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0-30 Vdc Stabilized Power Supply


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Hi Ta,
You have the collector and emitter of Q2 reversed. Its collector should connect to the positive unregulated 31.8V supply. Q2 might be destroyed since it had a high power in its base-emitter junction that isn't designed to handle power. U2 also might be destroyed since it probably got too hot feeding a high current to the backwards Q2. >:(

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The original circuit should work fine up to 15V at 1A if you replace the old opamps with the newer higher voltage ones. You probably should recalculate the resistors that set the maximum voltage and c

Hi, as promised I made an English translation of my working. Maybe there is few mistakes and I am sorry for that ! Good reading. ExplicationEN.pdf

February 23 above on this page has the latest schematic of the revised 3A lab power supply.

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Audioguru you are trully a guru!Thank you!Q2 was wrongly soldered i changed emmiter-collector and the psu works great now!Everything works!Output voltage is ok so as the current limiter!!!Q2 was indeed destroyed but fortunately i had a spare tip31c.U2 was also a bit hot before but i could touch it while hot so i did not mention it(i thought it was normal).Audioguru thanks a lot  for your help and your time!!!I will post images when i will put it in a case.Thanks again!!!  :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D

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;D Of course i will test the psu and make a post.I have a dmm [a scope is a bit expensive for me :( (can i use my sound card as an osciloscope?)].I think it is accurate(0.5% accuracy in Vdc , 2% measuring Amperes). What measurements should i execute?

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Hi Ta,
You can use your sound card as a 'scope to see hum on the project's output when you turn-up its voltage with full load. With your 24V transformer I think its max output with full load is about 26V and you will see ripple at higher voltages.
I would like to know how good is its voltage regulation from no load to full load, at a few output voltages. ;D

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I found a 2.7Ohm 25W resistor.I connect it at the power supply,I put the volt pot at max and played with the current regulator.In the beginning everything is ok but after a while something happens and output jumps at about 5A,if i set high amps it jumps after a second or so(with lower amps eg.1.5A it takes about 10 seconds) and if i close the amp pot(or volt pot) it remains at 5A.If i disconnect the load and reconnect it (even fast,i disconnect it for half a second)amps drop to normal again and after a while they jump at 5A.R7 gets REALLY hot and so as Q4.Any ideas?

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Hi Ta,
Since the current regulator's LED turns on, then the output pin 6 of U3 must be going low like it should, therefore D9 should be pulling down the input pin 3 of U2, therefore U2's output should also pull down to reduce the output current by reducing the voltage.

Maybe Q2 doesn't have a heatsink big enough so gets too hot causing it to conduct when it shouldn't. ???
Maybe one of the two 2N3055 transistors is sitting on a bump so doesn't conduct its heat to the heatsink very well, getting too hot and conducting when it shouldn't. ???

I hope you used big heatsinks and thermal grease between each power transistor and its heatsink.
With a few voltage measurements, you'll find the problem. Good luck.;D

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Well...end of fun for today.Q4 damaged!I used a quite large heatsink for Q4(with mika insulators) but it became really hot during tests(i could barelly touch the heatsink which was 15x10cm).Q2 was all the time cool, it did not heat up(4x2cm heatsink). I didn't use thermal grease.I will do volt measurements tomorrow when i get some new 2n3055(i will buy a stock of them  ;D )

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Hi Ta,
OOps, your output transistors melted! :(
Just think how hot the original project's single output transistor would get.

Your heatsink (only one?) is way too small for such a powerful project like this one. I would use your 15x10cm heatsink for about 15 to 20W. Since your faulty project was supplying 5A into a 2.7 ohm load, your output transistors were trying to dissipate a total of about 107W.

Thermal grease must be used to fill the microscopic valleys in the transistors and heatsink. Don't use a mica insulator, mount the transistors directly to the heatsink with thermal grease then insulate the heatsink from the chassis. Maybe it is best to use a separate big heatsink for each output transistor.

If you used a 30V transformer and calibrated the current regulator's pot at 3A, if you short the output with the voltage at 30V and the current at 3A then the output transistors will dissipate about 61W each.

I hope you use a much bigger heatsink with thermal grease but without mica insulators, and turn it on for short time periods until you fix the current jumping to 5A. ;D 

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Using a fan blowing over the heatsink could be a solution, too, if you don't want to buy a new heat sink. You need a very big heat sink to get 123W away without active cooling and even than the thing will get hot as hell. That is not good for the caps and will spoil the accuracy of voltage and current regulation, because the heat will spread inside the case.

