Hi
Here is the last schematic with adjustments I did after some measurements and thinking about the schematic.
I had already explained in the beginning that one of the characteristics I like to see in a power supply is good DC stability and few interference signals.
These interference signals can be noise from the LM723 itself and the extra -5V rail, but also interference signals from the 230V grid here in Amsterdam.
The tests I did this week showed that the sensitivity to voltage variations due to the 230V mains voltage variations and the load variations at the output of the power supply, I found the variations too large.
This means that I will have to use more parts than I had intended.
But the parts used are just like the LM723 "Old School" and I allow myself to use these parts.
What I have done now is not so much different than that you see more in well designed power supplies, the LM723 itself is powered by "clean" power lines.
The -5V power supply line.I have done extensive tests on the LM337 to see how good the hum suppression is and what the sensitivity is for variation of the DC level at the input of the LM337.
The output of the LM337 which is about -5V is in series with the reference voltage of the LM723, so you see every deviation of this -5V as variation in the output voltage.
After some tests with zener diodes I switched to a LM7912, it costs almost nothing and has a much better performance than a resistor and zener diode.
Now there is no hum or DC variation at the output of the LM337 that is present at the input of the LM7912.
The ripple on C3 is suppressed by more than 120dB if you do the wiring properly.
For a power supply with good properties, the wiring technique is important!
If you look at the diagram below, you can see that the ground (which is on J2 output bus) is split into multiple connections to this node.
There is a thick horizontal line in the diagram to J2 connection, which is the return connection of the energy coming from the transformer and the rectifiers and capacitors.
The same DC level of the -5V power rail (The GND connection of the LM7912 and this has a thick green line) must be connected directly to the J2 socket, as I drew in the schematic.
A third important connection to J2 is the resistor R21 in the diagram below, this is the negative "sense" connection.
Ignoring this wiring technique means that you end up with a worse power supply than is possible with the used circuit.
+23.5V power railAlso the DC level on the Vs and the Vc connections of the LM723 are sensitive to variation.
When loading the power supply from 100mA to 1.9-Ampere, the voltage at C8 can drop 1V. (This is due to the Ri of the transformer)
This drop in the voltage at C8 is then seen as an extra drop in the output voltage, making the DC Ri lower than necessary.
The level of 23.5V will probably still change, as it depends on the Power output section.
I'm going to make two connections on my test print as shown in the schematic, the normal output which is pin-10 and pin-13 which is the direct connection of the difference amplifier in the LM723.
This direct connection goes beyond the Darlington in the LM723 IC.
I hope that with my high gain of the Sziklai pair I will have both a low DC Ri and a better phase margin, but the test wil show if this use of pin-13 is usefull.
The DC level of 23.5V therefore depends on the Power stage configuration and whether it is possible with the used transformer to reach 20V at 2-Ampere,
if it becomes 1.8-Ampere at a low mains voltage, then I will settle for that, no problemo.
Because this power supply is for fun and teaching.
I have chosen for the 23.5V regulator to use a TL431 shunt controller.
The measurements I did on my test setup, which supplies the voltage to LM723 and the current that supplies the reference output of the LM723 to P-1 and R11 is between 5 and 6mA.
This is not the only current, also the current source made with Q3 which is about 1mA together with the current through the red LED are about 2mA total,
which should be added to the current consumed by the LM723, in total this is about 8 to 9mA.
C8 is now also enlarged to 2200uF to make the ripple as small as possible, this gives some more space for the TL431 controller.
later I can replace D3 with a Schottky version for about 300mV extra margin.
How to calculate the resistors arroud the TL431b?
I use a TI speadsheet which you can download via the link below.
www.bramcam.nl/NA/NA-723-PSU/TL431-Calculator.xlsmResistor R9 and R10 are setting the voltage voor de LM723 IC.
R6 determines the current that can be drawn that still falls within the control range of the TL431b.
Don't forget that you don't exceed the maximum dissipation of the TL431b!
I'm going to test the resistor R6 which is now listed as 120 Ohm in the schematic with the power supply connected to a 2-Ampere load
and 20V output voltage, to see if it still works at 220V mains voltage, at the two Power section configurations.
