combined + and - supply voltages beyond their ratings. Your engineer did not see the opamps' - supply and regulated their + supply to keep it from exceeding their commonly-used 30V supply, which is much too low for this project, as I explained earlier.
44V supply voltage rated opamps are recommended here and on the other post, and will not need their + supply to be limited, if a 28VAC, 8A transformer is used.
3) A 30V transformer would produce about 40VDC with load due to rectifier and other losses, and much more without a heavy load, which is extremly high and unecessary. A 28VAC, 8A transformer would be OK.
I was incorrect earlier when I explained that the transformer must produce 30VDC, 5A, 150VA. A 28VAC transformer will actually produce 39.5VDC, 5A, 198VA, fully loaded, before rectifier and ripple losses. An 8A transformer would be OK.
4) 0.27 ohms is correct for R7, since it will produce nearly the same voltage drop at 5A as the original 0.47 ohm did at 3A.
But a little 10W rated resistor will get extremely hot at 5A, so use a bigger (higher power rating) one.
5) Your engineer assumed that the original capacitor value for C1 is correct. Use 10,000 microfarads to keep the ripple down, so that the supply can still regulate when fully-loaded and set to a 30V output, which the original circuit cannot do.
6) The original circuit has Q2 operating at its high temperature limit with a low-voltage 3A load. It is small and cannot be easily heatsinked, and would get even hotter if the supply has a 5A load and/or higher transformer voltage. TO-220 cased substitutions are posted here or at the other post.
7) R15 with the original value of 1K wastes a lot of valuable voltage. Change it to 100 ohms.

MP,
The engineer did not earth-ground the circuit's 0V output. He was simply showing a common link between the 7824's zener and system 0V buss.
Let's fix this project to make it work properly for 30VDC, 3A output, then tweak it a little (adding a second 2N3055 and changing R7) for 5A output. ;D

 

[MP1]

MP,
The engineer did not earth-ground the circuit's 0V output. He was simply showing a common link between the 7824's zener and system 0V buss.

The red line with a ground marker on the bottom of it which is tied to the power supply low side output indicates that this line is grounded. If he was only indicating where the zener connects, he used the wrong symbol.

MP

 

[Xenobius1]

??? I dunno im not that technical. I can only follow a schematic and build it .. I don't know what the circuit would be doing thou.

Um .. so you suggest to try and find a circuit that takes 30v transformer 5A?

Il try to find something than .. any help would be really appriciated since you are all more thecnical than me.

thanks and if there is a post that may confuse someone please let me know so i can modify it / delete it.

10q

 

[audioguru2]

Xenobius,
What do you mean by this:

Um .. so you suggest to try and find a circuit that takes 30v transformer 5A?

I said that a 30VAC transformer voltage is too high. I said to use a 28VAC transformer.

I said to use an 8A transformer.

You don't need to find another circuit, this one will be fine with a few modifications that we have discussed.

 

[Xenobius1]

But I have a transformer with 4 outputs and 2 of them are:

30Vac 5A (both of them)

As you can see I cannot change the transformer because it costs a lot (110 USD) and it was customly made for me.

Im stuck with a 30Vac 5A transformer :-\

About the modification to make this circuit work iwth a 30Vac input, what should be done then ?
.... boy im way back in technical speaches. :'(

 

[audioguru2]

Xenobius,
I am sorry that you ordered an expensive transformer without having any technical considerations:
1) Your 30VAC, 5A winding will supply only 3.5ADC, due to its VA limitation. This project can use very-high supply voltage opamps, mentioned much earlier, and most of the recent modifications.
2) Your 12VAC, 12A winding will supply only 8.5ADC, due to its VA limitation. It will supply only about 10.5VDC regulated, due to its voltage limitation.
3) Your 5VAC, 5A winding will supply only 3.5ADC, due to its VA limitation. It will supply only about 2VDC regulated, due to its voltage limitation.