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filtered linear ps take2

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Original thread https://www.eevblog.com/forum/beginners/noisy-linear-ps-design/25/

Attached is take 2 of the design. I'm still trying to use 12v transformer. The design I have from the previous thread works well with 18v transformer, but drops voltage on 12v transformer.

This time around its not noisy at all. The problem I'm having is that output voltage changes both from input voltage, as well as load. Non of this happens on the simulator. When potentiometer is properly adjusted it works with 12v transformer too.

It's likely to be oscillating. Scope the output of the opamp.
Try a 100 ohm resistor between B-E of Q3. If that doesn't fix it, then frequency compensation will need to be added to the opamp.

That will be still orders of magnitude worse than a LM317.
Glueing together internet circuits and guessing what will happen is not the way.
Also the amp you showed looked terrible.
If an amp causes hummmm because there's 20mV ripple in the power supply, it's a simply a bad design.

So the issue was TL431 is giving input-dependent voltage, instead of fixed (based on U6/R1 only). And I am not sure why, since the application seem to be by the datasheet. Adding small capacitor around its anode\cathode does not change anything.
Replacing with say 7812 addresses the issue, but the voltage drop is still too high to use 12v transformer :(

If it were to work properly, the output voltage should be within a few millivolts of U5's Cathode voltage.


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