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My computer case has a built in fan which uses a Molex 8981 connection for power, but before reaching the fan, the wires split to form another male molex connector. Here is a diagram of what I mean.

My problem is that if I was to use the male Molex connection, surely it would short circuit the fan or vice versa? If not why?

Thanks in advance.

Similar fan:

Here is an image of a similar fan.

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migrated from Feb 6 '12 at 5:22

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

+1 for the great drawing! – zdan Feb 6 '12 at 5:43
WHY the heck, in the second image, does the red wire go to the YELLOW wire on the molex connector? – sinni800 Feb 6 '12 at 8:22
up vote 2 down vote accepted

What you have here is two "outlets" connected together in parallel. You are plugging a fan into one of the outlets and perhaps something else into the other outlet.

This is just like when you plug your computer into the power outlet in the wall. Most wall outlets (in the US anyway) have more than one place where you can plug in an appliance, and they are connected in parallel just like this.

While it is possible to "short" the outlet by plugging in a defective appliance (and thus blowing a fuse or tripping a circuit breaker), it is also quite possible to operate more than one appliance (like your computer and a monitor) at the same time.

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Unless you aren't using the Molex male as intended, it should be safe, since the fan and anything you connect to the Molex are connected to the same source in parallel.

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Thanks, but in theory if I was to connect something with much less or more resistance would one short circuit the other? – deebee396 Feb 5 '12 at 18:03
You would have to have 0 resistances to short circuit. This would probably cause something in your PSU to trip. – Kellenjb Feb 5 '12 at 18:34

It has the pass-through connector specifically so that you can use it. It is just a way to allow you to add the fan without taking up a power connection (useful for PSUs with few connectors). Plugging something into the other connector is the whole point.

It won’t short-circuit the drive or whatever you plug into the other connector unless the fan somehow shorts in itself, but then at that point, everything that is connected to the PSU is at risk (but fortunately PSUs are designed to shutdown as fast as possible when shorted, specifically to prevent damage).

The only real-world problem (as opposed to hypothetical) that you might need to consider is what you are plugging into the other connector. It is rarely an issue, but personally, I like to put my floppy and drives on the same cable as fans and give hard-drives and video-cards their own cable. The fan is unlikely to draw enough power to starve the hard-drive, but if you have a free cable, why not use that for the drive and have something less critical share with the fan?

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