Molex connector on / off switch ? anyone done it.

Jamin280672 · 26 Mar 2025 at 21:29

Im not too great with electrics, obviously building PCs is like putting lego together, but when it really comes down to electrics its time to call in a qualified electrician, however, this is something completely different and doesnt require an electrician.

So ive just brought 2 x 200mm noctua fans and a phobya 70w 4amp PSU, its about the size of an average laptop PSU but the difference is it has a molex connector coming out of the PSU, this is in preparation for a Mo-Ra 200 IV radiator I have on the way from Germany, I also brought a splitter cable which will convert the single molex connector from the PSU into 2 x 4 pin fan connectors for the 2 fans and a male molex connector for my D5 pump, the pump is a D5 Vario so I can control speed with the dial on the back of it, the fans only run upto 800rpm and are silent at 100% so I dont have an issue with them running at 100% all the time.

........but when testing things tonight I realized its all missing an on/off switch, once its on, its permanently on unless I pull the kettle lead out of the PSU, and then of course, theres remembering to plug it back in and power on before I switch the PC on, so what I thought was to cut the yellow wire on the splitter cable and integrate a switch into it, only now I really dont know what sort of switch I would need, any guidance would be greatly appreciated, the alternative would be to just grab an alexa plug and plug the PSU into that the ask alexa to switch it on and off, buggered if the wifi goes down and they're not the most reliable of devices......but, would this still be the easier option ?

Armageus · 26 Mar 2025 at 21:41

I'm no electrician so wait for someone with a bit more experience, but in theory yes just putting a switch on the yellow line should be sufficient.
Make sure the switch is rated for the appropriate voltage/Amps though

Too Tall · 26 Mar 2025 at 22:07

If you want to go down the switch route instead of using a device which will allow PWM control, get a rotary 12v rheostat which will allow you to control the fan speed manually.

Personally I’d get some 4 pin fan extensions and connect the fans directly to your motherboard. At least then you have software control/monitoring.

Mr Evil · 27 Mar 2025 at 16:48

Why not run it all off the PC's PSU? But if you need a separate PSU then I would probably add a switch on the mains side, so the PSU is truly off when switched off (equivalent to pulling out the kettle lead). Tht will save a small amount of money in electricity, and make the PSU last longer (because it won't be heating itself up all day every day). You can find suitable switches by searching for an "inline 3 core switch", and they're about as easy to install as a mains plug - anyone can do it with a little practice.

Jamin280672 · 27 Mar 2025 at 17:37

Mr Evil said:
Why not run it all off the PC's PSU? But if you need a separate PSU then I would probably add a switch on the mains side, so the PSU is truly off when switched off (equivalent to pulling out the kettle lead). Tht will save a small amount of money in electricity, and make the PSU last longer (because it won't be heating itself up all day every day). You can find suitable switches by searching for an "inline 3 core switch", and they're about as easy to install as a mains plug - anyone can do it without a little practice.

Because the radiator isnt really going to be close to the PC, maybe under my desk or on the window ledge by a window, my PSU is at the front of my case (Lian Li A3) and I cant imagine running 3 meters + of cable to run it off the internal PSU, it would do my nut in lol, hate cables as it is lol, but i'll defo have a look at the inline 3 core switch, thanks.

EDIT: Would the switch need to be 13 amp 240v ? as most of the ones im looking at are 240v but 2, 3, 6 or 10 amps, but im thinking although the output of the psu is 70w 4 amps, before the psu........ie the kettle lead, theres no reduction of power / current, it has a 13 amp fuse in the plug and obviously 240v, this is the kettle lead the PSU was supplied with, it had 2, a UK one and EU one, but I threw the EU one away.

another way I could do it is just grab one of these which are 10 amps 240v, they are pretty cheap, not much more than a switch on its own, and then just change the fuse in the kettle lead for a 10 amp fuse too ?

hornetstinger · 27 Mar 2025 at 17:41

Molex to USB exists, but that would be 5v.

LuckyBenski · 27 Mar 2025 at 20:18

On the mains side your 70W power supply would be drawing less than 1 amp (power = current x voltage so 240 volts x 1 amp = 240 watts). So if switching the mains side, any rating should be fine.

I'd personally add the switching on the fans side, maybe even have a 5V/12V switch or some other speed adjuster. But I'm an electronic engineer - a no tools solution is great too!

Jamin280672 · 27 Mar 2025 at 21:14

LuckyBenski said:
I'd personally add the switching on the fans side, maybe even have a 5V/12V switch or some other speed adjuster. But I'm an electronic engineer - a no tools solution is great too!

This is what I was originally going to do, but as someone else stated, that would mean that the PSU would be on all the time, I was going to buy one of these pictured below, and intergrade it into the molex splitter cable yellow 12v line, just simply cut the 12v line, add each side to the wires coming out of this switch so only the positive is switched and just leave the negative direct connected, none of the 3 devices i'll be powering will be using the 5v (red) line so theres no need to switch it, its just going to be 2 x 200mm noctua 12v fans, no issue running these at 100% all the time and a D5 Vario pump also 12v, which can be speed adjusted on the pump itself.

This switch is 12v 6amp, the PSU outputs 12v 4amps, so its close enough.

So it would end up looking something like this, all the wiring can be hidden in the bottom of the radiator except for the switch which can be attached somewhere with 3M tape.