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I have a power supply with +12v, +5v, and GND rails.

Is it good practice to put a 7v motor across the 5 and 12v rails?

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  • \$\begingroup\$ No. I doubt it even gets counted as a practice (let alone be best). I would rather term it 'cruel', however it would work( if both 5v and 12v share the same ground). \$\endgroup\$ – vvy Jan 6 '16 at 17:26
  • \$\begingroup\$ @vvy what if the power supply has a -12v rail, and you put a motor from +5 to -12? I can't think of other reasons that it would have one. \$\endgroup\$ – user86234 Jan 6 '16 at 17:28
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    \$\begingroup\$ That would only work if your 5V rail can sink current. Most supplies would not work in this way. \$\endgroup\$ – brhans Jan 6 '16 at 17:30
  • \$\begingroup\$ @brhans what about a pc power supply? \$\endgroup\$ – user86234 Jan 6 '16 at 17:31
  • \$\begingroup\$ almost certainly not. \$\endgroup\$ – brhans Jan 6 '16 at 17:33
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That is a good way to destroy everything connected between +5 and GND.

The motor will draw a lot of current (especially on start-up) and that will typically cause the +5V rail to rise up towards +12.

Most (positive) switching supplies and linear regulators are not designed to sink current from the output, only to source it, so as soon as the motor current exceeds the draw of everything else on the +5V line you'll have disaster.

Below is part of a schematic of a 200W PC power supply from here:

enter image description here

As you can see if you connect a resistor between +12 and +5 SBD1/D83-004 will become reverse biased and there is nothing keeping the +5V rail from rising other than capacitor leakage (the feedback might switch the supply down in time, if you are lucky).

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