Once again, the 555 PWM question...

Thread Starter

iONic

Joined Nov 16, 2007
1,662
I'm looking for a simple way to control the duty cycle of a 555 that will eventually drive a 12VDC, 400mA Computer fan based of the ambient temperature. I thought of the LM34, but the output is a temperature to voltage relationship. A thermistor might work, but I am a am stumbling on how to setup the 555 to accomplish a wide range of duty cycle. I want it to reach ~100% duty cycle but am not sure on how low the duty cycle will be needed to keep the fan spinning yet. I also want some sort of Hysteresis as I do not want the fan turning on/off all the time.

Any idea's on how to setup the 555 for this sort of arrangement?


Thanks
 

Wendy

Joined Mar 24, 2008
23,415
A simple thermistor making a variable voltage comes to mind. You could shape the temperature response curve (such as slope and endpoints) with op amps, or possibly use the thermistor by itself.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,662
A simple thermistor making a variable voltage comes to mind. You could shape the temperature response curve (such as slope and endpoints) with op amps, or possibly use the thermistor by itself.
In other words, replace the variable resistor with the thermistor. This in itself would not give a on/off delay (hysteresis) effect, would it? how might I achieve this effect?
 

SgtWookie

Joined Jul 17, 2007
22,230
I used a thermistor as part of the voltage divider for the feedback input to an LT1171 switching regulator IC. I set it up so that the output of the switcher had a range of 7v-11v under the expected operating temperature range, which was within the fans' specifications.

The fan is a 92mm x92mm 85cfm fan that draws around 800mA with 12v in, and it's mighty noisy when running full steam - but it moves a LOT of air. Running at 7v, it's barely audible; moves just enough air to keep it flowing over the thermistor. If the CPU starts heating up, the fan automagically compensates.

I went that route rather than PWM because computer fans are BLDC (brushless DC) rather than brushed (they used to be, many years ago - and broke quickly.) Trying PWM on them probably won't work terribly well.

Don't have my schematic handy; I've been away from home for a couple days.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,662
Then the LM34, having a linear voltage output could be used??

Sgt. I could probably do a similar thing as the fan does function even at 5V and PWM may not be necessary. Just a comparator with Hysteresis to vary the voltage maybe.

I have bee out of town also and just had my best buddy (my dog) put down. He had cancer(sarcoma).
 
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