I am new to electronic circuits so I might not have the correct nomenclature for some items.

  • Description:

    I have 3 high power 12V PC fans (NON PWM) connected to a hub that draw power from MOLEX connector. They can't be controlled by any means except with a on / off IR remote coming with the hub (No hard switches). They draw some high current. They can't be powered over 5V USB directly.

  • Issue:

    I need advice for simple circuit idea (without Arduino) to connect a USB from Motherboard to control the on/off of these fans (while drawing full power from Molex).

    i.e. when the motherboard is powered off, the fans are turned off and when i switch pc on again they turn on automatically (without having to remove the molex connection / using remote every-time i try to switch them off).

  • \$\begingroup\$ I am confused. Which "molex"? The one that comes from PC PSU? Or elsewhere? When PC is "soft-OFF", 12 V is also OFF so the fans won't spin. Where is the problem? \$\endgroup\$ Apr 17, 2019 at 17:35

1 Answer 1


You could use an NPN transistor with the base connected to the 5V (by 1k resistor) coming from the USB, collector connected to the negative terminal of the fan and emitter connected to ground. This way whenever the usb gets power the fan will power on (providing 12v is present)

This configuration:

NPN switch

  • \$\begingroup\$ Thanks alot. Excuse my shallow knowledge in electronics. But i have questions. First: the 1k resistor with the base will connect to which ( +ve or -ve) wire of the usb. and where will the other remaining wire connect to. second: confirm if i get this right "the +ve from the fan will connect to the +ve from the molex. Where does the -ve of the molex connect to ? \$\endgroup\$
    – Mena G
    Apr 16, 2019 at 7:53
  • \$\begingroup\$ No problem, the base of the transistor will connect to a 1k resistor, the 1k resistor will then connect to the +ve (5v) wire of the usb connector. the +ve of the fan will connect to the +ve of the molex connector (12v) and the -ve will connect to the collector of the transistor. the emitter of the transistor will then connect to the -ve of the molex (ground) \$\endgroup\$
    – andowt
    Apr 16, 2019 at 8:02
  • \$\begingroup\$ youll need to spec the transistor to suite, a bc182l will go up to 100mA but you probably need more than that, any idea what power / current requirements the fans have? \$\endgroup\$
    – andowt
    Apr 16, 2019 at 8:03
  • \$\begingroup\$ 3 fans with 0.5 A each, we can assume the hub control taking 0.1 A. Hence we could say 1.6A in total. \$\endgroup\$
    – Mena G
    Apr 16, 2019 at 8:34
  • 1
    \$\begingroup\$ That was very Helpful. Learnt some good stuff. Thanks alot for ur effort. \$\endgroup\$
    – Mena G
    Apr 16, 2019 at 9:51

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