Duet Wifi 5V Current Usage?

  • Ok I've searched around and oddly I can't find an answer to my personally important question, so figured I'd ask it here 😄

    As with a prior thread I'd opened, it has been pointed out to me that my current power supply, while already knowing it is pretty much a pos, is just not cutting it. It can not maintain a steady constant voltage and will start off around 14-15v then once a print starts, go down to around 10v and remains there until I turn the power supply off for a good couple of minutes… A few people have pointed out that this could be a cause for some probe issues I've been having, and occasional wonkiness in the printer in general.

    So with that said, I've finally decided to 're-purpose' my HP Server's psu. It is a DPS-700GB A and there seems to be little to no documentation about this PSU other than 2-3 pictures of how to wire the psu so it'll turn on.

    My issue with this, is that the psu has:
    +3.3VSB at 3.5A
    +10.5VSB at 1.5A
    +12.15V at 56A
    -12.0V at 0.3A

    ... There's no 5VSB ...

    I was hoping that I would have a 5VSB spot to tap into so I could take advantage of the WiFi's "Turn on/Turn off" feature of a psu... But without a 5VSB I can't do this.

    BUT, through some research, I can just make a voltage divider with two 10k resistors and it'll split the 10.5V pretty close to half. Giving me around 5 to 5.5V to use!

    So... my questions are thus:

    • What is the current draw for the Duet WiFi when running off of external 5V?

    • Thinking about it, the voltage divider works by basically shunting half the current to ground while letting the other half flow on to whatever device is being used. So does this mean that the psu may just stay on all the time anyway because half its voltage is being grounded? Or will it only trigger a psu to turn on once the full voltage is grounded?[*/]

    • Is my thought process behind this even correct? Will a voltage divider that splits the 10.5V down to roughly 5V actually work and function how I want it to?[*/]

    • Is this even how the Duet WiFi triggers a psu to turn on? By taking a psu's 5VSB and grounding it out? If not then how does it work??[*/]

    Sorry for such a rough and long going post. I'm just really interested if this psu will work in the manner that I wish it to, before I end up soldering stuff onto it and making it unusable to my server… Having the ability to turn the psu itself off via the Duet WiFi would be a 'must' for me if I switched over to this psu... Its not exactly a loud psu, as it actually is pretty darn quiet. But I'd rather simply just 'shut it off' when not in use to save on both a small bit of electricity, and noise pollution...

  • administrators

    Does that server PSU even have a remote on/off input?

    Unless the 10.5V rail is an always-on output (like 5VSB on ATX supplies), deriving 5V from it to power the Duet won't help.

    There are other solutions to achieving automatic switch off. For example, use PS_ON to drive an SSR that supplies AC power to your PSU. To start the system, connect a mains-voltage push button in parallel with the load contacts of the SSR. Or supply 5V power from a small USB power supply.

    A Duet WiFi sitting idle draws about 150mA.

  • The PS_ON and SSR thing spiked my interest. To be honest, I have no idea how PS_ON functions on a power supply. My general assumption used to be that a computer, when you press the power button, simply supplied a voltage to PS_ON which told the PSU to turn on. But I have a feeling it is more along the lines that the PS_ON pin supplies some sort of voltage and is grounded to tell the PSU to turn on.

    As for if the 10.5V rail is always-on, I have no clue. From things I've looked up, when something on a PSU is labeled "VSB" it usually means it is a 'stand-by voltage' meaning it is supplying a voltage even if the main PSU is switched off internally. My concern about "VSB" labeled pins tho, is that I've read that some PSUs turn off their VSB pins once the PSU is 'switched on'. I imagine that the PSU I have doesn't turn off the VSB pins because as of 2004, a majority of general computer/server power supplies use their 5VSB to supply 5V to USB devices to allow things like keyboards and mice to 'wake up' a computer. I don't know how effective such things would be on a server that generally is either already always on 24/7, or is simply remotely turned on or off via something like a iDRAC.

    I've been hoping to figure out some way to do the switching on/off thing prior to actually doing anything to the PSU. Sadly the HP PSU I have, doesn't use pins, but large traces on a PCB that just slot into a socket inside the server. So I have to physically solder wires onto the PCB for me to do anything. And once I do that, I highly doubt I will be able to reliably remove the solder to a good enough extent that I could reuse the PSU in the server if the 10.5VSB doesn't actually work as intended.

    I personally have been trying to stay away from using a USB style power supply for the board ever since having used a RAMPS 1.4… It was so frustrating to have to deal with multiple power supplies.... one for the 360W AC to DC power supply that currently is terrible, a 2A USB charger for a raspberry pi running octoprint, and a USB charger for a camera. Would love to just keep it how it is and just use a single plug in the wall to power everything that is used for the printer...

    Just kinda one of those "I know I have things around the house that can be used to solve X, but I just don't know how it can be done reasonably, and without buying a slew of other items"... D=

  • It's better to stick with an electromagnetic relay for a power supply - you're not going to need to switch it that often and they don't have any trickle current in OFF state.

  • So ignoring my ignorance with this atm… How would one go about using either a relay or ssr to remotely turn on/off the power supply, without using any external power other than the power supply itself? aka no usb.

    My only real idea on this, is finding out if that 10.5VSB actually stays on while the PSU is active, and then using a voltage divider to drop the voltage to 5V. Then, find out how the Duet WiFi electronically 'switches on/off' the power supply, and hooking up a relay to the 12V outputs and the fan control, so that although the PSU technically will always be on, it wont deliver 12V power to the Duet WiFi. But that almost nearly defeats the purpose, as my idea was to have it go into its 'stand by' sort of state so it uses as little power as possible. Which would mean that I would need to connect the relay to the AC input for the PSU... but doing that would mean the 10.5VSB isn't active at all since the whole unit would be physically off. I may have to just say screw it and find one of those 1000W Server PSUs if I wanna go down the Server PSU route... I've read Ebay generally has them for under 30 bucks for usually a pair of two. And all the ones I've seen posts/references to, all have an actual 5VSB that stays on. But then that defeats my idea of 'using stuff I have around without spending money on more things' XD

    Gotta love them "Catch 22's"

  • administrators

    You can't use a voltage divider to power the Duet 5V rail from 10.5V, you need to use a 5V regulator instead.

  • A regulator I can do. I previously purchased a set of 5 when I first got my capacitive sensor, being told by a friend online with the same probe that it was easier to use the 5V regulator between the probe and the sensor input pin on a RAMPS, than it was to 'fiddle with resistors and such'. I bought them, tried it, and didn't work. Then I found out that my probe would just work running directly from the board without any modification lol. So I should still have four 5V L7805CV regulators in my '3D printer box-o-parts'.

  • @dc42 said in Duet Wifi 5V Current Usage?:

    A Duet WiFi sitting idle draws about 150mA.

    My Duet WiFi is version 1.03 and my Mean Well HRPG-450-24 provides a 5V 300mA standby current.

    1. Is this enough to use it for EXT 5V if no display will be connected and all fans are 24V?
    2. Is it necessary to set a special jumper to use EXT 5V?
    3. Does the Remote Control function of the PSU work properly with PS_ON if RC- connected with GND and RC+ connected with PS_ON or do I need a special switch?

    Thanks in advance.

  • administrators

    @It does not look like the RC connection is compatible with the ATX power on supported by the Duet:


    The Duet expects to turn the power on by connecting the RC signal to ground. It looks like this power supply works the opposite way:
    1 RC+ Turns the output on and off by electrical or dry contact between pin 2 (RC-), Short: Power OFF, Open: Power ON.
    2 RC- Remote control ground

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