josko

I have built this supply long ago, it sits in old AT case, but today I found problem, that on negative (-) supply I am having -40V DC against wall plug ground. It's not problem in general, but if you want to supply something that is for example connected to computer through USB it'll blow the fuse if not USB port of computer.. What could be causing this problem?

So it should be voltage between (-) output of supply and P2 (+ input of U3) or am I wrong?

Thank you for your answer. I'll explain you my setup: I have separate winding on transformer for 9V supply of fans and panel meters so it is completely separate from measured voltage (voltage meter works OK). I also have voltage divider set up to divide 1.65V to show as 5A = that is 50mV on 200mV panel with adjusted decimal point. I think that problem is that I thought that I should measure voltage on that + input if U3. But if I measure current on R7 (shunt resistor) I'll get current in circuit, but I would like to see current of limiter, because if I use this power supply I need to set up current limiter to some value...

Is it much of a problem if I have 0.33Ohm 15W (three parallel 1Ohm 5W) shunt resistors instead of 0.27? I think that it's ok, but I have calculated voltage divider for 0.33Ohm to my 200mV current panel meter that makes 1.65 volt show as 5Amp which I think is OK - 5Amp through 0.33Ohm is R*I that is exactly 1.65V. But in fact when it shows something around 3.5A on my panel I get 5Amp on my multimeter when I measure current through 4.2Ohm resistor (on my output terminals). And what is even stranger is that on the other module with the same panel meter (exactly the same I have switch on it to choose left or right module) when I set 4Amps I get exactly 4.5 Amps.. Is there any way to correct it? I measure reference voltage between + input of U3 and GND is it right?

I have one last question (probably). As I understand using only two TIP3055 transistors for 5A power supply is not possible, how many transistors do I need for 5A versions and then what modifications to picmasters circuit do I have to make? I mean I know that I have to add parallel transistor or better another two, but do I have to change emitter resistors for lower value too? And also assuming that I'll probably use 4 parallel transistors do I have to change driver transistor too? I'm asking because shorting circuit with two transistors on 5A obviously blow up transistors (although with no sign of overheating - a have two fans inside old AT computer case which makes good airflow through heatsinks) and I want to make that power supply reliable.

I have probably broken circuit.. I was trying current sensor and shortened output and now it is not working at all. I mean full rectifier voltage is on output. I see no smoked parts, nothing like that. It just doesnt work. What could possibly gone wrong? I have to note that before this experiment I changed shunt resistor from 0.5 ohm to 0.3 ohm. It seems as if current sensing is still operating.. because if I turn down current pot low enough limiting diode lights up, but still voltage doesn't goes down.. therefore I suspect driver transistor... //Edit: I have figured it myself, I have blown up power transistors and driver transistors .. only one power transistor is still OK.. I have replaced all power transistors and driver transistors and it's working now :) Now I know what I did wrong: I have replaced shunt resistor and leaved trims set up for 0.5 ohm shunt resistors (for 5 Amps) that made it something about 8 Amps for 0.3 ohm resistors on full load and of course it blew up power transistors... Now I have replaced 10 Amp fuses for lower 6.3 Amp fuses so now I can't blow it up :) Only surprising thing on it is that there was no smoke or any sign of broken transistors, I had to measure them to see that they are broken..

Thank you. So it is voltage between 3 and GND? I understand that it compares voltage between voltage which is set from reference voltage by P1 pot and voltage on shunt resistor. So I understand that it has the same voltage range on shunt resistor as across shunt resistor? That is what I am trying to figure out: On which point I could measure voltage that is directly comparable to voltage across shunt resistor? I see that there is + and - input of opamp U3, but also there is 10k R12 and also other resistors which makes me think that there will be slightly different voltage range directly on 2 input of U3.

Where should I measure reference voltage that is used for current limiting? And correct me if I am wrong: To get current displayed on 200mV panel meter on which circuit limits I need to just measure shunt resistor and from it's value determine voltage range for reference voltage?

Gosh I found it.. I have forgotten to solder one interconnection... stupid me.. it is when you solder at night :D Thank you for your help. And you are right 38 VDC was measured at no load at all I have not tested full load yet, but I have 10Ohm 100W resistor lying around so it's to to do some burn in test :)

I found one difference here and it's that I have -5V DC on C10 on defective circuit and -8.2 on C10 on working one.. but R8 is correct and diodes (D4, D6, D3, D5) seems to be OK too.. I'll try to replace diodes tomorrow, I'll have to go buy some new. Zener voltage on ZD1 is the same on both circuits do I have no other suspects.. Do you know what voltage should be on C10? And another question if I eject U3 is there any threat to turn on circuit? I want to measure voltages without U3 to check if there is no problem in it..

Thank you for your answer, I have 2x 29,7V AC transformer (it is 1200W this makes it 600W for one 29.7V AC on C cores so it doesn't drop under any load, actually when I shorted it once it blow up secondary (and also primary) circuit breakers instead of transformer itself :D). Thank you for information about 30VDC I have managed to get 38VDC by adjusting RV3 (I have adjusted it to get 0 V DC when I turn voltage potentiometer to min) is it dangerous for correct operation of my circuit? You found it! It is that -1.3V supply on defective circuit I measure something about +0.53V (I have -1.45V on correct one) Do you have any clue what could be wrong?

Hi, I have made two modules of picmaster's version, but I have a little issue. First of my two modules is working flawlessly regulating voltage from 0-38V and so the current limiter. But the other visually looking the same (all parts are the same and as well interconnections) is also regulating voltage but only 0-31V and also current limiter is not working at all in one position of current setting potentiometer I have managed to get limiting diode shut (it's around 1/4 from lowest current setting) but if I move it slightly higher or lower limit indicating LED begins to shine but it is not sudden blink as in the first module, but linear, like some bulb dimmer also it doesn't matter if I move it to max current or to min current and also voltage drops only by 2-3 volts when it limits current. Any suggestions what might be wrong here?

redwire: I know that I'll have to redesign that PCB to have wider traces. I wanted some starting point, just didn't want to design PCB if there is one already designed...

Ich Will: Is it for this one: http://www.electronics-lab.com/forum/index.php?action=dlattach;topic=7317.0;attach=16065;image And do you have better resolution? Audioguru: What about original PCB... are there many changes from original design?

pyrohaz: If you don't need too much power you could build something single-ended from this page (it is in czech language so I used Google Translator for you): http://translate.google.sk/translate?hl=sk&sl=cs&tl=en&u=http%3A%2F%2Fhlava.webpark.cz%2Fjednocinne.html Especially this one: which is single-ended triode amp so it has original tube sound of 30's. It is using quite cheap valves (PL509)

Thats OK... actually, my transformer have separate windings. I draw it wrong on schematic. This is photo of my monster transformer (it has 12 kg), that 9V battery is just for comparison sake. And what about that PCB design?

To audioguru: Thanks for answer! So is it possible to do something like this? To use it as symmetric power supply... I don't want to have 2x 12A now, 2x 5A is enough for me, but transformer is capable of deliver such power so I want only know that there is such possibility with this circuit. And also is there PCB design for this last version?

Hi, great design... but I need even more powerful one ;D How to modify this circuit to by usable for 2x 0-35 V DC at 0-12A ? Because I have unused 1000VA transformer with output 2x 30V AC so it is about 42V DC on caps so I think that it would be sin not to use it. I think that using more powerful transistors should be enough (do you know some more powerful sort usable in this circuit?), but what about that OPAMPs, do I have to use some sort for higher voltage?