0-30 Vdc Stabilized Power Supply

[Sallala1]

Hi all!

I'm, who start this topic some years earlyer.
My parts are in a box one years ago, now  there is the time to build the PSU.
I think, I visit the topic, before start building. It was a good idea

I readed through the topic. Thank you everybody, who help to correct the circuit, especially audioguru! I updated the parts list, TI shipped the three free OPA445 today.
(I ordered it July 5 evening, and FedEx knocked the dor July 8 in the morning Whoaa. (I'am in Europe/Hungary)

Maybe I will finish it the end of the next week. (I will busy in the weekend) I will post my experience.

Gabor Salamon

 

[ta03]

I measured the volt pot and it is ok.the other pot varies from 0v to 1.6v .
When i connect a 30 ohm load the out put voltage drops (without current limitation)
and varies from 0.5 to 0.8v . ??? ??? ??? :-\Current limiting also seems erratic.

 

[audioguru2]

Hi Sallala,
Welcome back! ;D
You are going to end up with a power supply that works very well.

 

[audioguru2]

I measured the volt pot and it is ok.the other pot varies from 0v to 1.6v.

Then your pots are OK.

When i connect a 30 ohm load the out put voltage drops (without current limitation)
and varies from 0.5 to 0.8v . ??? ??? ???

If you have the output voltage adjusted for 30V without a load then with a 30 ohm load (1A) what voltage does it drop down to when the current pot is set to max?

With a 30 ohm load, when you adjust the output voltage upwards, is there a point where the voltage doesn't increase any more when the voltage pot continues to be turned up?
Then measure the voltage at pin 6 of U2. If it doesn't increase to about 34V then your transformer can't supply enough power.
What is the rating of your transformer?

Current limiting also seems erratic.

With the output voltage adjusted to 30V with a 30 ohm load, doesn't it drop to 15V when the current pot is set to about 17%?

 

[ta03]

with 30ohm load and pot set to 30v the output voltage is 0.81V(without  load it is 31V)  .With the volt pot set at min output voltage(with load) is 0.5V(without  load it is 19V) .By turning a bit the volt pot it jumps instantly at 0.81v and stays there independently from the pot(without load jumps from 19 to 31v the same way) .Pin 6 of u2 is at 31V  and i use a 24V 5A transformer. With the output voltage adjusted to 30V with a 30 ohm load,  it doesn't drop to 15V when the current pot is set to about 17%

 

[audioguru2]

Hi Ta,
Your project has a few problems. Maybe its -5.6V supply section isn't working. Isn't the voltage of pin 4 of U2 -5.6V with and without a load?

with 30ohm load and pot set to 30v the output voltage is 0.81V(without

 

[ta03]

The voltage of pin 4 of U2 is -5.58V without load and with load, with current limiting on it is -4.35V.should i measure the voltage at the transistors?

 

[audioguru2]

Hi Ta,
Your negative supply is fine.
Let's fix the output voltage without a load when it is 19V instead of zero:
1) With the voltage pot turnsd down to zero, the slider's voltage is 0V, isn't it?
2) Then the input pin 3 of U2 is also 0V, isn't it?
3) Since the output voltage is 19V, then the emitter of Q2 is about 19.6V, isn't it?
4) Then the output pin 6 of U2 is about 20.2V, isn't it?
5) The voltage divider of 56k R12 and 27k R11 from the 19V output results in 6.18V on pin 2 of U2, doesn't it?

 

[ta03]

Audioguru thanks a lot but before i do anything i just discovered that the 2 2n3055
are probably burned and they need replacement(strange they didn't smell bad...LOL).The collector of 2n3055 is its case right?I will check what you have told me right after.Thanks again for your patience.

 

[audioguru2]

I just discovered that the 2 2n3055 are probably burned and they need replacement(strange they didn't smell bad...LOL).

If you have a big heatsink and the current regulator works, they shouldn't burn in this modified circuit. I never smelled a burnt metal one. Aren't you using transistors with the metal TO-3 case?

The collector of 2n3055 is its case right?

Yes.

 

[ta03]

I replaced the 2n3055.Now the its varies fron18.1 to 31 volts but now it is proportional to the volt pot.By turning the volt pot from max output to minimum,the voltage drops until halfway of the pots turning ability,and after halfway it remains steady at 18V.Now to your questions:
1)Yes
2)It varies from 0V to 10.7volts if i turn the volt pot from one end to the other.
3)Emmiter of q2 is 31.8V, base 18.6,collector 18.4 with output voltage at minimum
4)31,2V at max output ,14.4 at minimum
5)5.85 at minimum and 9.88 at maximum

 

[audioguru2]

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. >

View attachment 37306

 

[ta03]

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!!! 

 

[audioguru2]

Horray Ta,
I'm glad you fixed it and it is working well. ;D
Do you have a 'scope and an accurate dmm?
I hope you don't mind thoroughly measuring its performance and posting it.

 

[ta03]

;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?

 

[audioguru2]

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

 

[ta03]

Ok, i will use my sound card as an oscilloscope.In a couple of days i will hit the shop
to buy some resistors to construct a voltage divider first(I think my soundblaster will burn if i give it 30V input ).

 

[ta03]

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?

 

[ta03]

I forgot to mention that if i set volt and current pots so as the current limiter is not on, after a while amps jump at 5A and the led turns on. :-\ :-\ :-\

 

[audioguru2]

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