0-30 Vdc Stabilized Power Supply

[subumann]

Thanks Audioguru !
But i was triyng to replace the ops with cheaper versions because, for a dual suply the costs of the entire project will be very close (or beyond) to a "store ready made" suply 0-30V/0-3A.
If i replace 081 with LM 301 (with compensator caps) will there be a problem ?
Or maybe LM318 ?

PS: 6*OPA...=6*14$=84$
      2* TRAFO=2*22$=44$
      the rest=~20$
TOTAL=~150$ - displays not included, heatsink not included, case not included...
And a suply with 2*LCD is 140-150$ !!!

 

[audioguru2]

If i replace 081 with LM 301 (with compensator caps) will there be a problem ?

The very old LM301A is 33 times slower than a newer TL081 so the circuit might oscillate. It also has a high input bias current.

Or maybe LM318 ?

It has a high input bias current. The TL081 and OPA445 have FET inputs.

PS: 6*OPA...=6*14$=84$

Where in the world are you? You are being ripped off! Digikey sell them for "only" $9.50US.
View attachment 40344

 

[subumann]

Did Romania tell you something ? 

So, how about a ( toroid ) 18V / 75VA trafo ?  ( 2*18V / 150VA for a dual power suply - i allready have this one ). I dont mind if the maximum voltage will be a little less then 24-30 V.

 

[audioguru2]

With an 18VAC transformer, the max regulated output voltage at 3A is about only 16.5VDC. If the max output current is 3A then a single transformer should be rated for 76VA so yours is fine.

 

[ensteinx]

How I can up to voltage to 50V in outpup and current up to 10A, thanks.

I am from Lithaunia and my English not very good...

 

[audioguru2]

How I can up to voltage to 50V in output and current up to 10A?

It would be a lot different from this circuit. It would have more than 500W of heating to be cooled.

 

[subumann]

Just some pictures.
In a few days tests and maybee case with meters and stuff...

View attachment 40354

View attachment 40355

 

[subumann]

One more.

View attachment 40356

 

[subumann]

Last one.

View attachment 40357

 

[ante1]

Hi subumann,

That looks just great! I have to ask; do you plan the layout for the board on paper or do you have some software to do the work?

Good work! 

 

[subumann]

Milimetric paper to be exact.
I do that for years.  ;D

 

[ante1]

Well, you are doing a nice and tidy layout the hard way then!  ;D

 

[subumann]

Doing it the hard way gives more satisfaction.  ;D

 

[subumann]

Now seriously, i start building electronic things back in 1984 ...+- 1 year...  The computer is a normal way to do it now but when i am in a hurry i prefer the paper. I'm more used to it.

 

[ante1]

I know what you mean; I made my first PCB in the early seventies.
Keep up the good work! ;D

 

[kcc]

Hi All:

I've been studying this schematic and reading the two primary threads about this power supply for a few days now (and may I say special kudos to Audioguru and Kain for their fortitude in producing the 5A version).

The circuit makes sense except for the current limiting function.  I'm hoping somebody can explain to me how the current limiter acts in a linear manner.

As I understand the project specs I can set the current limiter to a specified amount (e.g. 2 A).  If the load's resistance causes it to draw more than 2A, then the voltage is reduced so that the current is maintained at 2A. 

What I don't understand is how this linear effect is achieved.  It seems to me that U3 is the op amp (wired as a comparator since there is no feedback) used to kick in the current limiter when the voltage drop across R7 exceeds the the preset value assigned by the voltage divider of R18, P2, and R17 (11.2 Volts across 66.033 KOhms is not a lot of current).  When V of R7 is greater than the wiper of P2, the comparator output goes negative to just a volt or so above -5.6V (-4.6V).  This will forward bias D9 and make the voltage on the non-inverting input of U2 about -3.9V

This is a low voltage which I'm surprised doesn't turn off the output completely but in any case I do not see how the voltage into U2 could be anything other than this value when current limiting is on.  The output of U3 is near the negative rail (as expected from an comparator) and there is a fixed voltage drop across D9 resulting in a fixed value into U2.
There doesn't seem to be any components that can vary their voltage or current to reduce the bias into the pass transistor just enough to maintain the 2A.

I've looked closely at the parallel return path thru R9, R8, P1, R7, and D7 but the resistance thru D9 and U3 is much lower than the other path and should dominate the value of the voltage into U2.

I do realize that this circuit has been built, does work and obviously my analysis is the problem here.  If somebody could explain to me the voltages around the current limiter when it is on I think I'll understand the circuit completely.  I'm pretty sure that R9 and R8 will play into this somehow but if their purpose can be highlighted I'd be very grateful.

Thanks in advance for the help!!!!!

Kevin

 

[audioguru2]

Hi Kcc,
Welcome to our forum. ;D
U3 operates as a linear amplifier and does not switch like a comparator.

Set the current for 1.0A and the voltage to 25V or 30V. Put on a 20 ohm load then the output voltage drops to exactly 20V, not zero.
Put on a 10 ohm load and the output voltage drops to exactly 10V, not zero.
Remove the load and the voltage goes back up and U3 doesn't regulate the current anymore.

 

[kcc]

Good to be here.  I see some very interesting projects!

How is it that U3 acts as a linear amplifier with no feedback?
Or am I simply missing the feedback loop?

Without feedback a difference of a few mV should force the output rail to rail (or as close as the op-amp can come).  Hence my thought that U3 is wired as a comparator.  Can you describe the feedback loop?  What is the gain of U3?

Thanks,

Kevin

 

[audioguru2]

R7 is the current-sense resistor. It feeds the inputs of U3 with the current-setting pot making an offset voltage.

When the voltage across R7 causes U3 to become active then its output begins to be reduced which causes it to reduce the output voltage until the current is at the set amount.

U3 has a voltage gain of about 200,000. The voltage amplifier has a gain of 3.07, so the total gain is 615,000. An increase of 1uA in R7 causes an increased voltage across it of 0.47uV which causes the output of U3 to drop 0.29V.

It is a little more complicated because of the interaction with the current-set pot.

 

[kcc]

Audioguru:

Sorry to continue on about this but I'm really trying hard to understand.

It sounds like what you are saying is that the circuit is actually looking to use the open
loop gain of the opamp of about 200,000 (110 dB) as per the spec sheet?

The implication is that the total offset voltage between the two inputs could be about
70 uV before the output would be driven to either rail.  But if you have a max of 3A
across R7 the potential difference is 1.41V or about 20,000 greater which should drive
the output to one rail or the other.

The current-set pot appears to be a simple voltage divider between 11.2V and 0V which allows the non-inverting input to vary from 5.6mV to 1.7V.  This is close to the range that R7 should drop (2mA to 3A should drop 940uV to 1.41) What interaction would there be with the output?

I'm not at all sure what I'm missing here.  I know the circuit works.  I just can't get my head around how the current limit is controlled.

Thanks for the help!!!

Kevin