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


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You might find it better to use this one for the specs you have described.

http://www.electronics-lab.com/projects/power/027/index.html

and start a new thread on modifications to this one, although from your description, it sounds like it might be ok as is.

MP

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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.

???

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Mastrilla,
Sometimes a variac is not isolated from mains and in that case it is dangerous to use for a PSU.
If you take a look here http://www.action-electronics.com/variac.htm#Vary you can see that only one out of five is insulated. When it is not insulated means that the secondary winding is part of the primary and if you connect live to the wrong side you can kill yourself! :o And we don

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Yes, ante brings up a good point. A good addition would be an isolation transformer. A 1:1 transformer or a similar circuit that would not change your voltage but which would guarantee that you are isolated from the mains. This isolation might even help with the problem you are seeing, depending upon the circuit of the variac.

MP

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hey all

I am/mostly have built this power supply, but i've made some changes along the way:

C1 is now a 4700uF 50V (Maybe change this later...)
The 3 feedback resisters are metal film (R4/5/6) for the lower tolarence (1% compared to 5%)
And i'm changing Q2 to a BD139 (if you want to make the circuit 5A then you'll have to change Q2 to one which dissapates more heat, BD139 can also be connected to a heat sink)

The transformer i'm using is a half-half (half the primary and half the secondary on each side), not sure about the VA, as it is one which my dad made himself a while ago.

I should be able to get access at uni to a oscilloscope, and test the supply for any problems.

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ginji, thanks for the update on the project. I would also be interested in an easy method of building transformers if your dad would not mind sharing. There is a post and an article on this website but it is very difficult. If you could do this, please add as a new post so that the subject is easily found by others.
Thanks!
MP

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ginji, thanks for the update on the project. I would also be interested in an easy method of building transformers if your dad would not mind sharing. There is a post and an article on this website but it is very difficult. If you could do this, please add as a new post so that the subject is easily found by others.
Thanks!
MP



it's been a while since he's actually built a transformer (last time was at least 16 years ago if not more) so i think what ever he can actually remember is a bit out of date. The article in articles section explains the process quite well (from my understanding of it) although i can understand that people get confused by i

the printed circuit board i'm using was made by my uni (yay for my uni) using protel and some CAM stuff (CAM is computer aided manufacturing) so it isn't etched but has the copper round the tracks cut away.
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Hello all,
I have been following this discussion with interest as I wish to build this power supply as my first 'home brew' PSU project.
I have a transformer rated at around the suggested 'upgrade' level, but I dont want to start collecting other parts until I am sure there is nothing else to change.
I would like to make mine the 'upgraded' version with a 5A output, so I need to get to the bottom of the changed components list.
Im a beginner to electronics, so if anyone has a finalised upgrade list I would love to have a copy. Im not too worried about keeping the costs down on this one as I want this to be the only bench supply I build for a while, so it has to be good.
Does anyone have the suggested component list yet? Hope you can help and thanks for a really good forum.
Ian (bigpetespc)

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C1 upgrades to 4700uF or higher, 10 000uF is suggested i believe

The rectifer bridge as a single 10A or higher part, this way it can be attached to the side of the case

I'm changing Q2 from the 2n2219 to a BD139 (you can use a BD137)
WARNING! the pins on the BD139 is different to the 2n2219

Feed back resistors (R4/5/6) to metal film type for the lower tolerance

Other suggestions (not made in this thread, but the other one)
Replace R2 and R3 with 1/2W resitors
R7 with 10W resistor
R1 with 2-3W
P1 is a many-turns wire-potentiometer (10 turns), and P2 is a "cermet" single-turn

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Thanks Ginji,
I was a little lost to be honest with all of the suggestions that had been made in the two forum topics.
Youve saved me a bit of head scratching with the components youve listed.
I will now start to get some parts together and hopefully I will have a working PSU in the not to distant future.
Ian

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Hi Jingi,
Now that souds like a really good idea. Ive seen it mentioned in the threads, so If I understand it correctly, it would allow a super fine tuning of the output voltage.
Would this second pot be 10% of the original value? and is it easy to wire the two pots together?
Thanks for the answers by the way.
Ian

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mastrila,

Have you measured the voltage at Q4 collector when doing these tests? Is here enough voltage to supply the output or are the transformer and rectifier dropping too much? If there is plenty of slack here the problem is in the controlling part. Measure the base to emitter voltage under these load conditions. This is where I would start the troubleshooting. Please let me know.

Ante ::)

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Hy,

Im new in this forum, and just wanted to make a good power supply for my projects. I already made the PCB for this supply. I dont really understad that the U2 and U3 opamps are good, or not? In this or the other thread somebody said, that it has a voltage of 38V instead of the maximum 36V. Is it true, or do i really need to get some other high voltage amplifier?

thx

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tuzvihar,
That comment is still in debate. The person who made the comment has not built the project. Those who have already built this project have not given me a reason to believe there is a problem with it as yet.
Welcome to the forum. I am glad to see someone who is actually building a project instead of redesigning it in their head. Please keep us posted on how it performs for you.

MP

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