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LM317AHVT and Heatsink

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mike_mike:
Should I test again the modified schematic with the oscilloscope ?
The only difference between the tested and the attached (current) schematic are the 3x8.2K parallel resistors (which was initially only one 3.3K resistor) and the 100R resistor (which was initially 122R resistor).

And I am facing another problem. If I set the output voltage to 26.4V when I connect a 9Amps load, the voltage drops by 0.5V, reaching 25.9V. I did not find this issue last year when I tested this power supply. Why this voltage drop appears ? What should I do to correct this voltage drop ?


If I set the output voltage to 19.53V and I connect a 4.74A load, then the voltage drops to 19.27V. I did not found this behavior of the power supply at previous versions.

I re-tested using an older PCB and I obatined 25.9V with 7A load, and 26.0V without load. So, probably the PCB is the problem.

David Hess:

--- Quote from: mike_mike on October 19, 2018, 11:22:33 am ---Should I test again the modified schematic with the oscilloscope ?
The only difference between the tested and the attached (current) schematic are the 3x8.2K parallel resistors (which was initially only one 3.3K resistor) and the 100R resistor (which was initially 122R resistor).
--- End quote ---

That change should not matter.


--- Quote ---And I am facing another problem. If I set the output voltage to 26.4V when I connect a 9Amps load, the voltage drops by 0.5V, reaching 25.9V. I did not find this issue last year when I tested this power supply. Why this voltage drop appears ? What should I do to correct this voltage drop ?

If I set the output voltage to 19.53V and I connect a 4.74A load, then the voltage drops to 19.27V. I did not found this behavior of the power supply at previous versions.
--- End quote ---

It looks like you have a high enough minimum load on the output.  Make sure that you are not falling below the dropout voltage of the combined regulator and transistor and make sure there is not a parasitic oscillation.

C3 should be located at J2 and not near U1.  R3 should not be required if R6 is low enough to provide the entire minimum load.


--- Quote ---I re-tested using an older PCB and I obatined 25.9V with 7A load, and 26.0V without load. So, probably the PCB is the problem.
--- End quote ---

It could also be a layout problem.  The lower part of the divider should terminate at a single point near J2 but the top of the divider should terminate at a single point close to U1 for best load regulation.

mike_mike:
What is the difference between A and B from the attached schematic, besides the direction I need to rotate in order to increase or decrease voltage ? Is there any difference between A and B ?

David Hess:
There is no difference.

mike_mike:
Thank you for the reply David Hess.
I tested the power supply and the results are (the schematic is the one from reply #27, with C4=2200uF/63V instead of 1000uF/63V)
without load V=19.80V, with 6.17A load V=19.79V
without load V=26.0V, with 7.24A load V=26.0V

I don't know if the wire that come from the pin 1 or another pin of LM317 to the PCB was making good connection with the screw terminal located on the PCB.
If there was a bad connection, could the power supply work good and if I repair the bad connection by tightening the screw could the power supply to work bad ?

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