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I'm wondering if there is some trick to power my OctoPi with the power supply of my 3D printer. I'm using an Geeetech I3 Pro W.

The power supply itself should be able, but the output is as far as I'm aware of 3.3 volts. Not my desired 5 V for USB, it would be a shame if I really would need to buy a new power supply when I have a strong one actually running. My current power supply causes a lot of "Under-voltage detected!" warnings.

After thinking a little about the specs, there are cigarette lighter adapter for cars they use 12 V. Has anyone experience with using that on his printer?

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  • $\begingroup$ Presumably, the PSU is 12V or 24V (for extruder and steppers), and there is a lower voltage regulator on the printer which you are trying to use. Depending on the version of PI you use, up to about 2A is a good spec for the supply you need. $\endgroup$ – Sean Houlihane May 5 '19 at 19:45
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What you are looking for is called a "buck converter" or a "step down module". These literally cost about half a buck/Euro a piece. These converters convert a high voltage into a low voltage, the better ones are able to draw 2 to 3 Amps, which is required for stable operation of the Raspberry Pi.

If you have an old computer power supply of a decent brand (probably not as you refer to a kit/assembled printer, but added for completeness), you can even use the standby 5 V line out and switch the power supply on using a relay to short the green wire of the PSU to ground. This is how I use it on one of my printers.

Note to power the Raspberry Pi through the micro USB port, to not bypass safety features.

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Where are you plugging in the USB power to the Pi? If you are back powering it from the data connection, you will bypass the fuses and potentially ruin your Pi or worse. Look at this wiki under the power section:

Back-Powering; (powering the Raspberry Pi from a USB hub through the uplink/data port, single cable) Back powering is possible on the Raspberry Pi, but not advisable. Revision 1.0 boards have to be modified to back power, this is due to the 140 mA "polyfuses" that are installed in the USB port circuit. Revision 1.1 boards do not need modifications to back-power, they have replaced the polyfuses with 0 ohm resistors in their place. Revision 2.0 boards do not need modification, they have neither resistors nor polyfuses. It is advised that short (12" (.3 meter) or less) USB cables be used for back-powering a Raspberry Pi. Cable resistance plus connector resistance can quickly reduce operating voltages below the proper range (5.25 V to 4.75 V). But do note that if you do not power the Raspberry Pi in the "official manner", that is through its micro-USB port, but use any alternative way (such as through the GPIO header, the test points TP1 and TP2), but also by back-powering it, you are actually bypassing the Raspberry Pi's input polyfuse protection device! This can have extreme consequences if ever you manage to put more than 6 V on the Raspberry Pi, even for a very short period. As this causes the overvoltage device D17 on the Raspberry Pi to trigger and short the 5 V supply! Without the polyfuse limiting the current through D17, it will burn out, probably melting the Raspberry Pi's enclosure with it, (if you have any) and possibly causing a fire-hazard. It will probably also create a permanent short of the 5 V supply! So be warned, and if you use back power make sure your hub or its PSU has a fuse to prevent this from happening. If not, add your own fuse.

As far as powering the Pi through a 12 V to 5 V converter this will work as long as the current is rated above what the Pi will use, preferably a lot higher. You will also have to consider how this option will cut the power abruptly when you switch the printer off and the Pi will not boot down properly.

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  • $\begingroup$ It's always best to include the info from the link, for two reasons: (a) link-rot, and (b) saves people time having to click links and search for that to which you were referring... :-) Interesting info about the back-powering anyway. $\endgroup$ – Greenonline May 4 '19 at 19:11
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Thank you guys for your help. I ended up with a suggested "Step Down Power Module" which works fine for me. I connected that module directly with the power supply of the printer with a blade connector. The first startup showed me a couple of low voltage warnings, but the second start worked fine without any warnings. I didn't know how the UI looks like without the warning on the top.

So I avoided that fancy and risky back powering since I power it now as intended. Now I have one power supply less which is great out of my opinion.

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    $\begingroup$ You should have marked the person who posted your solution as the Accepted answer so that they get reputation for it. $\endgroup$ – juniorRubyist May 13 '19 at 6:02
  • $\begingroup$ I gave both an upvote, but when that way is here more common I can also accept their answer. However in the end I chose my own solution. Since I don't gain reputation for accepting my own solution I saw there no problem with it. $\endgroup$ – rekire May 13 '19 at 6:28
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I’ve been using a buck converter for quite awhile with no problems. However just as importantly as proper voltage and amperage is using a good cable. I had my step down properly set, but was still getting low voltage warnings until I switched to a thicker cable.

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