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New Fnirsi DPS-150 mini lab power supply (30 V, 5 A, USB PD powered)

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DaneLaw:

--- Quote from: jwr on December 02, 2024, 10:05:21 pm ---Hmm, interesting… none of these other images show the behavior I'm seeing: negative voltage when the PSU is OFF. That's what I'm most concerned with, although a ripple of >750mV peak to peak at 5V output is pretty concerning, too. Perhaps I got a faulty unit after all?

(I tried with two different USB-PD power adapters, one from Anker and one from Ugreen, but that shouldn't matter — I might expect a little more HF noise with a crappy adapter, but not negative 6V)

--- End quote ---

Try with a clean battery source.
just to rule out, your DC source, as some of these fast charge handshake protocols, can get a little confused, but it obviously shouldn't to the extent your witnessing.

Aldo22:

--- Quote from: jwr on December 02, 2024, 10:05:21 pm ---Perhaps I got a faulty unit after all?

--- End quote ---

I tried it again with a 45W PD Charger.
I see the same thing as you with this one.



I probably hadn't noticed because it had a 50 Ohm load resistor.
The negative voltage disappears with the LR. It is about 0.13mA AC




Normally I use the linear power supply as the source for the DPS-150, when I work with the oscilloscope.
You don't see any major problems with this one either, just a small ripple that disappears after switching on.




I think your device is fine.

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