LM723 output voltage and sensor linesTo keep the phase marging stable on the output voltage between "0" and +20V, I decided to test the regulating the +input of the LM723 after a remark of kleinstein.
The bottom side of the potentiometer P-1 has a DC level of "0V" compared to J2.
The reference voltage(pin-6 LM723) is also with respect to J2 about 2.1V.
Note, I am not saying that the reference output of the LM723 is now suddenly 2.1V instead of 7.1V!
This 2.1V is due to the reference shift of the -5V regulator relative to J2, this because pin-7 of the LM723 is connected to the -5V regulator.
The trim-1 potentiometer in the -5V regulator ensures that the output voltage can be adjusted to "0V" when P-1 is turned all the way counterclockwise.
It is not that important that the -5V rail is exactly 5.0000V.
Noise, hum and stability are important here for this power supply rail.
It can also be -4.2V, -4.85V or even 3.765V or as kleinstein suggested 2.5V.
2.5V I found a bit on the low side by some testing I did at this low commonmode voltages for the inputs.
It's no problem to sit in the "sweet spot" of the input commonmode range for the best performance.
By using two regulated supply lines in this diagram, the LM723 can never get too high a supply voltage again.
As shown in the diagram, P-1 controls between 0V and +2.1V, this value will have to be amplified by the differential amplifier in the LM723.
As with a normal opamp, the gain is controlled by two resistors connected to the -input.
In the schematic these are R17 of 10K and R21 of 1K, I have a small trim range applied so I can set the maximum output voltage, this is done with Trim-2 potentiometer.
A few remarks about the values of the resistors and potentiometers that are attached to the + and - inputs of the LM723.
With the chosen schematic it is not possible to precisely equalize the impedances.
The impedance at the -input is fixed but that of the +input depends on the position of the potmater P-1.
At the +input there is always a 1K resistor present that together with C12 of 4u7 form a low pass filter, witch filters the reference noise and when changing the output voltage.
At this 1K, in the worst case with the potentiometer on "0V" the parallel resistance of P-1 and R11 must be added up which is around 700 Ohm.
At this low value of impedances, the bias currents and noise are less important.
Schematic version 0.7ResumeSo what has changed in the schematic:
1e
The output voltage is now controlled by variation of the + input, phase marging remains stable over the entire output voltage range.
2e
The LM723 is powered by two clean power supplies that are much more stable and cleaner than the first setup.
This makes the final performance a lot better, I love to design clean power supplies, which is one of my abberations.
3e
That there is now the possibility to choose the output to which the power section can be connected with D7 and capacitor C16
which give a DC shift for the output commonmode without adding phase margin.
Still to be doneTesting of the Power section connected to the compentation connection.
Select the current limitation components well, it involves R20 and R24 and perhaps replace the current source version as proposed by kleinstein.
If the phase margin and dynamic behavior are good, the timing of the Power On/Off circuits should be further tuned so that no abberations occur.
And some more explanation why I made certain decisions for this power supply.
But, to say it again, this is a power supply for me, and not meant to be a building project.
And the second reason I started this topic is that I learn to express myself better.
This is mainly intended as a learning project for those who do not yet have much experience in building or designing power supplies.
Many who start with electronics want to build/design a power supply without sufficient electronic knowledge. (buy
The Art of Electronic)
I am no exception, I made many mistakes and have read very many hours in the first versions than The Art Of Electronic
Power supply are difficult circuits, and yes I know the Internet is full of "simple" schematics.
This almost always means that the "designer" himself does not know what he or she is working with at that moment.
A common mistake is that the person who is going to rebuild a schematic will replace parts without much thought because they have them in stock.
Please do this in you want to have a poorly functioning circuit.
For your information.This piece of text with the adjustment of the schematic took me almost 6 hours today.
Gaining knowledge and transferring it takes a lot of time and effort, if you want to learn you will have to make a real effort
and this effort for good learning can not only consist of just looking up something quickly via Google.
I see more and more that people want to know everything at the push of a button.
And I think, who are you fooling?
Like Louis Rossmann I want to say now, I hope you learned something from, I dit.
Kind regards,
Bram