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0-30V Stabilized Power Supply

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If a project is low powered <1.5A then it's a good idea to add a fuse because the lowest rating plug fuse you can normally buy is 3A and won't be enough to protect the circuit.


Unfortunately most cheap transformers have one shot thermal a one shot, non-replaceable thermal fuse which is why it's important to add an external fuse.

Another thing is that lots of projects use regulators such as the LM317 or LM78xx which have built-in thermal protection so there's a very small risk of them going wrong.

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The input voltage here is 120V. My trafo is 30V, 5A. 150VA / 120V = 1,25A Fuse. Is this the correct thinking?

Another thing: I've noticed some of the few pics of the assembled boards in this thread show the ceramic resistor (10W) soldered like other components, I mean they're touching the board. Since this thing is gonna heat significantly, wouldn't it be better to solder it higher, like 1cm from the board?

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3A it will be then. Gotta make some changes, including this one, to the PCB. Eagle is kicking my ass right now.

A new problem: I'm going for the 5A project and accepting Redwire's suggestion of three 2N3055, each with a 0R27 5W. The problem is that no shop here has these resistors with that aluminium encapsulment (you call it wirewound in North America?).

Hell, out of curiosity I checked Farnell USA and  they have one of it: http://export.farnell.com/welwyn/wh25-0r27-ji/resistor-25w-5-0r27/dp/9507213 but its a 25W thou. Farnell-Newark here in Brazil doesn't have it. No one does. They're blaming the "Worldwide shortage of components due to the global crisis"...

I ended up getting myself 0R27 5W ceramic... But now I have no idea on how to add that to the heatsink. This things are gonna melt. You guys have any tip?

Thanks,
Effenberg 

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Don't forget that the purpose of a fuse if to prevent the cable or transformer from melting adn catching fire down if a fault occurs.

In this case there are two things that are most likely to cause this: a shorted turn inside the transformer (hopefully the internal thermal fuse will kick-in), the capacitor or rectifier going bad.

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Don't forget that the purpose of a fuse if to prevent the cable or transformer from melting adn catching fire down if a fault occurs.

In this case there are two things that are most likely to cause this: a shorted turn inside the transformer (hopefully the internal thermal fuse will kick-in), the capacitor or rectifier going bad.


Yeah, I'm kindda worried about the transformer in particular.. I couldn't find a 30V 5A unit readily available at a decent price, so I ordered a unit from a very small shop that manufactures them... It took them only 5 hours to make me one... God only knows the quality of this thing...

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 I ended up getting myself 0R27 5W ceramic... But now I have no idea on how to add that to the heatsink. This things are gonna melt. You guys have any tip?



I used a 0R27 10W cement resistor not just because I wanted a 5A version but to also reduce wasted power.   The 10w version is longer than the 5W resistor so it dissipates heat better.  Although you can not see it on the first post of this thread, the cement resistor is not sitting on the board but raised above it.  Below it on the bottom side of the board  is a large ground pad that helps dissipate heat.  At 3A it only consumes 2.4 watts.   I also built the pcb so that the resistor could be mounted to the frame by using an aluminum housed resistor.  I found that in normal operation the 10W board mounted was not a problem.

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I think I'll have no option but to solder it very raised and hope for the best.

I am giving up on these aluminium cased units... they are just a myth lol, no one has them at reasonable prices. Just so you have an idea, a standard resistor costs about USD 0.17 here (one unit). If I buy 10, 100, 1000 I would get an even smaller price (up to USD 0.02). But when I look for these more specific components:

10,000uf Electrolytic Capacitor: USD50.00 / Unit
0R33 10W Aluminium Case Resistor / USD68.00 / Unit (!?!?!?!?!?)
Heatsink for 3 TO-3: USD131.00 / Unit.

Its just a hobby lol... You know that spam from Africa or whatever about international funds transfer, etc? Maybe they know someone interested in a kidney?

