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I have a 120V to 12V adapter with a molex end that I want to use to power a PC fan controller. However, the pins on the female molex (the 12V adapter) doesn't have holes that fit with the pins from the male molex (the PC fan controller molex connector). This figure shows the problem I'm having. enter image description here Is this normal? From my understanding of molex connectors, the 12V adapter molex connector 12V ground pin is missing, and for the PC fan controller molex, the 5V ground pin is missing. Also, when connected like that, the molex connection would be in a 7V configuration. If I'm correct, the fan controller needs 12V + 5V to function correctly (which my 120V to 12V adapter is supposed to provide). So why both connectors pins are not correctly aligned?

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  • \$\begingroup\$ Is it working ? \$\endgroup\$
    – Eugene Sh.
    Jul 20, 2016 at 13:59
  • \$\begingroup\$ @EugeneSh. No, it is not working. The fan controller is not powered on. \$\endgroup\$
    – m_power
    Jul 20, 2016 at 14:17

2 Answers 2

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Both manufacturers have assumed that whatever will be plugged into the other end of their connector will have both ground pins fitted & connected together (usually a safe assumption).
They therefore assume that they can save 1 or 2c on each unit by only fitting 1 ground pin.
Unfortunately for you, they each picked a different pin to fit.
If you have any access to the back of one of the connectors, you may be able to gently tease the ground pin out of the hole its presently in (you may need to depress/bend some little tabs on the side of the pin) and fit it back into the other hole such that it now lines up with its mate in the other connector.

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    \$\begingroup\$ Quite laughed when I read "Unfortunately for you, they each picked a different pin to fit." \$\endgroup\$
    – m_power
    Jul 20, 2016 at 15:33
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    \$\begingroup\$ Astute analysis of the root cause! \$\endgroup\$
    – Spehro Pefhany
    Jul 20, 2016 at 16:15
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    \$\begingroup\$ I changed the ground pin location and everything is working now! Thanks again. \$\endgroup\$
    – m_power
    Jul 21, 2016 at 14:12
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The ground pin is not connected... One solution would be to move the ground pin on the fan controller cable to the proper position. If you look at the pins closely, you can see 2 small tabs bent out from the pin itself that keep the pin in place. With a small screwdriver, you can gently bend them back to be level with the pin itself, and pull out the pin gently with the wire attached.

Now you have to bend the tabs so that when you put the pin back in, it will keep the pin in place. (Be sure to double check which hole you put it in...)

Video on dismantling a molex connector

In normal cases, this should not have posed a problem, as the power connector should have both ground sockets populated and connected together.

For why the panel does not work

The hackish 7V wiring you included is irrelevant in this case. The panel most likely does not work that way...

That trick would use the voltage difference between the 5V and 12V rail to get 7V. However, as you can see from the pictures that means actually connecting the negative supply lead of the load ("black wire" in this case) to +5V, and the positive supply lead to +12V.

Your panel has 3 wires going to it: the GND, +5V and +12V. Most likely it is designed to have all of them (or at least GND and one of the positive supply rails). It can not just "find out" what you plugged into it and adapt to what it has, you have to have it connected properly - but without a schematic, or at least a board picture it is very hard to guess...

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  • \$\begingroup\$ So there is no difference between the 12V ground and the 5V ground? And yeah, I don't understand why my power adapter connector doesn't have 4 holes. I know that it is used to power, normally, PC hard drives, so is it because of that that it only has 3 pins located at 1 3 and 4? \$\endgroup\$
    – m_power
    Jul 20, 2016 at 15:12
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    \$\begingroup\$ @m_power The two middle pins on the molex are (as in "normally should be") tied to the same GND. And I think it is fair to expect a power supply to have both lugs populated... Well, some cost-cutting was made, as most Molex powered peripherals have both pins there, and joined to the GND plane on the device's PCB. And then the other cost cutting was on the adapter side: they left out one pin - designing for both sockets to be there... However, the latter should not be an issue, as the standard for a PC peripheral PS says that the two middle sockets should be there and tied to GND... \$\endgroup\$
    – ppeterka
    Jul 20, 2016 at 15:18
  • \$\begingroup\$ Since I tried to connect them, could the 7V provided have damaged the fan controller? \$\endgroup\$
    – m_power
    Jul 20, 2016 at 15:32
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    \$\begingroup\$ @m_power There is the possibility of damage but many circuits would cope. As the voltages were pretty low and likely the fan did not run the currents would have been small and substrate or protection diodes may have saved the day. The 5V rail will have tried to go negative. \$\endgroup\$
    – KalleMP
    Jul 20, 2016 at 19:38

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