Question about power supplies

A

Abbie

Guest
Hello all,
I have some question about a 15V 3A variable voltage/current
power supply circuit. Firstly, the op-amps in the circuit have
a negative feedback capacitor. Does this mean, since power
supplies are dc, that these op-amps will actually be comparators
and not amplifiers. The other question is this: There are two
different power sources, one is the main one, which is to be
regulated, and the other is from another set of windings on the
same transformer, which are regulated with zener diodes and
a fixed voltage regulator, and are used to power up the op
amps and the controlling circuit. The funny thing is that the GND
of this other source seems to be connected to the plus of the
regulated output of the supply, does this sound o.k. ?
 
Abbie wrote:
Hello all,
I have some question about a 15V 3A variable voltage/current
power supply circuit. Firstly, the op-amps in the circuit have
a negative feedback capacitor. Does this mean, since power
supplies are dc, that these op-amps will actually be comparators
and not amplifiers.
Click to expand...
No. A capacitor in negative feedback makes the opamp act as an
integrator. That is, an amplifier that has lower gain at higher
frequencies (and very high gain at DC. This is a very common circuit
block in feedback controls (search PID controllers). If this opamp is
being used as the integrator function in a negative feedback loop, it
provides more accuracy if you are willing to wait longer (amplifies
the error voltage more and more as time passes, forcing the error to
eventually approach zero. Eventually, here, may be measured in
microseconds, milliseconds or seconds, depending on the size of the
capacitor.

The other question is this: There are two
different power sources, one is the main one, which is to be
regulated, and the other is from another set of windings on the
same transformer, which are regulated with zener diodes and
a fixed voltage regulator, and are used to power up the op
amps and the controlling circuit. The funny thing is that the GND
of this other source seems to be connected to the plus of the
regulated output of the supply, does this sound o.k. ?
Click to expand...
This is not at all unusual. It all depends on which side of the
supply the regulating device in series with, and which side has the
current sense resistor. The output of the supply is not a positive
voltage or a negative voltage as far as the control circuit is
concerned, but a pair of terminals with a voltage between them.
--
John Popelish
 
Here it is, see if it makes any sense:

http://wwwwww.users.btopenworld.com/UNTITLED.gif





"John Popelish" <jpopelish@rica.net> wrote in message news:3FBC30CC.2E0184E9@rica.net...
Abbie wrote:

Hello all,
I have some question about a 15V 3A variable voltage/current
power supply circuit. Firstly, the op-amps in the circuit have
a negative feedback capacitor. Does this mean, since power
supplies are dc, that these op-amps will actually be comparators
and not amplifiers.

No. A capacitor in negative feedback makes the opamp act as an
integrator. That is, an amplifier that has lower gain at higher
frequencies (and very high gain at DC. This is a very common circuit
block in feedback controls (search PID controllers). If this opamp is
being used as the integrator function in a negative feedback loop, it
provides more accuracy if you are willing to wait longer (amplifies
the error voltage more and more as time passes, forcing the error to
eventually approach zero. Eventually, here, may be measured in
microseconds, milliseconds or seconds, depending on the size of the
capacitor.

The other question is this: There are two
different power sources, one is the main one, which is to be
regulated, and the other is from another set of windings on the
same transformer, which are regulated with zener diodes and
a fixed voltage regulator, and are used to power up the op
amps and the controlling circuit. The funny thing is that the GND
of this other source seems to be connected to the plus of the
regulated output of the supply, does this sound o.k. ?

This is not at all unusual. It all depends on which side of the
supply the regulating device in series with, and which side has the
current sense resistor. The output of the supply is not a positive
voltage or a negative voltage as far as the control circuit is
concerned, but a pair of terminals with a voltage between them.
--
John Popelish
Click to expand...
 
Abbie wrote:
Here it is, see if it makes any sense:

http://wwwwww.users.btopenworld.com/UNTITLED.gif
It makes some sense, but I think there are errors in the schematic.
Click to expand...
R11,12,13,14,15 do not make sense as part of a current limit function
and I suspect a ground connection (the common of the +-12 volt
supplies) is missing on R1,2. There has to be a return path to the
+-12 supplies for the base current they deliver to Q1 and 2.

--
John Popelish
 
I have made a few mistakes which I hope I have corrected,
please find the diagram at

http://wwwwww.users.btopenworld.com/UNTITLED3.gif

Again, all I want to know is if it looks o.k. , and if so how
exactly is the voltage or current controlled. I have omitted
several capacitors because they just clutter the whole thing
and I don't think they affect any analysis.

Please answer even if you are not sure.

"John Popelish" <jpopelish@rica.net> wrote in message news:3FBD764F.2CFB22E9@rica.net...
Abbie wrote:

Here it is, see if it makes any sense:

http://wwwwww.users.btopenworld.com/UNTITLED.gif
It makes some sense, but I think there are errors in the schematic.
R11,12,13,14,15 do not make sense as part of a current limit function
and I suspect a ground connection (the common of the +-12 volt
supplies) is missing on R1,2. There has to be a return path to the
+-12 supplies for the base current they deliver to Q1 and 2.

--
John Popelish
Click to expand...
 
Abbie wrote:
I have made a few mistakes which I hope I have corrected,
please find the diagram at

http://wwwwww.users.btopenworld.com/UNTITLED3.gif

Again, all I want to know is if it looks o.k. , and if so how
exactly is the voltage or current controlled. I have omitted
several capacitors because they just clutter the whole thing
and I don't think they affect any analysis.

Please answer even if you are not sure.
Click to expand...
This is quite different that the first version. How sure are you that
this is now correct? I hate to waste a lot of time trying to
understand how a mistaken schematic functions. Did you discover a
connection between OPRED and the common of the +- internal supplies,
or did you add this because I said it ought to be there, somewhere?

By the way, any feedback capacitors around the opamps may not bee
needed to figure out the control concept, but they are essential in
understanding the stability of the control loop.
--
John Popelish
 
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