# Choosing the right NTC

##### Ash
Feb 9, 2013
424
Hi Guys,

I have a 12VDC fan hooked up to a Pwm module, which is controlled using an NTC. Shown in this link: https://www.ebay.co.uk/p/12v-PWM-PC...le-High-temp-Alarm/835782064?iid=302358014128

After adjusting the trim pots the Dc fan stays off below 25 Degrees C (room temperature) which is perfect. Then when the Xbox turns on the fan rotates. However, the fan isn't as fast as I'd hoped.

So I'm hoping to find a 50k NTC, with a lower b constant or something to replace the 50k NTC 3950 provided. Any ideas what I should get?

Thanks,
Ash

#### kellys_eye

Jun 25, 2010
6,514
How do you determine the fan isn't spinning fast enough? Did you measure the heatsink temperature to prove the fan speed is sufficient to keep it within the range required? Or are you just basing your assumption on 'the speed'?

The system should (ideally) be designed to maintain the heat sink at a pre-determined level (say 45C) and the hotter it 'tries' to go the faster the fan spins to keep it at 45C.

Unless you actually measure the temperature you can't be sure that the low speed it appears to be running at isn't the CORRECT speed.

##### Ash
Feb 9, 2013
424
How do you determine the fan isn't spinning fast enough? Did you measure the heatsink temperature to prove the fan speed is sufficient to keep it within the range required? Or are you just basing your assumption on 'the speed'?

How.. Because it's too hot and I can feel it with my hand. Plus based on the speed of the fan. I wouldn't call this an assumption, but an educated guess.

However, I've just taken some measurements to give you an idea. The NTC measures around 53k at 23 degrees C my current room temperature and the fan doesn't spin . When the Xbox is switched on the ambient temperature in my TV unit rises to about 30 degrees C and the fan spins at this point, but at minimal speed.. It's right on the edge of stalling about 3V I think, I can't get the probes in to measure it.

So I've obviously set the module to keep the fan off when it's not needed. I don't want to waste energy. But that means it won't accelerate anymore even if the Xbox reaches 45 degrees or more. I'd like it to keep the ambient temperature in the TV unit at 40 degrees.

I could do with an NTC lowers the resistance more exponentially with the increasing temperature. So I can set the trim pot to keep the fan off.. While having a low enough resistance at high temperature to keep the fan at a nice speed.

P.S I must add that even though I said the temp increased to 30C, I wasn't playing any games and it usually gets hot quickly.

#### dorke

Jun 20, 2015
2,342
Look at the description of the module.
It has two set points acceleration temp and full speed temp.
Have you tested them both?

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