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  #1  
Old 08-09-07, 03:31 PM
Uncle_Fluffy Uncle_Fluffy is offline
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Posts: 48
DIY 7v switch.

Hi guys,

I'm wanting to make a switch that switches between 12v and 7v for the 3 fans on my rad.

I know there are 2 methods, I can either use an inline resistor/s to drop the voltage or the less refined method of using the 5v line as the negative.

I've done all teh calculations for the first method and would need a 28.5 Ohm resistor that can take 0.875Watts. I have not been able to locate such a resistor or combination in series that can meet both requirements.

This leaves me with the down and dirty method of using the 5v rail. I have heard that his can cause the death of some PSUs and i cant afford to replace muy new Infiniti.

Has anyone used this method and how great is the risk to mu PSU?

Any advice would be great!

Thanks.

Ian.

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  #2  
Old 08-09-07, 06:20 PM
Dav0s Dav0s is offline
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i blew up my psu by doing it the dodgy way, if you do it that way, make sure you ground it or something.
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  #3  
Old 09-09-07, 10:18 AM
Uncle_Fluffy Uncle_Fluffy is offline
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That's exactly what I'm wanting to avoid.

Need to find a good source for resistors!
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  #4  
Old 09-09-07, 10:38 AM
FarFarAway FarFarAway is offline
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SpecialTech sell them for fans I believe mate, failing that Kustom PC's do
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  #5  
Old 09-09-07, 01:25 PM
Uncle_Fluffy Uncle_Fluffy is offline
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Yeah, I see what you mean.

Those cables will work for 1 fan, but 3 draw three times the current and will need a lower resisitance. They would burn out with the 3 fans

Looking on Maplin I have found 10Ω and 18Ω resistors so I could put them in series to get the required resistance but they are only rated at 0.6W. My thought was to then have the same again in parallel with the first.

I think this would give me the wattage rating of 1.2W but maintain the resistance at 21Ω?

If someone could confirm this or set me straight I'd be mighty grateful!

:EDIT:

I think I've solved it! I found a calculator that works out the total resistance for resistors in parallel. It shows that my original theory was complete pants!!

I'm now awaiting a resistor pack from Maplin

Ian.
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