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uSupply Problems
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redder558:
For a while now, I have been wanting to build my own bench PSU. Recently, I have come across Dave's μSupply, but I have seen posts that say that there are some major flaws with it. I want to know what these problems are and if they are present in the pot-controlled design shown in Part 3 (image is attached).

Also, what does Dave mean when he puts '4x10k matching' on the schematic?

If the problems are major ones, can you also recommend me a better discrete (because they are better according to https://www.eevblog.com/forum/chat/power-supply-using-discrete-components-vs-regulator-ic/) PSU schematic?
daubmaso:
Hi, I believe most of the issues with this iteration lie with the current limit function of the μSupply. It can cause oscillations that don't exactly create a very clean output when the current limit is triggered. What fixed it for me is changing the value of R2 on the schematic (near pin 2 of the lt3080) from 1k to 100R.

By 4x10k matched he means that the 10k resistors should be hand picked and measured to make sure each value is identical across all resistors. This is so that the differential amplifier for the current shunt resistor has as close to a 1:1 gain as possible to ensure an accurate current limit.

If you haven't made any power supplies before, I would probably recommend to just use an lm317 without any current limit. You can make a nice little supply with it and a cheap panel volt/ammeter from ebay, for a very small amount of effort. If you do choose this route I would recommend at least a fuse on the input in lieu of any current limiting
redder558:
Thanks for the very simple explanation! Also, on your suggestion about the LM317, I was thinking about building something similar (with a few additions) but I kept reading that current limiting was invaluable to many people.

I just have two small questions, can I simply replace the lt3080 with the lt3083 without any additions? Also, if I increase the resistance of the voltage / current pot, then will the max theoretical output also be higher?
daubmaso:
Current limit would certainly be a very nice feature to have, but when it comes down to it just having a variable bench supply at all is incredibly handy.

If you still want the current limit function to work there are 2 methods for doing this. You can either add an additional 3v reference and use that as the input to the current limit pot, or you can replace the 2.5v reference with a 3v one, feed the current limit pot directly from that and modify the gain of the voltage control op amp (for example changing the 12k resistor to 8k would give roughly the same output range as the original schematic).
But before you modify anything in this circuit I strongly recommend you watch the μSupply videos very carefully, Dave does a great job explaining every aspect of the supply and how the control works.

Here is my version of an lt3083 supply, it was meant to be controlled by a 3.3V PWM signal, but can easily be replaced by a potentiometer.
daubmaso:
Sorry I should mention that the file I attached is a LTSPICE simulation in case you're not familiar with it's file extension. So here is the schematic all by itself. Please excuse how messy it is, I threw it together sometime last summer and completely forgot about it until now.
V1 is the set voltage for the current limit, and V2 is to set the output voltage of the regulator.

If you have any questions about the functionality of this schematic just let me know, I'd be happy to help.
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