help with fan control

1-3-2-4 · Nov 27, 2008

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

ericgibbs · Nov 27, 2008

1-3-2-4 said:
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.

1-3-2-4 · Nov 27, 2008

it's listed here..

**broken link removed**

ericgibbs · Nov 27, 2008

1-3-2-4 said:
it's listed here..

**broken link removed**

hi,
Are you using the IRFZ24N FET.?

1-3-2-4 · Nov 27, 2008

ericgibbs said:
hi,
Are you using the IRFZ24N FET.?

no the IRF510 N channel FET

*edit

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

ericgibbs · Nov 27, 2008

1-3-2-4 said:
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.

1-3-2-4 · Nov 27, 2008

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

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

danielsmusic · Nov 27, 2008

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

ericgibbs · Nov 27, 2008

1-3-2-4 said:
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.

1-3-2-4 · Nov 27, 2008

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

but then I would also need to change my potentiometer as well right?

ericgibbs · Nov 27, 2008

danielsmusic said:
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.

danielsmusic · Nov 27, 2008

It would depend on the thermistor, but yes, you'd probably have to replace the pot.

ericgibbs · Nov 27, 2008

danielsmusic said:
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

danielsmusic · Nov 27, 2008

Yes, I did. You could vary the pot to compensate, though.

1-3-2-4 · Nov 27, 2008

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

audioguru · Nov 27, 2008

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.

1-3-2-4 · Nov 27, 2008

it's not shorted it's 9kohms

ericgibbs · Nov 27, 2008

1-3-2-4 said:
it's not shorted it's 9kohms

I thought you had a made a typo, thats why I ignored it.

Lets know how the project goes.

1-3-2-4 · Nov 27, 2008

audioguru said:
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.

ericgibbs said:
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!

audioguru · Nov 27, 2008

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.

help with fan control

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