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Old 02-25-2009, 03:46 AM
Buffalobiian Buffalobiian is offline
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Default 12v to 7v fan reducers - do they harm power supplies?

I've been looking at undervolting my fans at 7volts to run them more quietly.

The method for achieving this can be shown here:

Basically, you wire the fan so that current comes from the +12V into the fan, and out to the +5V connection instead of GROUND, creating a 7V potential difference.

However, the SPCR warns that when feeding current back into the 5V lead, you have to make sure that other components use more-than-or-equal-to that amount of current to avoid feeding any current back into your power supply.

I've looked around at fan reducers you can buy as well, and they generally fall into 2 categories:

1) Resistor based reducers: use a resistor to chew up voltage, leaving you an undervolted fan


2) Non-resistor based reducers: effectively, these do the same thing as the above article's method, wiring up the 12V and 5V leads to create a 7V difference.

Example of item: (note: very last item at the bottom of the page)

Thing is, these fan reducers don't come with any sort of warning whatsoever about feeding current back to your 5V line, or making sure other components are using more current, despite being the exact same thing.

My questions are:

1) Are the two methods (7 volt "trick" as per first article and the 12V to 7V reducer as per second link) the same thing?

2) If so, is there any worries about current feeding back into my PSU if THERE ARE NO COMPONENTS UTILIZING THE +5V RAIL. <- for argument's sake, let's assume that's true for this question.

3) If the answer is YES for (2), do I have to make sure that there are components that consume sufficient current on (i) the entire +5v rail or (ii) the molex cable which the fan adapters are attached to.

Note: I'm considering wiring up 3 Nexus fans in said method. The fans are rated at 0.3A max each, through testing shows them to use 0.62W @ 7v, meaning roughly 88.6mA.
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Old 02-26-2009, 12:52 PM
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RAM GUY RAM GUY is offline
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I have never heard of this causing any problems with the PSU, however this is not something that we have tested, nor is it anything that I have personally tried so I really couldn't guarantee anything. There is always going to be some sort of load on the 5v rail as long as you have the PSU connected to a motherboard.
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