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I have a couple of questions related to a project based on the Raspberry Pico. I'm using the USB port of the Pico only to power it, not for communication.

  1. The Pico has 47uF of capacitance on the VSYS rail (which is about 4.7V, a diode drop away from the 5V USB rail). Doesn't this introduce a large inrush current when plugged into a USB port? Let's say the effective capacitance on VSYS of the Pico is 20uF. AFAIK USB ports have about ~120uF of output capacitance. So when I connect it to a USB port, the 120uF output capacitor will transfer energy to the 47uF input capacitor of the Pico, right? And that transfer will only be limited by the cable inductance?

  2. I want to add ~60 to ~100uF effective capacitance to the VSYS rail of the Pico. Does this necessitate a current limiter? With a current limiter the USB input would look like this: USB input -> Schottky diode -> 47uF capacitor -> Current Limiter -> Bulk Capacitance (100uF)

p.s. the reason why I want to add capacitance is, because it turned out that with a certain USB charger this pico would not work. Current drawn < 200mA. I added 1000uF of capacitance for test purposes and the problems disappeared.

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  • \$\begingroup\$ Well, even the 47uF is 4.7 times more than the maximum allowed by standard. Why do you need to add more capacitance, what good would it do? It might still work with some USB chargers but might not work with all. \$\endgroup\$
    – Justme
    Dec 27, 2023 at 12:44
  • \$\begingroup\$ I've added the answer to my initial post. \$\endgroup\$
    – trevor
    Dec 27, 2023 at 12:51
  • \$\begingroup\$ Adding a capacitor does not add DC load. The surge is high but if the USB supply turns off when DC current is too low, it will still be too low. If a capacitor appears to help, the reason why it does is unknown. Obviously the problem is not solved by adding capacitance, but getting a proper supply. What is your current power supply, because not a single mains powered USB supply does what you describe, and most power banks are designed to turn off when it seems that a phone is full. \$\endgroup\$
    – Justme
    Dec 27, 2023 at 16:11
  • \$\begingroup\$ I never said that the USB supply turns off. Where did you get that from? I said the pico does not work. There are stability issues with the pico, that only appear with that charger. Code that otherwise works, sporadically does not work anymore. Obviously the problem IS solved by adding capacitance. If there is a lack of capacitance or even too high ripple, then added capacitance will fix this. \$\endgroup\$
    – trevor
    Dec 27, 2023 at 18:03
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    \$\begingroup\$ The full name is Raspberry Pi Pico and if you take the context, it does not matter if it is Arduino or whatever. Your Pico still has already more than 4 times the allowed capacitance that needs charging. \$\endgroup\$
    – Justme
    Dec 27, 2023 at 22:48

1 Answer 1

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with a certain USB charger this pico would not work.

USB chargers ARE NOT good POWER SUPPLIES! They are designed to charge internal batteries, which usually does not require high quality of voltage output. That's why the "chargers" are relatively cheap as compared to real AC-DC power supplies. They likely have unbearable level of ripples, and that's why your Pico needs an additional huge capacitor to work. Do not use "USB chargers" to directly power any microprocessors or other electronic devices unless your device has a "buffer" in the form of Li-Ion battery or some supercapacitor.

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    \$\begingroup\$ Technically, they are all power supplies. They are not chargers because they have no charging circuitry for any purpose as they are just 5V voltage supplies. So there is no such thing as "USB charger", people just call them that because they are used for charging things. \$\endgroup\$
    – Justme
    Dec 30, 2023 at 21:46
  • \$\begingroup\$ @Justme, sorry, there is a special class devices called "USB chargers". Usually they have some relaxed requirements for ripples, and they have a special I-V load curve. Formally the "USB charger" is a device with USB port that conforms to USB Battery Charging specifications for DCP. For funny-looking load curve, see Fig.4.1 of the spec. \$\endgroup\$ Dec 30, 2023 at 23:33
  • \$\begingroup\$ So it is an official term, I was wrong (I've even read the BC specs before as I've made a device with a BC port but a chipset implements it so I don't need to know the details). But it's still just a term for a 5V power supply which happens to provide the voltage and current according to a certain specification. My main point was that these "USB Chargers" still are 5V power supplies, and actual battery charger is inside the phone or whatever device, so it's nothing special except it fills a standard which requires certain currents with certain voltage drops and should work like 5V supply. \$\endgroup\$
    – Justme
    Dec 31, 2023 at 0:11
  • \$\begingroup\$ @Justme, I am not sure why you don's see the terminological difference here, OP is talking about "USB charger", not "battery charger". And again, "USB charger" is indeed a power supply but with intentionally poor load regulation (if you wish), so usually its voltage output drops significantly with high load. Various MCU boards usually have very spikey power consumption and may not like the sharp voltage drops, which is likely the cause of OP's problem. And that's why using a 1000uF filter capacitor makes it work. \$\endgroup\$ Dec 31, 2023 at 0:41
  • \$\begingroup\$ Well the OP did say the current is less than 200mA and below 500mA the device with a charging port has to provide 5V +/- 5%, with the exception of load changes not causing under/overshoots below 4.1V or above 6.0V. Also, I will state again that the Raspberry Pi Pico, if it has a 47uF capacitor on it's USB supply input, cannot be compliant with USB standards, and thus is not required to work with other standard compliant devices such as USB Chargers with DCP. For all we know, the OP's USB charger may be very sensitive to standards or it may have old bad leaky capacitors in it. \$\endgroup\$
    – Justme
    Dec 31, 2023 at 1:27

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