plugging in fans on +12V to +5V on molex to get 7V

[medbor]

Hi

My usual tool for lowering noise in my computer is to connect the fans red wire to +12V and the black to +5V, but i've realized that it will create current in the wrong direction on the 5V rail (or maybe i think wrong?).

Q1:I do this since the airflow is next to none on 5V and too loud on 12V, but will it damage anything?

 

Q2:And also is my motherboard reporting voltages wrong or can my computer really function normally with +12V at 9.55V? (i even switched PSU)

 

Q3:And my temp readings according to OCCT on my processor cores vs EVEREST is 10C. Right now CPU=41C and cores=(25,23,21,21)C in EVEREST, but in OCCT they are cpu=41C and cores=(35, 33, 31, 31)C. BIOS only show cpu temp, but even in OCCT the temps don't seem right?

 

Q4: How much PSU do i really need for a q6600 (G0 stepping) with a HD4830 and 3 HDDs? (I used the recommended calculator but it said 400W and that seems low i think)

[Waco]

My usual tool for lowering noise in my computer is to connect the fans red wire to +12V and the black to +5V, but i've realized that it will create current in the wrong direction on the 5V rail (or maybe i think wrong?).

That's correct - it'll push current back into the 5 volt rail which can cause problems. Most newer PSUs can detect this and won't run with too much back-current.

 

A fan controller is the right way to do it.

[medbor]

That's correct - it'll push current back into the 5 volt rail which can cause problems. Most newer PSUs can detect this and won't run with too much back-current.

 

A fan controller is the right way to do it.

But fans rarely pull even 0.5A and on 7V even less, so is this really a problem?

Could this perhaps screw with the 12V readings?

[Waco]

The backfeed can damage a PSU if you aren't pulling more on the 5v rail than the amperage you're pushing back through it. Modern machines don't really load the 5v rail all that much so it's a LOT safer to simply run a fan controller.

[paulktreg]

Opinions vary on this subject and it's probably true that power supplies don't like having current fed back into the rails but wiring fans in this configuration must of been done hundreds of times without any reported problem, I've even done it in the past. I can't say for sure whether or not it will damage the power supply but if more current is leaving the 5V rail than is entering then I can't see a problem. A 12V fan running on 7V will pull about 0.2A and that's considerably less than the current that will be leaving the 5V rail.

 

A "safer" way would be to fit a series resistor in the 12V supply to the fan but you'll need to calculate its value as described here.

 

Hope that helps?

[danieljury3]

An even simpler way would be to wire in a 5.1V Zener Diode. The ones in stock with my local electronics store are rated at 1 watt which at 5V is about 0.2A. A look through the catalog of a larger suppler yielded a 5.1V Zener rated for 5W which would give you almost 1A which would be much better suited to your purpose.

The only problems with doing it like this are that the power is dissipated as heat so you would be wasting a couple of watts of power as well as creating unnecessary heat though the resistor solutinon has the same problems.

The best solution would be to get a fan controller as it would contain more efficient power regulation circuits.

If you want the part number of the Zener I found its 1N5338BG

[medbor]

Opinions vary on this subject and it's probably true that power supplies don't like having current fed back into the rails but wiring fans in this configuration must of been done hundreds of times without any reported problem, I've even done it in the past. I can't say for sure whether or not it will damage the power supply but if more current is leaving the 5V rail than is entering then I can't see a problem. A 12V fan running on 7V will pull about 0.2A and that's considerably less than the current that will be leaving the 5V rail.

 

A "safer" way would be to fit a series resistor in the 12V supply to the fan but you'll need to calculate its value as described here.

 

Hope that helps?

The voltage over the resistor and the current through it would just be converted to heat, isn't that bad in other respects?

right now i have 2 80mm and 2 120mm fans connected in this way. Total amperage if they where on 12V would be about 2A, but i have 3 HDDs, and they pull quite a lot of power on 5V according to this review:

http://www.xbitlabs.com/articles/cases/display/system-wattage_5.html#sect0

but it might be that is because of the RAID 0?

 

An even simpler way would be to wire in a 5.1V Zener Diode. The ones in stock with my local electronics store are rated at 1 watt which at 5V is about 0.2A. A look through the catalog of a larger suppler yielded a 5.1V Zener rated for 5W which would give you almost 1A which would be much better suited to your purpose.

The only problems with doing it like this are that the power is dissipated as heat so you would be wasting a couple of watts of power as well as creating unnecessary heat though the resistor solutinon has the same problems.

The best solution would be to get a fan controller as it would contain more efficient power regulation circuits.

If you want the part number of the Zener I found its 1N5338BG

I had a fan controller (some variant of Zalman fanmate) and that was basically a transistor (which ran rally hot, it even had a heatspreader) and a potentionmeter. i know that you mean a better fan controller than that, but still the technology could be the same or am i wrong?

Isn't Zener diodes basically the same as putting a resistor (except that it has a constant voltage over it)?

[danieljury3]

I once built a simple power regulator out of a small transistor, diode and resistor I had lying around and the transistor ran too hot to touch. It might have been because I was using a small signal transistor rated at a couple of milliamps instead of a power transistor like I should have been using.

 

Also a diode would have the same effect as a resistor but you wouldn't need to make any calculations to find the correct resistance to use. The type of fan controller I had in mind was something more along the lines of something you shove into a spare drive bay.

I would get a better power supply (unless you already have the 400W power supply) because a 500W or 600W power supply probably wont cost much more.

 

This is the 3rd time I've had to write all this out, kept accidentally hitting Ctrl+W which kept closing this tab.

[medbor]

I once built a simple power regulator out of a small transistor, diode and resistor I had lying around and the transistor ran too hot to touch. It might have been because I was using a small signal transistor rated at a couple of milliamps instead of a power transistor like I should have been using.

 

Also a diode would have the same effect as a resistor but you wouldn't need to make any calculations to find the correct resistance to use. The type of fan controller I had in mind was something more along the lines of something you shove into a spare drive bay.

I would get a better power supply (unless you already have the 400W power supply) because a 500W or 600W power supply probably wont cost much more.

 

This is the 3rd time I've had to write all this out, kept accidentally hitting Ctrl+W which kept closing this tab.

I already have it (found it in the trash actually), but i will replace it as soon as i found one of my liking

Oh and for that type of controller i would have to pay like money, and it seems just so easy to do it this way

did you know that hardrives fit almost perfectly in 5.25" bays sideways, thats how one of my harddrives is mounted

Edited December 5, 2010 by medbor

[GabrielT]

Just buy a Lian Li 3 speed fan controller. They are $10 and handle up to 11w. Basically a 5v, 7v, and 12v setting end of story. You may need to by some 3 pin wire extensions though.

[danieljury3]

The fans that came with my case have switches on them. My 200mm Antec fan came with a 3-speed switch and the 120mm fan came with a 2-speed switch and I don't like either of them but that's because I want to have more control over them.

[medbor]

Just buy a Lian Li 3 speed fan controller. They are $10 and handle up to 11w. Basically a 5v, 7v, and 12v setting end of story. You may need to by some 3 pin wire extensions though.

Thanks for the nice find! Although that is just resistors and a switch really, its in a useable form so i will definitely look for it in sweden