0-30V Stabilized Power Supply

[redwire]

Hi DrinkH2O4,    I don't know if anyone is going to try to match up parts on the board layout provided, to see what Picmaster changed from the design being discussed on this thread.  It would certainly be easier with the sketch layout but you may need to pick a design and either ask Picmaster or this board for help.  I thought Picmaster used a PIC chip to do some of the control work.

 

[audioguru2]

Hi Sulphuric Acid,
I have Picmaster's schematic. He changed ALL the parts designation numbers.

 

[audioguru2]

i get constantly 28,8v on pin 6 of u2 although i turn the pontesiometer.

But the output of your project is at 36V which is 7.2V higher.

If the base or emitter of one or both of the output transistors is shorted to the positive unregulated supply then the emitter-base diode of the BD139 is reverse-biased and acts like a 7.2V zener diode.
After fixing it then maybe the U2 opamp is destroyed and needs replacement. 

 

[DrinkH2SO4]

Hi again, and thx for the replies. I think the pcb in the latest zip file (the brd file) is double layered pcb, for which i dont have both sides copper plated board at the moment. Also i wouldn't want the pcb i already made (from the layout picture i posted before) to go to waste. 

 

[_pike]

Hello well great news...finally got it work.I had a 2n3055 in my drawer that was "off" and caused all the problem.(Can't figure out if i had tested before solder it...)
It blown up (i think not sure ,but i changed it)the bd139 and i think that burn one of the inputs of the mc34071 and thats because when i changed the transistors, i noticed that when i was turning the pontesiometer from the half and clockwise the voltage was increasing from 15-30 from the half and anticlockwise it was happening the same...on the center position of the pontesiometer was the lowest voltage value which was 15 and turning it clockwise or anticlockwise the voltage was increasing from 15-30v.I am writing this down because somone would might face a similar problem.

Regards and thank you all for your precious help and ESPECIALLY audioguru which i propose you to change it To --->electronics_guru.. 

P.S I haven't  test it but what will happen if i short circuit the output of the power supply?  Will C.C take over or it will burn anything?

 

[audioguru2]

Hello well great news...finally got it work.I had a 2n3055 in my drawer that was "off" and caused all the problem.

Good, you fixed it.

I haven't  test it but what will happen if i short circuit the output of the power supply?  Will C.C take over or it will burn anything?

If the current regulation circuit works properly and the BD139 and 2N3055 transistors have enough heatsinking then the output current will be what the current-setting pot makes it from a few mA to 3.0A and the LED will light.

 

[_pike]

audiogury sorry for asking that but i need to solve it to my head (I am newbie so show understanding    ;D)  Why when the current limiter is enabled the voltage drops???.I know that is has to do with the ohms law but i need more spesific details... I mean why the voltage drops instead of staying steady ...
For example we have a device that needs 12v 3,2A to operate if we set the voltage pot to 12 and the current pot to 2,5 then we will notice that when the current will reach the 2,5A the voltage will start falling (don't know how much) why is that happening instead of preserving the voltage staedy to 12V and the current to 2,5Amps ????

Regards.

 

[audioguru2]

Why when the current limiter is enabled the voltage drops???.I know that is has to do with the ohms law but i need more spesific details... I mean why the voltage drops instead of staying steady ...
For example we have a device that needs 12v 3,2A to operate if we set the voltage pot to 12 and the current pot to 2,5 then we will notice that when the current will reach the 2,5A the voltage will start falling (don't know how much) why is that happening instead of preserving the voltage staedy to 12V and the current to 2,5Amps ????

You want 12V at 2.5A. Then Ohm's Law says the load is 12V/2.5A= 4.8 ohms.
You set the voltage to 12V and you set the current regulator to 2.5A and load it with 2 ohms. Then Ohm's Law says the voltage MUST be 2.5A x 2 ohms= 5V as shown with Ohm's Law again.

If the voltage stayed at 12V then the current in 2 ohms would try to be 12V/2 Ohms= 6A which is impossible in this supply with a maximum current of 3A.

Set the voltage to anything and set the current to 2A. Then short the output. The voltage will be zero and the current in the short will be 2A. 

 

[_pike]

Thank you very much!!!!It seems that you won't get away easy from me..... ;D  I am joking.....
Something i had forgot...Why we use 3 2n3055 for this power supply where 1 2n3055 can provide this current?

regards!

 

[audioguru2]

The 3A version of this improved project uses two 2N3055 output transistors. The 5A version uses three of them.
They share the heat.

When the output is shorted or set to a low voltage and the current is 5A then the output transistors must dissipate a total of about (40V - 1.35V - 0.55V) x 5A= 191W. Each transistor must dissipate 191W/3= 64W.

One 2N3055 transistor can dissipate a maximum of 115W if its case is held to only 25 degrees C with some kind of freezer. But then its chip will be extremely hot so maybe 90- degrees C would be more reliable.
Where will you find three suitable freezers or one big one?

