Wiring fans in series for lower volts?

[punosion]

I read a few suggestions in various posts about wiring fans together in series, and running them off of a 12v line from the power supply. This way, if you have two 12v fans, they would both run at 6v, and you wouldn't be unnaturally taxing the other rails of the power supply.

How is this done? Not sure of the specifics, as I wasn't able to dig up a clear answer...is there anything I should keep in mind if I wanted to do this?

Thanks!

 

[Daemonfly]

Well, since most fans have a red wire for 12v+ & black wire for ground:

Red lead from FAN1 is connected to yellow +12v wire from PSU.
Black lead from FAN1 is connected to red lead from FAN2
Black lead from FAN2 is connected to black gound lead from PSU.

This would give 6v on both fans, considering a solid 12v from PSU.

 

[punosion]

Cool, I was figuring it was that easy. This won't run one of the fans in reverse or anything crazy like that, will it?

 

[lazerin]

Cool, I was figuring it was that easy. This won't run one of the fans in reverse or anything crazy like that, will it?

Nope it won't. But if you have a big fan, reduced voltage may prevent it from spining up correctly

 

[D $hady]

OK, please help me out if this is wrong. It's been a few years since high school physics. Ohm's law (me thinks) says V(voltage) = I (current) * R (resistance). So if you double the resistance (two fans in series) you're really reducing the current by half, since the voltage is a constant (assuming the fans are the exact same). Is this right? And would fans running at half the amperage behave the same as fans running at half the voltage?

Ack.. sorry this came out more complicated than I wanted it to. Just curious about the answer. Maybe I'll just have to play with it and see.

 

[Cjwinnit]

Remember to use identical (or similar) fans......

Example: a 4W fan and a 2W fan in series..

The 4W fan has half the resistance, and so the 4W fan will have 8 Volts across it, and will dissipate 16/9 Watts.
The 2W fan will have 4 Volts across it and (if it starts up at all) will only dissipate 2/9 Watts.

 

[macklin01]

Thanks -- I was just going to bring that up. So long as the fans are identical, they'll both run at 6V. If one has a higher load than the other, than it will take a higher percentage of the total voltage when wired in series. CJwinnit has the proportions just right, and it should answer D $hady's questions.

BTW, welcome to the forums, D $hady! -- Paul

 

[LuckyBob]

you're really reducing the current by half, since the voltage is a constant (assuming the fans are the exact same). Is this right?

No, 'tis not correct in this case You'd be correct if he hooked the fans in parallel; voltage stays the same in parallel, while current stays the same in series.

 

[macklin01]

Resisting urge to pull out electronics books and goof off instead of writing code .... must resist soldering iron!

Thanks for the refresher, LuckyBob. -- Paul

 

[CrashOveride]

wow... you guys may have solved the problem i have iwth my loud PSU... i cant believe i have never seen a thead on this... or though of it yself.. lol

 

[macklin01]

Ah, a field test. Let us know how it goes! -- Paul

 

[Cjwinnit]

No, 'tis not correct in this case You'd be correct if he hooked the fans in parallel; voltage stays the same in parallel, while current stays the same in series.

In the example you are both correct for different reasons.

So if you double the resistance (two fans in series) you're really reducing the current by half, since the voltage is a constant (assuming the fans are the exact same). Is this right?

The potential difference across the fan system (both fans) is still 12v, whether they are in series or parallel. If they (two similar fans) are in series then each fan has a potential difference of 6v, both together have 12v.


With two identical fans in series, each dissipates a quater of the rated power.

Dont forget that if the fans are different, then one of them will have a potential difference below 6v, and one will have a potential difference above 6v. If then fan below 6v spins then you are ok.
I was thinking about this myself but this is what put me off a little bit.

 

[CrashOveride]

i wont be doing it too soon... kinda busy... HW and all

 

[macklin01]

@Cjwinnit:
Hmm, and if the one fan that's below 6V stops spinning, doesn't its current draw spike?

@Crash:
I totally understand. (finals time?) We have finals and research colliding over here. fun, but yuck! And CompUSA has run out of Bawls 12-packs! :grr:

-- Paul

 

[Cjwinnit]

@Cjwinnit:
Hmm, and if the one fan that's below 6V stops spinning, doesn't its current draw spike?

-- Paul

Not sure about it, I really meant as the computer is starting up. That's really something that needs to be checked with a multimeter and a half-hour spare in the lab

 

[macklin01]

Ah, as in failing to start up at all. (Stall speed < startup speed) Good point -- I hadn't thought of that. When I designed my fanbus, I actually designed it so that the voltage couldn't drop below roughly 7V on each fan channel for that very reason.

-- Paul

 

[a c i d.f l y]

Ironic you'd ever think I'd need that formula from DC/AC Circuits back in highschool... *shocked*

Its actually weird how much math I'm seeing on these forums.. some people have too much free time.

-Frank

 

[CrashOveride]

@crash:
I totally understand. (finals time?) We have finals and research colliding over here. fun, but yuck! And CompUSA has run out of Bawls 12-packs! :grr:

-- Paul

nah... just lots of HW tonight...


EDIT: well i decided to try it but couldnt cuz of the way i had my fan wires... i didnt have access ot both sorry i just 5 volted them adn lowerd the speed of my adjustable oens... sounds like a dell now... all quiet

 

[D $hady]

Macklin1 thanks for the greeting. And Bob thanks for setting me straight It's sad how in just a few years I've forgotten so much. I'm glad other people remember this stuff (or maybe you guys use these physics at work??) Either way it's nice to have so many people with so much knowledge about all of this.

-- Dave

 

[DarkJediSleikas]

With two identical fans in series, each dissipates a quater of the rated power.
B]

That is correct, as long as both fans have the same power rating.

I wouldn't recommend putting two fans in series for this reason. Unless those fans are Tornados, there is no need to run a fan at such a low power.