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Quiet capacitors

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sveinb:
I tried putting a 5 ohm resistor in series with the usb power. That didn't really help. I guess the problem is that most of the ripple is created by my own circuit, and is not coming from the outside. (Edit: maybe some of you are suggesting that I put the resistor between the capacitor and the voltage regulators, but I'm nervous about the stability of those regulators in that case)

jonpaul:
check for SMPS unstsable/sub cycle oscillation

j

EPAIII:
Just send me your oversized ones and I will run them through my hydraulic press. I guarantee they will fit.

Zero999:

--- Quote from: sveinb on February 05, 2024, 10:49:20 am --- (Edit: maybe some of you are suggesting that I put the resistor between the capacitor and the voltage regulators, but I'm nervous about the stability of those regulators in that case)

--- End quote ---
It's possible the ESR of the capacitors is too low, which is resulting in instability. Look up low-dropout regulator capacitor ESR stability, using a search engine. It's a very common and well-known problem, which needs to be ruled out.

magic:

--- Quote from: sveinb on February 05, 2024, 10:49:20 am ---I tried putting a 5 ohm resistor in series with the usb power. That didn't really help. I guess the problem is that most of the ripple is created by my own circuit, and is not coming from the outside.

--- End quote ---
Yes, sorry for suggesting this, it was nonsense :palm:
It would only help if the ripple were coming from other devices on the bus.

It is also unlikely that an oscillating regulator produces 8kHz, they usually oscillate faster. Your USB microframe theory is more plausible.
Besides, it's not that hard to find LDOs stable with any reasonable ESR.

If there are regulators on board, their output capacitors shouldn't see much ripple and produce much noise, so the problem is the input capacitor.

Since 100μF tant was still not good enough, maybe try a 10μF one rated for higher voltage. It may or may not be quieter. Lower capacity will not violate the USB spec. Increased voltage rating may increase its reliability. Pay particular attention to a transient voltage peak which occurs after the device is connected and all capacitors have just charged up but cable inductance keeps pushing more current into them. This can destroy underrated voltage regulators or capacitors.

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