10 Amps power supply with LM723

[mike_mike]

I know that it works.
But my circuit does not, and I am not capable of finding were is the problem...
Probably I could find someone in my city for help.

[iMo]

Be careful with grounds.
There are 2 "grounds" - in my simulation the left hand side of the R_Shunt is COM and the right hand side is GND. I often connect the input filter capacitor to GND (wrong).
Doublecheck how did you wire the stuff.
When the simulation works it should in HW too..

Enclosed the simulation source - try it and compare it carefully with your kludge

[mike_mike]

Thank you for your help. I will try to find the problem.
Meanwhile, a quick question: does the attached schematic have current limit down to 0A ?

[Wolfgang]

... in fact yes, but with low precision. The 723 is not very well suited to lab grade current limiting.
An obvious problem here is the Vbe tempco of the pass transistors when running under high power.

[mike_mike]

I finally managed to make the schematic from post #23 to work, but I found that the current limiting pot does not regulate linear the output current (I don't know how to explain exactly), so if I set the pot at half of its way, then the output current is about 0.5A, while the maximum current is 3A. The pot is a linear one, B500R.
If the pot is at half, then the output current should be 1.5A ?
Is that true ?

[iMo]

1. What was the problem, could you tell us?
2. See my previous post with 500ohm pot's wiper value (measured from its low side) vs. current limit. In the mid position it shows 1A limit. It is a simulation only..

[mike_mike]

The problem was the PNP transistor. There was a bridge between the C and E of this transistor, and I did not saw it when I made the inspection...

Later Edit: If I use 2 220R resistor in series instead of the 500R pot, then the output current is about 1A. The connection between the 2 200R resistor is connected to the point were the pot wiper was connected before.
I do not have any other 500R pot or trimmer to check if the first one was faulty, can there be used 1K pot instead of 500R pot ?


I checked again with 1k pot for current limit, and at the half of the pot, the current is 0A. Only on the other half the current starts to rise from 0A to 3A.

[iMo]

In case of 1k pot you have to increase the 4k7 resistor.
I would put, say, a 22k trimmer instead of the 4k7, set the 1k pot's wiper to the top position and set the minimal current limit with the 22k trimmer. Replace the 22k trimmer with a close valued resistor finally.

For example a simulation with 10k the top resistor, 1k pot, 120ohm bottom, AND a 10k resistor from pot's wiper to gnd
POT's wiper       CLimit
top                   23mA
mid                  1.8A
down                3.8A

[mike_mike]

I checked using a 22k trimmer, the 1k pot, and I found that the proper value is around 12.7K for the trimmer.
The current limit starts from 0A and goes up to 3.31A.
I found that when I rotate the pot, it starts from 0A, then if I rotate it a very little then the current suddenly increase to more than 200mA, and there is a very little "space" on the pot for 1mA or 5mA. Is that normal ?

Later Edit: only now I have saw your edit.

[iMo]

The pot setting is highly none-linear. It follows the Vbe characteristics.
The 10k (or similar) from wiper to gnd could make it "more linear"..

PS: or use 1k "logarithmic" potentiometer.

[mike_mike]

For example a simulation with 10k the top resistor, 1k pot, 120ohm bottom, AND a 10k resistor from pot's wiper to gnd
POT's wiper       CLimit
top                   23mA
mid                  1.8A
down                3.8A

I used the modification specified above and it works a little bit better, but at mid, the output current is about 1.20A, instead of 1.8A, and the maximum output current is 3.06A. The current starts to rise from only at about 1/4 of the pot course.
For the tests I used only one TIP3055 and only one 4700uF capacitor.v Also, for the tests I did not used D1 and D2.

Is the Rwiper connected correctly ?
Please have a look at the schematic and at the results above and tell me what you think.

[iMo]

I would play with those 3 caps see below.
Also I would make the Voltage setting at the Vref side around the +IN (fixed divider at the output).

[mike_mike]

The role of those caps is to filter the voltage, for example the 10uF polarized capacitors is to filter the the voltage reference for voltage regulation and current regulation ?
The results of the output current pot are good ?
What are the advantages of using the pot at the Vred side ?

[iMo]

Yes.
Yes.

The output divider values contribute to the feedback control loop dynamic parameters.
When changing them the feedback response is changing.
Therefore it is better to have the Voltage regulation at the IN+ side.

For example with fixed 9k/1k output divider (/10) the output voltage will be aprox. 10*V(IN+).

[mike_mike]

I tested the attached schematic, but the minimum output voltage is about 9V, and if I reduce the R3 to 0 ohms, the minimum output voltage is about 7V.
I also tested with R12=8k2, R10=2k2, R1=0R and R3=680, and the minimum output voltage vas 3.5V and the maximum was about 29.5V. The voltage pot has a part where if the voltage is at maximum (29.5V) then the output voltage does not go down until the pot is at 1/4 of the course. I found those values on a schematic on the internet: http://www.electroniq.net/power-supply/lm723-variable-power-supply.html

[iMo]

You wired the C9 10u capacitor at wrong place.
Look at my schematics above.

Try with R1=12k, R2=10k pot, R3=0 and with the divider by 10 at the output.
Wire C5=10u from the bottom of R1 against gnd of 723.

[mike_mike]

I tested the attached schematic but it goes only from about 7V to about 30V.
The voltage on pin 5 is 0V and on pin 4 is 0.74V, while on the output is about 7V.

[mike_mike]

I also tested with 4k7 and 3k9 resistor for R12. The value for R1 was 1k2.
The output voltages were:
with 3k9: from 3.7V to 29.8V, but when the output voltage was 3.7V, and if I started to rotate the pot, the voltage started to rise only when the pot was at 1/4 of the course.

[mike_mike]

I tested with the following values, and the output voltage varied from 3.1V to 25.9V. The maximum that I need is 25V, so 25.9V is OK. The lowest output voltage is also OK.
R1=1.2K, R2=10K pot, R3=680R, R12=6K8 and R10=2K2.
There is a problem that still persists, the 10K pot has a "dead" portion on the low voltage. When the output voltage is abut 3.1V, the voltage starts to rise only when the pot is at about 1/4 of its course.

[mike_mike]

I try to make a simulation of the attached schematic in LTSpice, but the program does not show the .raw file when I run the simulation.
At the beginning I set to DC op pnt, but after I run the simulation, I set it to transient.
Can you please tell me where I am wrong ?

[iMo]

GND is missing in your schematics.

[mike_mike]

Thank you. Now the simulation works.
Regarding the previous posts, are the results good ? Are there any problems ? I mentioned that the voltage pot has a "dead" portion, can it be solved ? It is not such a big problem, if the power supply works correctly...

[iMo]

It is many decades I messed with 723 PSUs. Afaik the minimum output voltage is 2V with 723.

[Wolfgang]

Correct. Below that the 723 inernal error amp does not work properly anymore. Below the emitters there is a current source, and its collector voltage is the error amp input voltage minus Vbe, so when you have 2V at the inputs, its about 1.4V Thats already not too much, so below that the current source is simply not working with a good and high output resistance.

[mike_mike]

Hello,
I have found the attached power supply schematic.
Can you please help me with part numbers for replacement transistors TR3-TR6 (A823) and TR2 (3223) ?
The replacement transistors are not really necessary to be pin to pin compatible, because I can redesign the pcb layout.
I would also like to know the voltage for ZD3, the part number for D4 (1N4148 ?) and the secondary voltage of the transformer.