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Hi Ta,
He, he. ;D ;D
I knew you would ask what size. I could crunch some numbers, select a suitable one, then you'll say you can't get that one.
Use a fan with two of your existing heatsink (no mica, use grease) like Thomas says.
Make it variable speed like my pc and like some hi-end audio amps. Use a digital display or bar-graph to show the temp. ;D

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Hi Zeppelin,
Welcome to our forum. ;D
If you are going to use two 2N3055 transistors and a 30VAC, 4.3A, 130VA stepdown power transformer and high voltage opamps as recommended here, then each transistor will have about 38V across it when the output is at 3A and the voltage is low or shorted. Therefore each transistor must dissipate about 57W.
In the forum of the 5A version of this project
http://www.electronics-lab.com/forum/index.php?topic=196.msg23794#msg23794
a heatsink 234-2520 from RS Electronics Israel was asked about. RS might be the same company as the one in the UK. Wakefield in the USA have a similar heatsink.

The thermal resistance of a metal T03 cased 2N3055 transistor is 1.5 degrees C/W and the thermal resistance of this heatsink is 1.4. Therefore with 57W of dissipation in a transistor and an ambient temp of 30 degrees C, the junction of the transistor will be 187 degrees C.
The absolute max for the 2N3055 is 200, so this is very close to operating at the absolute max.

Notice that I haven't included the 0.7 degrees C/W thermal resistance of a transistor insulator in my calculation since it would make the transistor overheat. But a fan could be used to increase the efficiency of the heatsink.

If I used heatsinks as small as this one, I would not use a transistor insulator but use thermal grease and a fan. The heatsink improvement by using a fan is discussed at www.wakefield.com . ;D

 

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I changed the 2n3055's.Now everything is fine :I checked the voltages you mentioned and they are ok,I put the 2 transistors in the same heatsink(I forgot to buy another one  :-\ ,tomorrow i will) using thermal grease and a 12V pc case fan..Everything seems fine and i am currently testing the psu with 2A output on 2.7ohm load.One thing i noticed is that one transistor get much hotter than the other one(so as the heatsink below the hotter,meaning that it does not heat because it doesn't come to good contact with the heatsink,I believe).Can someone (audiogugu  ;D ) explain it?

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Hi Ta,
Good, you fixed your current jumping problem by replacing the 2N3055 transistors that were destroyed by over-temperature. ;D

Since one new one gets much hotter than the other, you can measure the voltage across each one's 0.1 ohm emitter resistor to see how well they match. The low value emitter resistors help a little to match the transistors without much loss. You could match transistors better yourself, or use emitter resistors up to 1 ohm/5W to force them to be more equal with a loss of up to 1.5V at full load.

The thermal grease provides poor electrical contact from the transistor's collector to the heatsink, so I hope you are connecting to each collector with a terminal fastened to its mounting bolt.

A burr on a hole on the heatsink will prevent a transistor from making good thermal contact.

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Hi!

What about this heatsink?

http://www.wakefield.com/extrusions/index1.cfm?index3=yes&ext_id=578&c_row=5&ext_ref_num=&ext_type=0&ext_width=7&ext_height=

Does this one need fan too? (for 5A project?)

I think the upside of the heatsink in the picture should be outside of the case.

Maksar

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Hi Maksar,
Those heatsink extrusions are very long pieces that you must cut to length yourself.
It is difficult to mount them so that the fins are outside the case but the wiring is inside. I think they are mounted to the inside rear wall of the case with the top and bottom of the case having air slots.

The extrusions have their thermal resistance rated for 3 inches long pieces. For this 3A project where each output transistor must dissipate a max of 57W, a heatsink thermal resistance of 1.4 without a transistor insulator or a thermal resistance of 0.7 with a transistor insulator, for each output transistor, will heat them almost to their max.

Hi Ta,
Your 2N3055 transistors have a very poor match. Even with 1 ohm emitter resistors they won't match.
They are all different but usually match pretty well if they are from the same batch.
Maybe one of yours was made on a Friday afternoon by one manufacturer, and the other was made on a Monday morning by another manufacturer. ;D

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Hi audioguru!

As you can see the standard lengths are 15cm(5.8") and 29cm(11.5"). So it isn't so long and can be easily matched to the case dimensions (case's height and width are fixed and length may be variable for individual needs). Also I don't see problems to mount it as a rear wall, there are several options how to do this.

The heatsink has 0.8 C/W/3", so it is ~0.5 C/W/5.8". With fans (200LFM): ~0.2 C/W/5.8". Am I right?

The question is: Is this heatsink in length 15cm suitable for 3A project without fans and for a 5A project with 1 or 2 fans on the side wall of the case (and holes on the other side wall)?
I think we need to require for 3A project as well as for 5A, that the temperature won't rise above 100C. And of course we sould use termal grease with insulator pads. Am I right?

WBR,

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