0-30 Vdc Stabilized Power Supply

[audioguru2]

Hi Helder,
I'm glad to help you. ;D
The voltage from your transformer is lower than we thought, resulting in a total supply voltage low enough that cheap 44V-rated opamps can be used.
I think the negative supply will be fine when you replace the blown-up opamps that are installed now. ;D

 

[HelderS]

  Hi, Audioguru,

  Ok, I must wait now, to buy some 44V opamps. And after I change them, wich major changes to other components to make this PS Work ?? If I get an output of 20 V @ 1,5A, I really wouldn't mind...

  Regards, Helder Silva

P.S.: By the way where are you from, Audioguru?
        I'm from nearby Coimbra, Portugal. 

 

[audioguru2]

I recommend changing D8 and Q2. Change the values of R4 and R15. Then it will work as well as it can. If parts melt, upgrade them.
We don't know how low the transformer's output voltage will be with a heavy load, so we don't know what your project's max output voltage will be without any ripple or loss of regulation.

I'm in Canada, very close to Toronto. I live a 15 minute drive away from Lake Ontario where I walk my wifey and dog along its very long boardwalk nearly every day. The lake is so big that it prevents our winter temperature from dropping too low.

Hee, hee. ;D Here's a satellite pic of "my yaught" zooming on the lake: 

View attachment 37922

 

[mongseng]

Hi audioguru,
Can I replace D1, D2, D3 & D4 with general purpose rectifiers D25XB60 rated at 600V, 25A?

Thanks.

 

[audioguru2]

Hi Mongseng,
This low voltage project certainly doesn't need a rectifier bridge rated at 600V but it's fine if you have it. Bolt it to the chassis or a heatsink with thermal grease.

 

[pigster]

so at last i had time to inspect my supply further, and have little bit more info about turn-off peek, i have repleaced C1 (dont know if it was defective or not, i have just repleaced it, now it is 10 000 uF / 63V) , but i have also discovered, that if the voltage regulator is set to 4,5V or less, than i turn off the mains, and than i have about 36V on output (so C1 discharges into load on 36V), but if the voltage regulator is set higher than 4,5V (for example 5V), than all goes right and after turn-off i have 5V on output as C1 discharges. This was measured without load (just voltmeter connected to output).

Any ideas? And how does the voltage regulator works, if there is no negativ voltage (mains off, but C1 still charged)?

 

[audioguru2]

Hi Pigster,
Quote: "I said it before. "Q1 is supposed to conduct when the project is turned off and then it will immediately drop the output voltage to zero. The negative supply has low-value capacitors that discharge much quicker than C1, therefore Q1 becomes biased on."

Did you use a TIP31A power transistor for Q1or the little one from the original project?
It ain't working.

 

[pigster]

I use TIP31A, and i have its emmitor connected directly to (-) output of supply, as you suggested earlier in this forum. But thanks, i will repleace Q1 and we will see.

Thanks

 

[audioguru2]

Hi Pigster,
I am sorry I made a mistake that I corrected in a later post. The emitter of Q1 should be connected at its original position at the negative terminal of C1.
Check the pins on Q1:

View attachment 37950

 

[yoda64]

As a complete newbie wanting to build a variable power supply do you think this one would be fighting above my weight ?? ANY HELP WOULD BE APPRECIATED..

 

[pigster]

HURRAY, last problem solved, i have just changed connection of Q1 emitor and all is OK. First part of project completed (second one is adding a digital meter, but that will be another story). Thanks to all who helped me, i will post some photos soon.

 

[audioguru2]

Hi Pigster,
Nice going, you fixed it. ;D

How well does it perform?
If you can find a very big 10 ohms resistor or ten smaller 100 ohms resistors in parallel, use them to provide a 30V/3A load:
1) can it provide a 30V/3A output?
2) How much or how little is the AC ripple voltage at the output?
3) How much or how little does the output DC voltage change from 30V from having no load to having full load?

View attachment 37957

 

[Gruber]

one quick question : if i cant find OPA445AP and cant use TL081 is there any other op-amp which i can use ?
Very "funny" situation - the pcb board is done and all the other parts are here,only the op-amp cant be found to complete the PSU...
thanks

 

[audioguru2]

Hi Gruber,
See if www.farnell.com has parts distribution in your country, click on your flag.

Quote: The OPA445AP (Texas Instruments, Burr-Brown division) has a 90V rating, a 741A opamp, an MC34071CP (used to be Motorola, then ON Semi and now maybe another name), a TLE2141CP (Texas Instruments) and a National Semi opamp have a 44V rating, but they aren't guaranteed to work so high, just withstand it for a moment without damage. "Continuous exposure to this stress level may affect product reliability".

 

[Gruber]

thanks for the web link audioguru !
The strange thing in this situation is the fact that i was in the farnell's distributor in my country,but they said no such op-amp is available...
Seems i need to ask them again with some more details.

 

[audioguru2]

Ask your Farnell distributor to order it from themselves online. Then they'll have it for you the next day.

 

[Smitjuh]

Hi all,

I made the power supply and it 'all' works. The supply delivers a 30V/3A supply! I can change the voltage, and I also can change the current, but not over the whole range. And the LED doesn't light on any time. I haven't tested anything (I'm not at home right now) but maybe someone can give me the solution because he is known with the problem.....

 

[audioguru2]

Hi Smitjuh,
Welcome to our forum. ;D
It is good to hear that your project works. Did you make the original or the modified one?

You should be able to adjust the voltage from very close to 0V all the way to 30V.

The current regulator is supposed to reduce the output voltage and light the LED whenever the load current exceeds the setting of the current pot. So you need to check the few parts that perform the current regulation. ;D

 

[Smitjuh]

Hi  audioguru,

Thanx for the warm welkom 
I made the power supply with parts I still had in my hobby room. I created it in one weekend!
But not fully working.. whats a shame.

Pin 6 from U3 has a 33 volt potential. When I set pin 3 of U3 to 1V. as reference, what must pin 6 and pin 2 become? which values? My LED won't light... I checked all components in currentmaes. circuit. (resistors, Q3...) they look fine.

 

[audioguru2]

Hi Smitjuh,
The current regulator, U3, is used as a comparator with overall feedback through the entire voltage regulator. Its reference voltage from the pot is referenced to the rectifier's common voltage and one side of R7. Its input pin 2 is connected to the project's output's 0V terminal which is the other side of R7. When load current flows in R7 then a voltage is developed across it and when its voltage exceeds the voltage from the pot then U3's output voltage reduces and reduces the project's output voltage just enough to regulate the current at the setting of the pot.

The voltage at the input of U2 has a max of only 11.2V, so in order for U3 to reduce this voltage through D9 then the output voltage of U3 must drop from its resting voltage of about 33V down to 10.5V or less which turns on Q3 to light the LED.

When you set the pot's voltage to 1V without a load, then a load current slightly more than 1/0.47= 2.13A must flow through the load and R7 for the current regulator to become active.