Connecting fans in series or paralel?

Projects General Guidance
1

Hello,

I have a project where i use logic level mosfet turn on or off 7 computer fans.
These fans are simple computer fans that work off of 12V.

Now i have the fans connected to headers, and the headers are connected in parallel in groups of 3 and 4. then they are both connected to ground and the mosfet. I have attached a picture explaining this. SO essentially they ar all connected in paralel.
Now i have a 12V supply, that powers the fans. When only one fan is connected to any of the headers it works fine, however, when i start adding more fans the speed of the fans drops. Can anyone explain this?
Also, it seems that if i increase the voltage to say 18V, then bring it back dows to the same 12V the fans seem to run faster, than if i would just went from 0V to 12V.

So how do i get 12V to all of my 7 fans while using a mosfet to turn them on and off?

Thank you.

forgot to mention, the mosfet i am using is IRLZ34N

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2

use a voltmeter and check how many amps go through the fans if you increase the number from 1 ..4
Also check if the voltage drops when you increase the number of fans,

Then you will see why it happens..

3

When i have one fan connected the pins stay at 12v, when i have two its around 9V, when theres3 its at about 7.5V the strage thing is that the voltag seems to be slowly climbing up. for example when i have 2 fans the voltage starts at 9V but in about 15seconds its at 10V.

Why is this happening? Do i need another kind of mosfet for this?

4

also the same happens with the current through the fan, it drops when i connect more fans.

5

onix45:
When i have one fan connected the pins stay at 12v, when i have two its around 9V, when theres3 its at about 7.5V the strage thing is that the voltag seems to be slowly climbing up. for example when i have 2 fans the voltage starts at 9V but in about 15seconds its at 10V.

What you see is that the electrical resistance of the fan changes with RPM combined with initial friction.

this is a similar effect when you try to get a car rolling. You need to push hard to get from standstill to rolling,
but after it rolls it costs less energy to keep it rolling.

6

Ok so i have found out that my arduino pin only gives 2.1V when high. That keeps the mosfet from fully opening i believe.

Why is that?

Its pin 36 on arduino mega 2560. I also have 7 servo motors attached to pins 22,24,26,28,30,32,34.

Could the servo library somehow change my pins output because of the ammount of servos conneced, or the particular pins i chose?

7

2.1 V is quite low!
How is your mega powered?
An Arduino can only give 10-20mA per pin IIRC (check datasheet)

PLease post your schematic.

8

The mega is powered through a buck converter that gives me 5V from whatever input i have (i wire the input directly to the fans so that i can choose the fan speed by adjust the voltage, while the buck converter makes whatever is at the input into 5V for my arduino, servos and so on)

However i have found the mistake. First of let me say that i am sorry that i have wasted everyones time, however it did give me some new knowledge.

The problem was that i did not define the pin as output, so it only gave 2.1V as high, and so the mosfet did not fully turn on.
I have written my code a long time ago and so i assumed i k\made it right, goes to show how you need to check everything from the most basic things before jumping to conclusions.

Thanks again.

9

Please post your working code for future reference (than the time is not wasted after all :wink:

10

it appears your power supply is not capable of supplying the amperage for the fans.

take the modfet out, bring the fan ground to the power supply ground. run the fans directly. check the voltage.

if your fans are overloading the power supply, your power to the Arduino may be dropping as well. resulting in lower voltage out.

Use two separate power supplies.

11

onix45:
I have a project where i use logic level mosfet turn on or off 7 computer fans.
These fans are simple computer fans that work off of 12V.

Very strange. The mosfet you chose is just fine to use in that circuit. The Arduino can certainly drive the gate (in fact, the 100 ohm resistor in the gate circuit isn't needed - although that's not your problem).

I see no reason why you should be having this problem. Is your power supply staying at 12 volts at full load?