How big a 5v power supply needed for Duet 3 and RPI4?

N7JTI

I'm trying to plan the installation of my new Duet 3 mainboard and SBC. I'm going to be using a Raspberry Pi 4 for my SBC. I see that the RPI 4 wants a power supply capable of delivering 3A. According to the documentation the "Normal" method of powering the SBC is from the Duet 3. There are a few questions I can't find the answer to in the documentation.

How much current do I need to supply to the Duet 3 in addition to the 3A that are apparently needed for the RPi 4?
I can't find a 5v power supply with a micro USB connector that will supply the 3A, much less whatever the Duet 3 needs. So how do I provide enough 5v current? Is there some other pin or connector for me to insert it on? I'm not even sure that the typical USB power cord that terminates in a micro USB connector can provide that current since no USB port (other than USB-C) is specified to provide that much current.

This problems must be common, how have others solved it?

Thanks,

Alan

dc42

These days I am encouraging users to use the official RPi 4 PSU to power the RPi 4, and let the Duet power itself.

Powering the Pi from the Duet seemed like a good idea when we designed Duet 3, but that was before RPI 4 arrived with its higher power requirement.

Alternatively, you can power the Duet from the Pi through the ribbon cable. The Duet draws only a few hundred mA from the 5V supply, unless you power LED strips from it.

@N7JTI said in How big a 5v power supply needed for Duet 3 and RPI4?:

3A that are apparently needed for the RPi 4?

you don't need the full 3A if you're not going to add USB devices that also draw current from the Pi supply.

Is there some other pin or connector for me to insert it on?

you can supply 5v to pin 2 and 4 on the 40 pin gpio header if you build a custom cable f.ex.

the most important thing is to avoid voltage drop in the pi supply, or use a adjustable 5v source so you can account for the loss and have 5v with margin at the pi.

I'm planning on using a dedicated 5v supply, custom ribbon cable 5v --> Pi --> Duet and setting the 5v supply to 5.5v or whatever so I can have 5.25v at the Pi when idle to avoid the pi throttling due to low voltage.

Isn't this a ideal opportunity to use the external 5v in socket on the duet-3 with a suitable external 5v psu and jumper the board accordingly to have the external 5v in supply the Rpi with a high enough current?

Or is the external 5v there for another reason?

And another question @dc42 as you have recommended to have an Rpi4 use it's own dedicated psu and have the duet power itself.

What is the recommendation with regards the correct jumper combination on the duet-3 for that?

oozeBot

@CaLviNx said in How big a 5v power supply needed for Duet 3 and RPI4?:

Isn't this a ideal opportunity to use the external 5v in socket on the duet-3 with a suitable external 5v psu and jumper the board accordingly to have the external 5v in supply the Rpi with a high enough current?

This is currently how we have our machines setup. I am also interested in if this is a recommended setup.

@N7JTI

here is a pic of the Rpi-4 GPIO pin out if you need it

if i'm not mistaken that would put a diode drop in the path of the Pi, but if you can account for that then why not; I'd still prefer to have the Pi running if the Duet is disconnected, thus custom wiring.

Luke'sLaboratory

I use a 24V ATX supply to power my D3 + SBC setup, and wound up just directly soldering the 5v rails to the power in on the RPI and letting the duet power itself. I power USB devices from the PI, so I couldn't use the "only" 3A 5v Standby rails, which would allow me to turn off the duet w/out turning off the pi digitally.

Wally

@CaLviNx - which connector are you using for "external 5v in socket" ?
I'm having trouble locating this connection in the diagram here, https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Wiring_Diagram#main

oozeBot

@Wally it's circled in red.. but you'll need to modify the jumpers right below it.

Wally

@oozeBot - Thank you for the help. I'm using that currently to control a solid state relay to power on/off 120VAC input to my main power supply. So explains my confusion...

oozeBot

@Wally note that this is what I asked above for clarification on if this is still a recommended setup..

Wally

Ahh, thank you - yes this is a good question..

So, the big question is where else can we connect 5 VDC input to the Duet 3 if "EXT 5V" is consumed for PSON (power supply on)? In my case, applying 5 VDC here would power the solid state relay and defeat the PSON feature.

Currently, I power the Duet 3 via the micro USB connector (5 VDC) - is this they only work around? I'm not keen on soldering stuff to the expensive Duet 3 board.

oozeBot

I literally have one set up this way right now on my desk.. it appears to be the ideal setup for us. I just want @dc42 blessing after reading a few of his comments..

edit: and my photo is upside down.. lol

Wally

@oozeBot - thanks for the picture - literally worth a 1000 words....
PS: I have a very similar setup too!

T3P3Tony

This is the current documentation on the 5V options for the 6HC:

https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Hardware_Overview#Section_5V

Please let me know if it needs further expanding.

To answer the specific question of providing external 5V to the Duet and also to the RPI

While this is possible in theory:

EXT 5V -> Duet, and then Duet -> Pi over the ribbon cable

~~you then have multiple diode drops in the protection circuits~~
Edited as there is only 1 diode drop
its not recommended.

If you want to use an external 5V (suitably rated) to power both Pi and the Duet then you connect them in parallel:
EXT 5V -> Duet (on the EXT 5V header)
EXT 5V -> Pi

The safest way to power the Pi is through the USB C socket, the 5V pin on the SBC header bypasses some of the internal protection.

oozeBot

@T3P3Tony thank you for this clarification.. it was a concern we had that we had not been able to find a clear answer on.

@T3P3Tony said in How big a 5v power supply needed for Duet 3 and RPI4?:

This is the current documentation on the 5V options for the 6HC:

https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Hardware_Overview#Section_5V

Please let me know if it needs further expanding.

To answer the specific question of providing external 5V to the Duet and also to the RPI

While this is possible in theory:

EXT 5V -> Duet, and then Duet -> Pi over the ribbon cable

you then have multiple diode drops in the protection circuits so its not recommended. (and probably wont work)

If you want to use an external 5V (suitably rated) to power both Pi and the Duet then you connect them in parallel:
EXT 5V -> Duet (on the EXT 5V header)
EXT 5V -> Pi

The safest way to power the Pi is through the USB C socket, the 5V pin on the SBC header bypasses some of the internal protection.

@T3P3Tony

This being so are the 5v lines on the SBC header pins considered dead when you only have the jumper "Int 5V EN" shorted ?

Edit i have just remembered ( I will try to find the thread) that there was a thread about this very thing when the first batch of Duet-3 boards were pre-released .

EXT 5V -> Duet, and then Duet -> Pi over the ribbon cable

you then have multiple diode drops in the protection circuits so its not recommended. (and probably wont work)

It does work i have one wired that way (and wired it that way after the it was being discussed previously)

oozeBot

I just measured a .33v drop at the RPi based on the way its wired in the photo I posted above. By adjusting the 5v power supply, I was able to deliver 5v to the Pi. I do understand this is bypassing the RPi's power regulator though.. so I'm torn. If this was just for myself, I think I'd trust it, but if Duet can't recommend this setup, I am afraid we'll have to rethink it..

@oozeBot said in How big a 5v power supply needed for Duet 3 and RPI4?:

I do understand this is bypassing the RPi's power regulator though..

Pi doesn't regulate 5v, within lies the problem