Regards,
Effenberg

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Hi Effenburg,
Your transformer is too small for a 5A power supply.
It is rated at 30V x 5A= 150VA. But the project makes 42.4V so when the output current is 5A then the transformer will be overloaded with 42.4V x 5A= 212VA.

The original project had the same problem with its transformer overloaded.

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Hi Effenburg,
Your transformer is too small for a 5A power supply.
It is rated at 30V x 5A= 150VA. But the project makes 42.4V so when the output current is 5A then the transformer will be overloaded with 42.4V x 5A= 212VA.

The original project had the same problem with its transformer overloaded.


Oh Sh** ... Didn't thought of that.
Wait, I thought Redwire used 30VAC x 5A?

What's your trafo Redwire?

PS: Checking the thread historic again: Original project trafo was 24VAC/3A, updated version was 28VAC/3A, Redwire's was 30VAC/5A.
So 150VA / 42.4V = 3,5A (thats as far I I'll go, almost burning the whole thing)?
Ok, this is not happening.


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The latest parts list for the 3A power supply lists a 28V, 4.3A, 118VA transformer.


Yes, exactly. I am curious about Redwire's transformer. I thought he had a 30V/5A.

Anyway, I am considering no other way out now: I gotta use my two transformers and make it dual output somehow. I gotta study. Right now I am still thinking about the basics:

1) Primaries in parallel or in series?

2) Secondaries in series:
60V * 3A = 180VA
Sqrt(2) * 60V = 85V
85V * 3A = 255VA. Fire in the house.
180VA / 85V = 2,12A. Too little considering the initial goal and costs. Great changes in project (too much volts for components?).

3) Secondaries in parallel: (looks like the way to go)
30V * 6A = 180VA
Sqrt(2) * 30V = 42,4V
42,4V * 6A =  255VA. Fire in the house.
180VA / 42,4V = 4,25A at most. Looks like enough for me, even considering two outputs in the future.
But:
For R7, P = i^2*R = 4,25^2*0,47 = 8,5W
For Q4 emitter resistors, P = 6W per resistor = 18W
For 2N3055: 115W.
Its not looking good... Too much heat.


I need some help guys. Can you give me a hint on using these two transformers?
Are the above lines of thought correct?

The terrible thing about looking for transformers on the web is having to avoid 99% of websites about the actual movie / action figures lol.

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effenberg0x0

I home built my transformer from a microwave oven transformer.   I cut the windings and rewound it using copper tape.   My max voltage output is 58V unloaded.   I don't know the rated capacity is but I've held it at 7 amps for a short period, cut it off and check the transformer for unusual heat.    

You can use the transformer you have, simply adjust the current adjuster trimmer so that your maximum output does not exceed the capacity of the transformer.  That's the good think about the revised design.

Attached is a picture of the  LCD display I'm working on .  I have a older version actually connected that does not show the temperature or maximum current setting of the PS.

Picture.rar

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I've read this:
http://www.eng-tips.com/viewthread.cfm?qid=14767
http://electrical-riddles.com/topic.php?lang=en&cat=4&topic=282
http://ecatalog.squared.com/techlib/displaydocument.cfm?id=7400DB0701&action=view
http://www.ee.lamar.edu/gleb/power/Labs/Lab%2006%20-%20Transformers%20in%20parallel%20and%203-phase%20transformers.pdf

Considering my two transfomers were made by the same shop, same machine, same materials, in the same hour, I am gonna bet their % difference is lower than 1%. An idea that caught my attention was:



If you reaaly can not find appropriate transformer, you can consider next tip:
If you connect parralel multiple secondary windings on one transformer or even multiple transformers, as in your case, I suggest that you apply  separate four-diode bridge (Graetz) to each winding, and then connect together positive otputs and together negative outputs. In that way, if first winding has slightly lower voltage output, no current will flow, until second winding (with higher voltage) will be so loaded, that a voltage drop will appear; with even higher load current a lower voltage winding (first one) will start to conduct and thus help maintaining output voltage at increased current.
In that way you can connect as many windings as you like; transformers can be even with different power.
I do reccomend winding voltage differences are max. cca 5% , as the winding with higher voltage will be  maximum loaded all the time load is applied and winding with minimum voltage only when full load is applied.


