0-30 Vdc Stabilized Power Supply

[audioguru2]

Also that the LF411 can be used as a substitute for the TL081.

But the maximum supply voltage (only 36V) for the LF411 (and for the TL081) is too low for this project. Using an LF411A with a maximum supply voltage of 44V will be better.

 

[Kevin Weddle]

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?

Serial communications between external devices often have these same sort of grounding problems which are overlooked mainly because the data is transfered reliably. Your better off using an opto-isolator.

 

[vjedlicka2]

I think it was discussed here how to protect the power supply from destroying when you switch it off and forget to disconnect the load first. I cannot find the modified schematics; can someone please provide a link?

Thank you
Vaclav

 

[audioguru2]

I think it was discussed here how to protect the power supply from destroying when you switch it off and forget to disconnect the load first.

I don't think anybody said that before. How can it be destroyed when it is switched off?

I cannot find the modified schematics; can someone please provide a link?

The modified schematic has been posted on this website a few hundred times.
It uses a transformer with a high enough voltage so the circuit can produce 30VDC at 3A. The original circuit could not. Its new transformer spec is not overloaded like the original one was.

It uses modern opamps that have a high enough maximum supply voltage rating. The original opamps were over-voltaged even with the transformer that had a voltage too low.

The new opamps have input and output voltages that can go to ground so the negative supply voltage is not as high.

View attachment 41803

View attachment 41804

 

[vjedlicka2]

OK, maybe I did not understand (or do not remember) the purpose of the change. At the time I finished this power supply (2006), someone posted this diagram (attached) with the red diode. Can you please comment if it is of any use?

Thank you
Vaclav

View attachment 41805

 

[audioguru2]

OK, maybe I did not understand (or do not remember) the purpose of the change.

Many of the parts in the original project got too hot and/or were over-voltaged. Then it was not reliable.
It could not produce 30VDC at 3A,  instead it produced about 25VDC at 3A with lots of ripple.

someone posted this diagram (attached) with the red diode. Can you please comment if it is of any use?

Maybe somebody used this project to charge a battery and the diode prevents the battery from blowing up the transistors when the power is turned off.

 

[vjedlicka2]

Thanks!
Vaclav

 

[seb1982]

Hi everyone!

Wow, so I actually managed to make it to the end of the thread!  Incredible work by Audioguru, Mixos and others on here!

Just a very quick, simple query if anyone's able to help me - if I only needed a 2A output, would I need to alter any other components than the transformer to achieve it, as long as the circuit still had a 28-30v input?

What VAC rating transformer do you think I should get to achieve a 2A output?

I'm guessing that I wouldn't need such huge heatsinks on the 2N3055s at 2A either.

Thanks ever so much.

 

[mendimano1]

Hi seb,
If you have carefully readed the thread then you will have answers to all of your questions

 

[audioguru2]

If I only needed a 2A output, would I need to alter any other components than the transformer to achieve it, as long as the circuit still had a 28-30v input?

Use a transformer rated for 2.8A AC output.
Change R7 to 0.68 ohms/5W.

What VAC rating transformer do you think I should get to achieve a 2A output?

2.8A x 28V= 78.4VA.

I'm guessing that I wouldn't need such huge heatsinks on the 2N3055s at 2A either.

The power in each 2N3055 when the output voltage is low or shorted and the current is 2A, is 38W.

 

[seb1982]

Thank you so much, audioguru - you are a scholar and a gentleman!

I really appreciate your help.

 

[xristost]

He must be using liquid nitrogen and a huge fan to cool his single 2N3055 transistor.

There is not need for nitrogen neither for huge fan. It's a laboratory power supply which in my book is used for testing some projects, usually for short period of time. Dissipating power will be problem if power supply is used for extended period with low output voltage and high current . When I started the project I had already the enclosures so I had to take their sizes into consideration. There is a small fan which is switched on when temperature rises.

To mjvision : there is no need for emitter resistor if there is only one transistor.

P.S. Soon there will be improved version of the schematic in my blog. Stay tuned

P.P.S. The improved schematic: http://diyfan.blogspot.com/2013/03/adjustable-lab-power-supply-take-two.html

 

[audioguru2]

Your V3 design is very nice.

I noticed that on your V2 design the output of opamp IC1A cannot go low (because it is missing a negative supply) to regulate the current and things might burn up.

The old TL082 opamp has the "Opamp Phase Inversion" problem: If an input voltage is too close to the negative supply voltage (ground in your circuit) then the output goes high.
An input voltage is too close to the negative supply (ground) when it is lower than the common mode voltage limit of +3V to +4V in your circuit. But both inputs of opamp IC1A are always from 0V to +1V.

