<p>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.</p>

<p>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&nbsp;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.</p>

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

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.