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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Hi all, I am trying to come up with a small temperature controlled fan circuit for the thermal management of a project I am working on. So far I am just using the old comparator method which turns the fan on and off depending on the state of a thermistor.
Here is a basic representation of what I currently have.
Now that works fine but I would prefer having the fan come on at a high temperature and then turn off at a much lower temperature. Does anyone know how this could be done?
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Yes - make sure you use hysteresis in your design. Simulating in PSPICE is a good way to see how the hysteresis works with your particular project / thresholds. 5 degF of hysteresis is probably good.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Just for others to note: I think you've got your comparator connections flipped if you are indeed using a NTC resistor. If it works I'm assuming you hooked it up correctly in your actual circuit.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Billybobjoe wrote ...
Just for others to note: I think you've got your comparator connections flipped if you are indeed using a NTC resistor. If it works I'm assuming you hooked it up correctly in your actual circuit.
Opps, you are correct they are flipped. Here is the corrected one.
EasternVoltageResearch wrote ...
Yes - make sure you use hysteresis in your design. Simulating in PSPICE is a good way to see how the hysteresis works with your particular project / thresholds. 5 degF of hysteresis is probably good.
Thanks EVR. 5 degF sounds about right to what I have set it to. With out hysteresis the fan would come on for about 10 seconds and then turn off for the same amount of time, whilst this technically did the job it was quite annoying acoustically.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You can go super low-tech with just a thermal switch and fan e.g. My last hobby temp.controlled fan/heatsink I used a threaded thermistor passing about 1mA through the thermistor (<80C <600R, hotter >10k) across base-emitter of a transistor as a switch, emitter=ground and collector to load. I used it as an alarm/shutdown but it (or similar) could just as easily control a fan, don't forget, collector current=load=Fan current <= 1mA x current gain of transistor maybe a darlington with c2mA would be better for a fan controller.
Mechanical/Thermal switches have intrinsic hysteresis, adding hysteresis when the sensor has none is always a good idea, in this case the natural thermal inertia time constant of the fan/air/heatsink ( heating with fan off and cooling with fan on, with potentially varying heat input ) would probably be in the seconds range ... quite annoying. (hence variable speed fans)
Any transistor (e.g. npn with collector-base short) that can be fixed (thermally bonded) to the heatsink can be used as a temperature sensor (diode law) if you use the same transistor type as a 'cold junction' (pass about 0.1 to 1 mA ea.) you should get near zero voltage difference between the two diodes with heatsink at room temperature, the heatsink diode voltage will drop as temperature increases ..... I only mention it because physically attaching thermistors to heatsinks needs some thought, small transistors that can be screwed/clipped/etc to your heatsink are probably in your 'spare parts', if not very cheap. Using a similar transistor as the cold junction means it can dissipate 0.1mA at 600mV with negligible self-heating, or easily 'attached' to ambient (grounded metalwork, casing etc.) For a one-off comparing heatsink diode voltage with a reference voltage is good.
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Turning fan off at lower temperature?