BTW: The LCD looks nice dude

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I have checked my transformers more carefully today. They are exactly the same in terms of materials and construction. However, one is giving me 31.7V and the other is giving me 31.8, same digital multimeter...

Putting them in parallel will demand a rectifier circuit for each, with different resistors, to equalize outputs. Back to Eagle.

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I have checked my transformers more carefully today. They are exactly the same in terms of materials and construction. However, one is giving me 31.7V and the other is giving me 31.8, same digital multimeter...

Putting them in parallel will demand a rectifier circuit for each, with different resistors, to equalize outputs. Back to Eagle.



I would not worry about the .1V of a volt, this would  not make any difference at all.

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Hi Alex, that is exactly the idea. The calculation is at http://www.electronics-lab.com/forum/index.php?topic=19066.msg89507#msg89507 and the idea of rectifying the output is a couple posts later at http://www.electronics-lab.com/forum/index.php?topic=19066.msg89517#msg89517.

I was just worried about doing it because many technical docs mention the problem regarding small differences between transformers. But You and PicMaster convinced me.

Regards,
Effenberg

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OK, I'm going for 5A version with transformers in parallel. This is my first Eagle attempt. I'd like to hear some opinions. I'm specially interested if you agree to RV1, RV2, RV3, P1, P2 values (I'm using MC34071). And if you see any mistake.

Thanks,
Effenberg

Edit: Could not find P3, mentioned in Redwire's project in any schematics. Can anyone point me to it? Is it in series with P2?

post-48772-1427914402642_thumb.png

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Hi Effenberg,
Why is your schematic a hard to see negative? I changed it to a positive (white background) so i could see it like most other schematics.

1) Some of your opamps have the  pin numbers on the inputs backwards. Some of your opamps have the + and - input symbols backwards. Pin 3 is supposed to be + and pin 2 is supposed to be -.
2) U1 has R5 connected to pin 1 instead of to pin 2.
3) For C1 you have 3 capacitors in series instead of parallel.
I looked only for a couple of minutes.

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Why is your schematic a hard to see negative? I changed it to a positive (white background) so i could see it like most other schematics.

Sorry, I'm new to Eagle. It was setup like this and I didn't know I could put a white background. Now I know, thanks. I have attached the new image.

1) Some of your opamps have the  pin numbers on the inputs backwards. Some of your opamps have the + and - input symbols backwards. Pin 3 is supposed to be + and pin 2 is supposed to be -.

Thank God you saw it. I couldn't find TLE2141 or MC34071 in Eagle libs, so I used another opamp that has the pins inverted. Its fixed now.

2) U1 has R5 connected to pin 1 instead of to pin 2.

Yes, I was wrong to use pin1, but schematics I found in the thread show it at pin3. Is it 3 or 2?

3) For C1 you have 3 capacitors in series instead of parallel.

Fixed :) Lame mistake...

Questions:
1)I have attached the list of Potentiometers and Trimmers in the parts lists of the three versions (Original, 5A, 3A). I'm confused to be honest. I'm building my version with two 30V/5A transformers in parallel and MC34071 opamps. In this case, I guess I should use 10K for RV1, 50K for RV2, 200K for RV3?

2)About the third Potentiometer (1K), I placed it in series with P1. Can you tell me if it is correct?

3)A serious problem: All schematics show an unnamed capacitor (0.1uf) between R19 and U3. I named it CX. I cannot understand why it would be there. Can you give me a hint? I have highlighted it in the attached schematics.

Thanks


post-48772-14279144027306_thumb.png

post-48772-14279144027462_thumb.png

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