That is why transistor Q1 shorts the output of IC1B in your circuit when the negative supply reduces when the power is turned off. It prevents the output voltage from suddenly going up to maximum.   

View attachment 41846

 

[xristost]

Your V3 design is very nice.

I noticed that on your V2 design the output of opamp IC1A cannot go low (because it is missing a negative supply) to regulate the current and things might burn up.

The old TL082 opamp has the "Opamp Phase Inversion" problem: If an input voltage is too close to the negative supply voltage (ground in your circuit) then the output goes high.
An input voltage is too close to the negative supply (ground) when it is lower than the common mode voltage limit of +3V to +4V in your circuit. But both inputs of opamp IC1A are always from 0V to +1V.

That is why transistor Q1 shorts the output of IC1B in your circuit when the negative supply reduces when the power is turned off. It prevents the output voltage from suddenly going up to maximum.    

You are not quite right about IC1A - with IC1B they share common power supply pins and so there is negative voltage -3V.
And as I said in my blog both versions are tested and they work very well - I can regulate current limit almost to 5 - 10mA, below that is not possible because of input offset of the opamp.

 

[audioguru2]

You are not quite right about IC1A - with IC1B they share common power supply pins and so there is negative voltage -3V.

Sorry, I got mixed up about which opamp belongs to which opamp of the duals since all the other circuits used single opamps so they stay cooler.

And as I said in my blog both versions are tested and they work very well - I can regulate current limit almost to 5 - 10mA, below that is not possible because of input offset of the opamp.

Your circuit has a -3V negative supply and your opamps worked. But the datasheet for the old TL082 shows an input common-mode limit of 4V max, not 3V. So some opamps WILL FAIL!

That is why I selected common inexpensive ordinary opamps from two manufacturers that do not have the Phase Inversion problem, that work perfectly when their input voltages are at the negative supply voltage and have a max allowed supply of 44V.

 

[xristost]

Your circuit has a -3V negative supply and your opamps worked. But the datasheet for the old TL082 shows an input common-mode limit of 4V max, not 3V. So some opamps WILL FAIL!

That is why I selected common inexpensive ordinary opamps from two manufacturers that do not have the Phase Inversion problem, that work perfectly when their input voltages are at the negative supply voltage and have a max allowed supply of 44V.

I didn't find anything like that in the datasheet.
There is negative input voltage limit but in this schematic input voltages of IC1A NEVER go below zero, so I don't think there is such danger.
And if these opamps (TLE2141 and MC34071) are so common why there is so many question about replacing them with alternative opamps

 

[audioguru2]

I didn't find anything like that in the datasheet.

The datasheet for the OLD TL08x opamps lists the specs with a plus and minus 15V supply. The minimum Common-Mode Input Voltage Range (where its inputs work properly) is -11V. Its typical is -12V. So some opamps fail when an input is 4V above the negative supply voltage and MOST fail when it is 3V above the negative supply.
When the input voltage is 3V or more ABOVE the negative supply voltage then the output of the opamp will suddenly go as high as it can!

There is negative input voltage limit but in this schematic input voltages of IC1A NEVER go below zero, so I don't think there is such danger.

The maximum allowed maximum input voltage is not what I am talking about.

You have this OLD opamp with an input voltage THE SAME as the negative supply voltage or at 0.99V max above the negative supply voltage which is bad fort these OLD opamps.
 

And if these opamps (TLE2141 and MC34071) are so common why there is so many question about replacing them with alternative opamps

Are you in Russia? I am in Canada where ALL American and European ICs are available everywhere.
TLE2141 is made by Texas Instruments and MC34071 is made by Motorola/Freescale. They are HUGE companies.

Some students want to use the CHEAPEST and OLDEST opamps they can find at their school (741 opamps).View attachment 41847

 

[Kevin Weddle]

I'm looking at R4 in the circuit and it looks like the zener diode D8 at just over 1ma. Is that the feedback ratio of U1? RV1 seems to determine the output voltage. Adusting the offset to produce the desired voltage isn't familiar. Can anyone help explain this?

 

[audioguru2]

I'm looking at R4 in the circuit and it looks like the zener diode D8 at just over 1ma. Is that the feedback ratio of U1? RV1 seems to determine the output voltage. Adusting the offset to produce the desired voltage isn't familiar. Can anyone help explain this?

The BZX79C5V6 zener diode that I recommend for D8 is 5.6V when its current is near 5mA.
R4 also has 5.6V across it so its current and the zener's current are 5.6mA.

RV1 is the trimpot to adjust the output offset voltage. It varies the output voltage plus and minus a small amount. When the voltage setting pot is set to zero then RV1 is adjusted so that the output is exactly 0V.

 

[Kevin Weddle]

Opamp U1 is a very good circuit! I think it's absolutely great. Tell us what you think about this!