We use heatsinks to transfer heat from a power transistor to the ambient air. A heatsink is aluminium (conducts heat well) and it has many fins for a large surface area in the ambient air. Thermal grease is used between the power transistor and the heatsink to fill the microscopic dents with a heat conducting material.
A heatsink is not perfect since it and the power transistors will still get hotter than the ambient air but they work pretty well if each transistor dissipates only 64W.
Then a pretty big heatsink can cool the three power transistors. A smaller heatsink can be used with a fan. 

 

[_pike]

(40V - 1.35V - 0.55V) x 5A= 191W.

can you explain me the 2 values??? 1.35V and 0.55V...


And i can understand the part....
" But then its chip will be extremely hot so maybe 90- degrees C would be more reliable."

Do you want to say the it would be safer for the transistor to work in a temperature of 90 degrees C? What do you mean when you say "its chip" ?

Regards 

 

[audioguru2]

(40V - 1.35V - 0.55V) x 5A= 191W.

can you explain me the 2 values??? 1.35V and 0.55V...

5A in R7 that is 0.27 ohms= 1.35V.
5A/3= 1.67A x 0.33 ohms (the emitter resistors for the three output transistors)= 0.55V.

And i can understand the part....
" But then its chip will be extremely hot so maybe 90- degrees C would be more reliable."

Do you want to say the it would be safer for the transistor to work in a temperature of 90 degrees C? What do you mean when you say "its chip" ?

Yes.
It is safer not to operate the transistor silicon chip that is inside the case of the transistor at its absolute maximum temperature. Some manufacturers say so on their datasheets.

 

[Kevin Weddle]

What about the opamp differential voltage? U2 has almost none. Because the gain is low. The resistance is low.

 

[audioguru2]

What about the opamp differential voltage? U2 has almost none. Because the gain is low. The resistance is low.

Kevin,
Didn't you learn anything about opamps? U2 has a DC gain of 200,000 times. If one input is +2.0V then the other input is +2.000005V and the differential voltage is 0.000005V (almost none). The negative feedback sets the gain of the entire amplifier to 2.7 times.

What resistance is low? The input resistance of U2 is 70M ohms as listed in its datasheet. U2 drives the fairly high input resistance of the driver transistor.

 

[Kevin Weddle]

U2 doesn't have any differential voltage at higher output voltages. At zero volts ouput, it has more differential voltage because of the -3volt power supply. I'm not sure how this can affect the voltage regulation.

 

[audioguru2]

U2 doesn't have any differential voltage at higher output voltages. At zero volts ouput, it has more differential voltage because of the -3volt power supply. I'm not sure how this can affect the voltage regulation.

Kevin, you are completely WRONG!
1) The output voltage has only a slight effect on the differential input voltage of opamp U2. It increases slightly, it is not eliminated at higher output voltages because U2 is an amplifier.
2) The power supply voltage has no effect on the input differential voltage.
Opamp U2 does not have a negative supply voltage. It did in the original project that had many errors.
Opamp U3 has a -1.3V power supply voltage so that its output can go to about -0.7V to forward-bias D9 to force the output voltage of the project to zero when the load current of the project exceeds 3A. 

 

[redwire]

On my unit with an output of 30.0V,  U2 pin 6 has an output of 30.07, pin 2 11.46V,  Pin 3 = 11.44. No load

If U2, pin 6 has an output of 13.8V, pin 2 = 5.02,  pin 3 =5.00V

R12 is 55.6K , R11 with pot = 34.5 KOhms;  amplification = 1+ 55.6/34.5 = 2.61.  This is very close to  measured values.

Values measured with Radio Shack multimeter so digits shouldn't be taken too far.

 

[Kevin Weddle]

Thank you audioguru. I'm wrong. I forgot where 0volts differential occurs.

 

[denci]

The MJ15003 is a good power transistor.       The thermal Resistance, junction to case is about the lowest you can get but getting that to transfer to the heat sink is more difficult.  The power derating of 1.43 W/C is high if you are running hot but when you start at 250W you've got plently of  headroom.    Have you checked out the MJ11016 it is a darlington transistor and if you don't mind a 1v drop (or less) in the total output, it has a gain of 1000 and will reduce the load on the BD139. The thermal Resistance, junction to case is close to the MJ15003.  I did a test with only one MJ11016 driving a 5A-12v  load on a large heat sink (outside a case) with no fan for about 5 minutes without failure. The BD139 only had a small heat sink and did not get that hot.     I would feel very confortable with 3 transistors no matter what type (MJ15003, 2N5886 or MJ11016), running 5-7 amps.

A have been working on this a log time ago and i have just one question for you, which element in circuit is total allowed output current adepend on??
And just one thing, is project according to your parts list work properly at all or you did some changes on project, i am talking about 5A modified project, last table i have is below.
View attachment 42000

 

[audioguru2]

The entire output current flows through the output transistors and their emitter resistors. Since they have a high voltage across them when the output voltage is low or is shorted then they get very hot when the output current is 5A then they need good heatsinking.

The MC34071 is not available in a through holes DIL package anymore so we use only the TLE2141 opamps now.