Powering Pi for Octoprint

I am setting up my Mendel90 with Octoprint as the SD card holder on my Panellolu2, which are hard and expensive to buy, is getting a bit flakey and the Octoprint plus Pi camera will enable me to monitor prints more easily. The Pi (a Model 3A+) needs 5v on the micro USB socket so I thought that I would power it from the ATX power supply on the printer. Whilst I can use one of the +5v Red wires that I cut short when I built the printer I was wondering whether I could remove the 10R resistor from the Red and Black wires and wire the Pi into them. I assume, always a dangerous thing to do, that the resistor is there to give the ATX power supply a dummy load and keep it running so the Pi could be that load. Does anybody see any obvious flaws in my assumption?

Edited 1 time(s). Last edit at 05/28/2019 04:37PM by Bearded Flyer.

I believe that the required power to keep an ATX is the equivalent of a 8-20 ohm resistor on +5v, the minimum current required to keep an ATX supply from shutting down is 1.25 amps

A Pi ( Model 3 A+ ) idle power is 1.126amps though with any web camera you should be in the OK.

Edited 2 time(s). Last edit at 05/28/2019 07:03PM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Thanks for the detailed reply. I have measured the current draw on an uncalibrated USB meter and I am getting about 0.5 amp when booting and 0.3 amp when idling which does seem enough to keep the ATX PSU running. My figures appear to be similar to those on this webpage [raspi.tv] . If an 8-20 ohm resistor is required then that would be 5/8 (0.625 amp) to 5/20 (0.25 amp) so I should be OK but as it is a bit marginal and I do not want the 5v rail to float any higher I will keep the 10R resistor installed and use one if the shortened and insulated Red +5v wires to power the Pi.

Edited 2 time(s). Last edit at 05/29/2019 04:29AM by Bearded Flyer.

My reference was found here

[www.raspberrypi.org]

I too would have left the 10 ohm resistor...

Edited 1 time(s). Last edit at 05/29/2019 09:09AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

I don't want to criticise you but I have looked at your reference and I believe that you, unfortunately, have misread the units on the power graph. The Pi 3A+ is stated as drawing 1.126W not amps. This would give a current draw at 5v of 0.2252 amps which is lower than the figure I measured but I did have the Pi camera attached but I do agree that I should leave the 10R resistor in place to a oid problems with the PSU shutting down.

The 10R resistor is needed to get the 12V rail to be close to 12V. Without it the PSU throttles back to keep the 5V from being too high and then the 12V rails drops when the bed is on.

I would keep it in circuit.

I use the 5V standby rail to power my RPIs and then use GPIO to control the main PSU. See [hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]

Chris,
Thanks for the link to your blog. It did not come up on my Google searches but the information is just what I need.