TheBackShed.com

Forum Index : Microcontroller and PC projects : Powering Pico

Author

Message

asknik2022
Regular Member

Joined: 26/03/2022
Location: United Kingdom
Posts: 92

Posted: 06:18pm 06 Oct 2024

Copy link to clipboard

Print this post

If I power the Pico via the usb cable all is fine.

If i power the pico via mains supply regulated down to 5V DC by LM317T set via resistors to 5V DC to pin 40 of the Pico, then it works but randomly freezes .. Any ideas Please.

stanleyella

Guru

Joined: 25/06/2022
Location: United Kingdom
Posts: 2119

Posted: 06:47pm 06 Oct 2024

Copy link to clipboard

Print this post

where does your reg 5V go? vbus? don't plug usb into pc same time

asknik2022
Regular Member

Joined: 26/03/2022
Location: United Kingdom
Posts: 92

Posted: 07:39pm 06 Oct 2024

Copy link to clipboard

Print this post

It goes into pin 40 on the pico and no the usb is not plugged in when powered..

Andy-g0poy
Regular Member

Joined: 07/03/2023
Location: United Kingdom
Posts: 56

Posted: 08:03pm 06 Oct 2024

Copy link to clipboard

Print this post

I take it that when powered by the USB all is OK.

What is your power supply arrangement/circuit You mention a 317T but what feeds it, and is there any heatsinking on the regulator etc.

How much current is being pulled from the supply.

317's need at least 3V above their regulated output so for 5V you need AT LEAST 8V in, and like all three terminal regulators a couple of 0.1uF decoupling caps between the gnd and input and gnd and output pins are well worth while. They should be fitted as close to the regulator pins as possible. They prevent the regulator oscillating. If you have a scope you will see this on the supply very easily if it is happening. I've only come across it twice in 40-odd years, so it does happen if infrequently.

Also check that there is actually 5V getting to the PICO i.e. there is no excessive voltage drop on the power cables (unlikely but worth checking)

Andy

stanleyella

Guru

Joined: 25/06/2022
Location: United Kingdom
Posts: 2119

Posted: 08:37pm 06 Oct 2024

Copy link to clipboard

Print this post

a 7805 reg with caps each side and 8v supply but 12v ok depends on current, may get hot.
I've used these and similar https://www.aliexpress.com/item/32382955228.html

phil99

Guru

Joined: 11/02/2018
Location: Australia
Posts: 2135

Posted: 08:40pm 06 Oct 2024

Copy link to clipboard

Print this post

Manual p10 said The input voltage from either the USB or VBUS
inputs is connected through a Schottky diode to the
buck-boost SMPS (Switch Mode Power Supply)
which has an output of 3.3V. The SMPS will
accommodate input voltages from 1.8V to 5.5V
allowing the PicoMite to run from a wide range of
power sources including batteries.

So a slightly low voltage should not affect it.

The RUN pin has a weak pullup and is sensitive to noise and static.
Add a 100nF cap to ⏚ or/and a 10kΩ to 3.3V.

stanleyella

Guru

Joined: 25/06/2022
Location: United Kingdom
Posts: 2119

Posted: 09:36pm 06 Oct 2024

Copy link to clipboard

Print this post

ground 3.3v enable and power it from 3.3v to bypass the inbuilt buck psu which is noisy if you use a to d.

Mixtel90

Guru

Joined: 05/10/2019
Location: United Kingdom
Posts: 6778

Posted: 10:07pm 06 Oct 2024

Copy link to clipboard

Print this post

For simple powering it's best to feed the supply into pin 39 (VSYS). Pin 40 is connected directly to the supply from the USB socket and then feeds VSYS via a diode. The exception to powering VSYS is when you are using the Pico's USB port in Host mode. Then you have to power via VBUS in order to feed any USB device (such as a hub) that is plugged into the USB socket.

The Pico can handle a decent range of supply because of the switching PSU. In spite of this being noisy, do not disable it unless you definitely need lower noise operation. When it is disabled you have to use a regulator to power the Pico via the 3V3 pin. I've run Pico's quite happily from a couple of AA batteries. :)

If you are using a LM317 to get 5V you need at least 8V into it, preferably 9V or 10V and you *must* fit the recommended capacitors, keeping the leads reasonably short and mounting them close to the LM317. If you don't use them the LM317 can be unstable. They say 0.1uF on the input and 1uF on the output but I'd use 22uF for both. More on the input if it's noisy.

The data sheet for the LM3117 is almost always wrong! R1 is shown as 240R as standard. That is unsuitable for the 317 unless you can guarantee that the load will never drop below 5mA under any circumstances. You really need a resistor a bit lower than that, say 180R. Recalculate the output.

Mick

Zilog Inside! nascom.info for Nascom & Gemini
Preliminary MMBasic docs & my PCB designs

asknik2022
Regular Member

Joined: 26/03/2022
Location: United Kingdom
Posts: 92

Posted: 05:26pm 08 Oct 2024

Copy link to clipboard

Print this post

Many thanks guys for all your help

I sorted it by adding a 220uF Cap across 5V and Ground and also added a bigger heatsink to the LM317T

Print this page

To reply to this topic, you need to log in.