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My ongoing (beginner) case-modding efforts have led me to desiring a spinning globe within my computer case. I would like to use a low-RPM motor (something between 4 and 20 rpm) for that and connect it to the PSU to power it.

I have an IT background, but absolutely no clue about electrical engineering and the utmost respect for all of the magic happening there. Therefore, I've searched a lot about the topic, but failed to find definite info, thus I turned here.

I haven't yet bought a motor because I wasn't sure which one would be appropriate, but I think I'd need a motor like this one: Gear-Box Motor

My PSU is an EVGA SuperNova 80+ Gold G2 550W and has plenty of connection options.

My guess was that I could connect that motor to a 12V line, like the ones used for powering fans or components. I'm highly uncomfortable with trying though, so I'd really like to have an expert opinion...

  1. if it is safe at all (I'd like my computer to live long and prosper) ?
  2. which motor would be best (like the one depicted above?) ?
  3. which internal slot cable to use (12V processor, molex, fan, ...) ?
  4. how to connect which cable to which slot ?

Can anybody please help me?

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2 Answers 2

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You should use a separate power supply for fiddling around on the bench. Tapping power off of a working computer is adding a lot of unnecessary risk. You can probably harvest a working power supply for free from an old computer. Check the net for how to get it to power up, you may have to put a load on the 5V bus and to short a couple wires to fake the soft power switch. Add a fuse to the 12V connection or you could conceivably burn yourself from a short. Try a 5A inline fuse. You can find easily this as an automotive part. Photos from here and here:

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The motor you show, although specs would be nice, I can assure you will operate fine from a PC power supply. It probably draws less than 1A even when stalled.

You can use the Molex-style connectors that go to some fans and old-style IDE (non-SATA) hard disk drives etc.

enter image description here

The motor goes between the yellow wire and a black wire. If you reverse the polarity the motor will run in the opposite direction.

Mostly the motor you choose will depend on the torque and RPM you need. The one you show is 14RPM so it takes a bit more than 4 seconds to rotate through 360°.

It has a built-in gearbox so it will be difficult or impossible (depending on the type of internal gearing) to turn the output shaft manually without damage.

The torque rating is usually specified in N-m or oz-in and you will have to evaluate whether it's sufficient if it is called upon to drive with much torque.

This kind of motor is called a brushed DC motor and simply turns when voltage is applied. It draws more current when you start it and more current as you load the output shaft.

To get it to turn slower you can lower the voltage with something like PWM or a voltage regulator, however the maximum torque (stall torque) will drop commensurately. More sophisticated speed control methods measure the motor speed (with an encoder or tachometer) or the motor current and raise the voltage to maintain the speed more constant as the torque increases. Some schemes also use back-EMF as a sort of tachometer.

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  • \$\begingroup\$ Thanks very much for the good description. I'm absolutely okay with any speed that matches my requirements, so no big worries there. I'm more worried about your advice to not use the PSU, but another unit. This would complicate the build, of course, and add a big component to it. If necessary, I can use normal power from the wall socket, but how would I connect the cables then to such a motor? Or would I need an intermediate component? \$\endgroup\$
    – Franz B.
    Nov 23, 2016 at 17:30
  • \$\begingroup\$ The typical ATX power supply has a so-called IEC connector and switch on it, so you just plug it into the wall with a standard cord. The wires that come out of it are low voltage and will not shock you. I do not recommend opening the power supply case to add load/binding posts as some pages/videos will show you. \$\endgroup\$ Nov 23, 2016 at 18:03
  • \$\begingroup\$ @FranzB. -- regarding Sperhros advice on using other PSU, considering that you really dont need much power, maybe if you can find a motor that runs on 5V <1A, you can use a cellphone USB charger. They are cheaper and more compact, and seem to be enough for your project. \$\endgroup\$
    – Wesley Lee
    Nov 23, 2016 at 18:13
  • \$\begingroup\$ As @Wesley says you could find a 12V 1A or 2A wall wart or an enclosed power supply from a maker such as Meanwell that will supply 5A or 10A (or much more if you want). 10A is 120W, or about 1/8-HP, which is more than enough motor to take your finger off. The little motor you show will be very happy off a $5 12V/2A wall wart. \$\endgroup\$ Nov 23, 2016 at 18:18
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    \$\begingroup\$ I erad the asnswer as suggesting to use a separate/sacrificial PSU for testing. When you have it all working with a matching molex plug on the motor wires you can move it over to the main PC PCU. The depicted motor should be fine on any modernday PSU. \$\endgroup\$
    – KalleMP
    Nov 23, 2016 at 19:23
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  1. is it is safe at all (I'd like my computer to live long and prosper) ?

As long as the motor power consumption is not excessive then there is very little risk of damage to the computer / PSU. For such a low speed unless the motor gets jammed you should be fine. Even then it'll probably not be an issue but without motor specs it's hard to be sure.

  1. which motor would be best (like the one depicted above?) ?

Without datasheets etc. it's hard to be certain but the one pictured above looks like a good choice.
At those speeds you need one with a built in gear box, 12V is the obvious power supply rail to use and you're not going to be loading it up much so power isn't really an issue. That one seems to tick all those boxes. And be careful, some people may take that as a part recommendation request and try to close the question as off topic.

  1. which internal slot cable to use (12V processor, molex, fan, ...) ?

A fan connector seems like the logical choice since those are intended to also spin motors but it really doesn't matter, they will all go to the same place inside the PSU.
You can get fan connector cables (or molex if you prefer) off the shelf with the correct connectors on, get one of those, cut off the end you don't need and solder on your motor. That way you have something that will plug into a standard PSU output easily. Obviously take care to insulate the joins.

  1. how to connect which cable to which slot ?

Google "pc fan connector pinout" and then ground(0v) to the motor black cable, +12 to the motor red cable.

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    \$\begingroup\$ Thanks alot for the detailed answer! Where can I find the motor specs? I noticed that this one doesn't have the torque in the center, but I'd need that, so probably a motor like ASIN "B00DCARXL4" would be more appropriate (no clue if I am allowed to link directly to Amazon). \$\endgroup\$
    – Franz B.
    Nov 23, 2016 at 17:27
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    \$\begingroup\$ Links are fine. It's just "I want to do this, what part should I use questions" that are discouraged. Generally as long as you show you've made some attempt at research no one will complain. And yes, that looks fine. Finding specs for something like that is often hard, some times it's easier to just try it and see. \$\endgroup\$
    – Andrew
    Nov 23, 2016 at 17:29
  • \$\begingroup\$ Thanks for clarifying. Yes, unfortunately most of the so-called "details" pages feature nothing more than space occupied, voltage and RPM. Guess I'll have to try, as you said =) \$\endgroup\$
    – Franz B.
    Nov 23, 2016 at 20:56

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