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help with fan control

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I'm building a simple fan control which only has 3 parts

NTC thermistor 10k Ω

50k Ω spindle trim potentiometer

MOSFET Power transistor (N-Channel)

The fan speed should go up as the temperature gets hotter but my fan does not turn at all.

but the funny thing is while heating up the NTC I got as high as 7.80V then it started to go back down again.

for the pot do all 3 leads need to be connected? because pin 2 (wiper) is connected to the MOSFET source and Pin 1 of the pot is connected to the MOSFET Gate.

The MOSFET drain is connected to the fan's (-) lead And the fan's (+) is connected to the NTC which runs to fan's(+)

I don't understand why it wont work
 
I'm building a simple fan control which only has 3 parts

NTC thermistor 10k Ω

50k Ω spindle trim potentiometer

MOSFET Power transistor (N-Channel)

The fan speed should go up as the temperature gets hotter but my fan does not turn at all.

but the funny thing is while heating up the NTC I got as high as 7.80V then it started to go back down again.

for the pot do all 3 leads need to be connected? because pin 2 (wiper) is connected to the MOSFET source and Pin 1 of the pot is connected to the MOSFET Gate.

The MOSFET drain is connected to the fan's (-) lead And the fan's (+) is connected to the NTC which runs to fan's(+)

I don't understand why it wont work

hi,
Please post the circuit diagram.:)
 
no the IRF510 N channel FET

*edit

I have 9omhs on the thermistor at room temp and 2 ohms heated up.


hi,
Thats the problem , they have very different Vgs turn ON curves.
 

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I would scrap that thermister for a high ohmic one at room temprature. Then use a voltage divider.
 

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Thats odd that he would say to get that part but wont work..

and radio shack does not have that other MOSFET I know for sure as i went to a few stores yesterday.

and RS lists the specs on the MOSFET here

MOSFET IRF510 Transistor - RadioShack.com

hi,
I have the data for both types of FET.

For example:
if the IRFZ24 has a gate voltage of say 4.5V the drain current is about 2amps.
The IRF510 with a gate voltage of 4.5V has a current of 0.5amp.
 
I would scrap that thermister for a high ohmic one at room temprature. Then use a voltage divider.

hi daniel,
Wouldnt putting a 'higher' resistance thermistor in the top end of the divider reduce the Vgs when using a 10K pot in the bottom end.???

hi 1234,


Use a 100K pot with IRF510, see if it improves the turn on voltage at the gate.
 
Last edited:
It would depend on the thermistor, but yes, you'd probably have to replace the pot.
 
It would depend on the thermistor, but yes, you'd probably have to replace the pot.

hi,
Did you look at the two Vgs graphs I posted, the two FET's have different conduction specs.
Both in threshold and VgIds slope
 
ah I made an error I had two connections backwards I had the fan 12+V going to the gate and not the thermistor connected to the 12+V

and the ground was not set to the wiper and gate it was set up as the source.

It works now :)

thanks for those that helped :)
 
Your thermistor is shorted. It is only 9 ohms. The project says to use one that is 10k ohms which is more than 1000 times higher.
 
Your thermistor is shorted. It is only 9 ohms. The project says to use one that is 10k ohms which is more than 1000 times higher.

I thought you had a made a typo, that's why I ignored it.:)

Lets know how the project goes.

heh I had a little too much birthday drinks ;)

so far so good I will add the second fan a bit later when I grab a bit more rest, works really well it pretty much wont come on until around 25-30c which is good

that pretty much means on cold mornings no need to blow air on an already cold heatsink backing up for 5 seconds.

when my other 15,000 rpm fan arrives in the mail (that's the one going in) I will do some more testing man that little 40mm fan is almost 50dbA loud!
 
My pc has fans that change their RPMs depending on the temperature (how hard the processor is thinking about things). They are big enough to spin fairly slowly (but still blow a lot of air) so they are fairly quiet.
 
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