I Blew up Two 12V CPU fans.

[thirtyonemessier]

Hi All,

I had a 24V volt source to power on two 12V CPU Fans. I connected them in series but ended up bricking the fans.

I thought of it in terms of a voltage divider circuit, because I assumed they must be offering the same resistance:
Vo = Vs * (R2/R1+R2)

for illustration purposes, lets assume the resistance of each fan was 100ohm. Then:
12v = 24v * (100/200)

Each Fan would have got 12V but it fried the circuitry of both fans.

Can someone explain to me what stupid mistake I have done?

The fans are rated 12V 3W, i.e, 0.25A.

If I consider it to be a closed circuit, by KVL:
Vo = IxR1 + IxR2
Vo = I(R1+R2)
24v = I(100+100)
I = 0.12A

Here I < 0.25, the current drawn from the source is still less than what it should have drawn (0.25A). So why did the fans go bad?

 

[KlausST]

Hi,

they most probably are electronically regulated, thus they don´t behave as passive motors anymore.
--> you can´t connect them in series.

...or you add some circuitry to keep the center voltage at about half of total voltage.

***
After thinking about motors.. I assume even with non_electronic motors a series connection won´t work:
Let´s assume two identical motors, running with perfect matched situation in series. Everything is fine (at least in theory)
Now put some mechanical force at one motor: --> it will cause the motor to become lower ohmic. (on a constant current now the motor draws more current)
But with series connection the center voltage drifts in a way that the motor gets less voltage and the other (unloaded) motor gets higher voltage.
I is identical for both motors.
Now have a look at the power: The loaded motor draws less power (P = V x I, it should be the opposite) thus it´s RPM goes down while the other motor speeds up... with overly increased voltage.

Klaus

 

[betwixt]

If you want to repeat the exercise without blowing them up, add a 13V Zener diode across each fan before wiring them in series. If the fans draw equal current the diodes will do nothing but if one draws more than the other, the diode will take the strain. I assume the fans draw a steady current, if it changes as the fan rotates (cyclic by revolution) it might help to add some capacitors across each fan too. Something like 220uF to smooth out the bumps in the current waveforms.

Brian.