0-30V Stabilized Power Supply

[LEECH666]

More pictures.
Not quite sure which variant I want to use.
2N3055 on a L-profle mounted on the heat sinks or TIP3055 mounted flat on the heat sinks.

[audioguru]

R15 wastes supply voltage. 1k is too high but 100 ohms is fine.

When output transistors are connected in parallel so they share the heat then each one needs a 0.33 ohm emitter resistor so that their base-emitter voltage differences do not cause the one with the lowest Vbe to hog all the current.

The minimum current gain of one 2N3055 with a current of 3A is about 27. Then the max current in the BD139 is 3A/27= 111mA.
Two 2N3055 transistors each have a max current of 1.5A then their minimum current gain is about 42. Then the max current in the BD139 is 3A/42= 71mA.
You can calculate the current in the BD139 if there are 3 and 4 output transistors.

[denci]

As we know power dissipation of one of three 2N3055 output transistor is aprox. 65W and recommended thermal resistance for heatsink for one transistor is (200-35)/65 - (1.5 + 0.2) = 0,84 °C/W, but how can i calculate the required thermal resistance of heatsnik for three same transistor with the same power dissipation?
Is ther any equation for this or is eought that i get the heatsnik with thermal resistance three times smaller than thermal resistance for one of each used transistor??...sorry for my english!!
TNX

[donr]

Hope I am posting right here. What I would like is the revised schematic of this power supply and also info on where to purchase pcb. I have the revised parts list posted by audioguru. Thanks

[LEECH666]

The latest circuit diagram can be found here: http://www.electronics-lab.com/forum/index.php?topic=19066.msg1002666#msg1002666

According to this application note from Fischer Elekronik (heat sink manufacturerer)
http://www.fischerelektronik.de/pim/upload/fischerData/datasheet/base/technischeerlaeuterungen_d.pdf (sorry it's in German, there used to be an english version of this but it's gone) you calculate it like this.



Florian

[donr]

Thank you much, I will go from here.

[donr]

One more question. Is there a way I can get +/- outputs on this supply. I am going to build it anyway but would be even more handy for me if I had both. Thanks.

[audioguru]

One more question. Is there a way I can get +/- outputs on this supply. I am going to build it anyway but would be even more handy for me if I had both.

Make two completely separate projects (with separate transformers or separate transformer windings).
One can be a positive supply when its negative output is common and the other can be a negative supply when its positive output is common. Connect the commons together and to earth if you want.
Then you will have positive 0V to +30VDC at up to 3A and have negative 0V to -30VDC at up to 3A.

[donr]

Thank you audioguru for the info and please don't think I question your answer because I know next to nothing about electronics. Here's the rub, I just put a small voltage bench supply together, two 14v outlets, one plus, one neg, using center tapped trans. I was curious as to whether this one could be modified so as to have more output v and amps than the first one and have +/-. I have the parts for this 30v supply on order and thought maybe with some changes I could do what I asked about. Probably to late for that now?

[audioguru]

It is simple to make the modified 0v-30V/3A power supply produce up to 5A instead of 3A. There is a thread about it.
It uses a transformer that has a 7A AC output, 3 output transistors to share the heat and the 0.47 ohm current-sensing resistor is changed to 0.27 ohms.

[madafakamw]

Hi again.
I've just finished the PSu work and I got some problems. U3 gets instantly very very hot after i plug it in. Also The current control potentiometer acts like voltage regulator and it regulates between 15 and 38 V. I really need some help.
Ty.

[audioguru]

Hi again.
I've just finished the PSu work and I got some problems. U3 gets instantly very very hot after i plug it in. Also The current control potentiometer acts like voltage regulator and it regulates between 15 and 38 V. I really need some help.

U3 has a load of only 3mA through R20. There is no current in D9. Therefore U3 will not get warm.
The current control normally reduces the output voltage when the output current exceeds its setting.

[madafakamw]

Is normal to have 45V at the + and - of the bridge diode?  
LE:
I use 28V ac transformer.
If P2 shortcuts Pin3 of U3 with R18=U3 gets very hot;
if P2 shortcuts Pin3 of U3 with R17=U2 gets very hot, led also turns on;
I don't know too much electronics and I need someone to help me troubleshoot this power source. I've spent too many hours just to throw it away.   

[audioguru]

Is normal to have 45V at the + and - of the bridge diode?  
LE:
I use 28V ac transformer.
If P2 shortcuts Pin3 of U3 with R18=U3 gets very hot;
if P2 shortcuts Pin3 of U3 with R17=U2 gets very hot, led also turns on;
I don't know too much electronics and I need someone to help me troubleshoot this power source. I've spent too many hours just to throw it away.

Your 28V AC transformer might produce 30VAC when it has low load current. 30V AC produces 42.4V peak. The rectifier bridge reduces it to 41.0VDC. Your 45V is a little too high but is OK for opamp U3 if the 10V zener diode that feeds it is not connected backwards. The cathode of the zener diode is marked with a black bar and must be connected to the positive supply. Then the positive supply pin 7 of the opamp will be +31V which is well below its maximum allowed voltage of 44V. Yours might be +35V which is also fine.

P2 is 10k, R18 is 33k and the current-calibration trimpot is about 45k. The trimpot connects to the output of U1 which is +11.2V. Then if P2 connects pin 3 input of U3 to R18 its voltage is only 11.2V x 10k/(33k + 45k)= 1.4V which is fine for U3. With a low load current on the project then the pin 2 input of U3 is 0V. Then the pin 3 input is higher than the pin 2 input so its output is high which turns off the LED and it does not limit the project's output current. U3 will be barely warm. If you are using the tiny surface-mount package then it will be obviously warm but not hot.

When P2 connects pin 3 input of U3 to R17 then its voltage is almost 0V which is fine. a load on the project that has a currernt higher than a few mA will cause the output of U3 to go low which causes 3mA in R20 which turns on the transistor driving the LED. The low at the output of U3 also causes D9 to reduce the voltage to the input pin 3 of U2 which reduces the output voltage of the project so that the output current is reduced. U3 gets a little warmer but not hot and U2 does not get hot unless the driver or output transistor is connected backwards (collector and emitter pins swapped).

You also had the output never less than 15V which might be caused by the driver or output transistor with its collector and emitter pins swapped. Then U2 and the driver transistor will get very hot.