Parallel DC Computer Fan Circuit Problem

[cyrusthevirus]

I have four fan connected in parallel. The fans each draw .12 A and operate at 12VDC. They are connected to a 12VDC .5A unregulated wall wart. The problem I am having is that when I connect the 3rd and 4th fan, all of the fans slow down. When I measure the voltage, without any fans I get about 17VDC. With fans 1 and 2, I get about 10 VDC. With all fans connected, I get about 8 VDC. Is the problem the unregulated wall wart? Do DC fans have a forward voltage? or is it because the wall wart needs to have a higher Amp rating?

Thanks for everyone's help.

 

[bychon]

It seems pretty obvious that the wall wart is providing lower and lower voltage as you increase the load. That does not mean the fans are bad. It means the wall wart is losing the struggle to provide proper current.

 

[cyrusthevirus]

I thought that was the problem, but before I went and picked up a new wall wart, I wanted to check with the experts.

 

[Sceadwian]

The voltage/current rating of the supply should supply more than that. Generally the 12V 500ma rating means that at 500 you get 12 volts. four of those fans should draw .480 amps so you should be getting almost perfectly 12 volts out of it not 8, probably just a dirt cheap transformer. I don't like wallwarts anymore, I tend to check around thrift store for old laptop switch mode supplies, and cell phone chargers from people that get rid of their old phones.

 

[cyrusthevirus]

One last question before I cut the end off the regulated wall wart I purchased. Is it possible to wire the fan backward? If so would it be obvious because it would spin backward? Would this impact the voltage used?

 

[bychon]

When I opened a computer fan, it had a bunch of electronics in it so it could run off DC and not have brushes. If you plug those in backwards, it probably won't run and might smoke. Wanna try it and let us know how it turns out?

 

[mneary]

There is a forward and backward.

If you wire it backward, the common brushless DC motor will not run, and might get damaged.

 

[MrAl]

Hi there,

If your wall wart does not put out the full 0.5 amps with all the fans connected perhaps you can try adding another capacitor to the output. The extra capacitor will usually increase the average DC output voltage in a rectifier/capacitor power supply. If it is loading too low mostly because of the transformers internal resistance the extra cap will not help as much. Note also that some DC wall warts do not have any capacitor at all inside, so it may help. No guarantee here though.

 

[cyrusthevirus]

Okay so the new wall wart kinda worked. I am using a 12 VDC 1A wall wart. The problem I am having now is that three fans will run fine, but four fans will not. That is unless I wait for 15-20 seconds before I connect the 4th fan. Any thoughts on how to overcome this?

 

[shaneshane1]

Hi, i would recommend a smoothing capacitor across your DC output, say 1000uf... cant hurt to try hey!

 

[MrAl]

Okay so the new wall wart kinda worked. I am using a 12 VDC 1A wall wart. The problem I am having now is that three fans will run fine, but four fans will not. That is unless I wait for 15-20 seconds before I connect the 4th fan. Any thoughts on how to overcome this?

Hi again,


Sounds like it could be the startup current, but then again i would not expect it to be that high.
Maybe someone else here has measured small fans as they start up and see what the current is.

What else you could do is measure your open circuit output voltage and then load your 12v supply down with a known resistance and measure the voltage again and report those two numbers (and the resistance) back here. We could tell more about what is going on that way.

When i do a quick calculation with a 12v transformer with Rin=1 ohm and RLoad=24 ohms and a capacitance of 3000uf i get 13v for the peak and 12.06v for the valley (in the dc output) but with only 1000uf i get a peak of 13.7v and a valley of 10.9v, so the cap does make a difference.
With Rin=4 ohms i get a peak of 10.9v and a valley of 9,1v, which is clearly under 12v now.
From this you can see that it would help to know your Rin which we can estimate if you take those two measurements.

Maybe you can do those two measurements and let us know...the resistor should be about 12 ohms or something like that but needs to be a power resistor (you dont have to keep it connected for very long though just to get one reading).

 

[shaneshane1]

I have a 12volt/5amp DC power supply and it trips at about 3amps... nothing i can do to get the extra 2amps i "paid" for... if nothing works you might have to get a larger PS (power supply) it seems you are getting "better" results with your new power supply... so fi the capacitor suggestions arnt working then you might have to get a larger current supply...

 

[MrAl]

Hi again,

If you take those two measurements we can tell more about your problem, or you can use the old wall wart for the fourth fan and the new wall wart for the other three

 

[cyrusthevirus]

Hi again,

If you take those two measurements we can tell more about your problem, or you can use the old wall wart for the fourth fan and the new wall wart for the other three

I will try the capacitor and take measurements tonight. I had already moved to the two wall wart option, but figured I would look for additional suggestions.

Do I put the capacitor between the two leads from the wall wart, but before the fans?

 

[shaneshane1]

considering it is an unregulated DC output have you tried limiting the current for each FAN? some of the fans might be drawing to much current... lets say the fans are rated at 12V and 120mA.. put a 100ohm 5Watt resistor across each fan (each FAN will draw about 1.44Watts)

 

[MrAl]

Hi again,


You can connect the capacitor across the DC output of the wall wart, probably anywhere, but near the wall wart is probably best.
It should be a significant size, say at least 3000uf and rated for at least 25 volts dc. Of course you have to get the polarity right or it could blow up the capacitor.

 

[cyrusthevirus]

Sorry for not clarifying, the new power supply is regulated and maintains 12VDC until the last fan is added. The fan I am using is a Silenx IXP-64-14.
**broken link removed**

 

[cyrusthevirus]

Well I got the answer to my problem. From tech support:

The spec sheet lists the rated current as .3A but the information that came with the fan lists .12A. Do you know which is correct? .3A is correct, some of the packaging out there has the wrong information as the packaging company made a mistake on it.

Damn. Time to purchase another PS.

 

[Sceadwian]

Should just use an old ATX power supply, they cost 20 bucks brand new and will drive dozens of fans.

 

[shaneshane1]

use a miltimeter... set it to Amps and put it in series with your fan! that will show you how much current the fan is drawing...test all the fans 1 at a time, some of the fans might be drawing more than others.