Automatic Cooling Fan Speed Control? Possible?

It sounds like you are looking for an NTC - Negative Temperature Coefficient thermistor.

Here is some info to look over.

Larry

An automobile radiator mounted sensor delivers power (grnd)  to the fan  at X°(some are variable) About the size of a quarter x 3/8"?.

So a two speed setting depends a lot on motor variable speed ability; but...

Use 60-80ohms feeding the fan outright. Once at X° the thermo delivers full power downstream of the 60-80ohm; (at the fan bridging over the 60-80ohm ....or over other voltage reduction method 🤔

In other words; pos. thermos may work too if the circuit can be done "2spd".(or 3sp 4sp, etc) (yep-Thermistors , they come in MANY forms, probes to board mount)

???

Here's a simple circuit that's purpose build to cool a power supply.

Temperature Controlled DC Fan

Although this doesn't vary the fan speed, is that really necessary?  That seems like overkill for a simple power supply.

You might look at the KSD9700 temperature switches available on ebay, available in different temperatures. I stole the idea from one of Stan's posts. I forget what it was about, but was obviously about doing something at a desired temperature. 

Tom, those are also "bang-bang' control of the circuit creating the high temperature, not the fan.  They turn off when the limit temperature is reached, and are restored when the temperature drops sufficiently below their rated temperature.  With these, the fan would be turned off when the item got hot, not exactly the desired operation!

KSD-9700 Series Thermal Protector

Thanks for all the ideas so far.  I searched on ebay and found a line of KSD301/302 temp switches. 301's are normally closed whereas 302's are normally open. They are available in 5 deg C increments from 40C to 160C.

So I plan to order a couple of 302's each, at 40C and 50C. They are simple inline hookup, 10 Amp rated, surface mount, with spade lug connectors, and only about a buck each. Perfect for what I need. Simple on-off switching, but that should be fine as grj suggested.

Thanks all, Rod

gunrunnerjohn posted:

Tom, those are also "bang-bang' control of the circuit creating the high temperature, not the fan.  They turn off when the limit temperature is reached, and are restored when the temperature drops sufficiently below their rated temperature.  With these, the fan would be turned off when the item got hot, not exactly the desired operation!

KSD-9700 Series Thermal Protector

Hmmm... You are exactly right. That datasheet is not what I was thinking they did. 

I was looking around at this stuff on ebay yesterday. Must have either gotten my devices mixed up or mis-understood the translation to English in the descriptions?  

But wait, here's what I was going by, ebay item 183364095688 says they come in either N.C. (break on temp rise) or N.O. (make on temp rise).  Some of the ads list them as available in either configuration from a drop down selection when ordering.

They come in both styles, NO and NC.  For this application, he'd want the NO version that would be on when the temperature exceeds the rating of the device.  The NC models are to protect a device against over-temperature and to cut power when the temperature exceeds the rating of the device.

Yes, the NO version is what I was thinking of. About everything else I found was 3 or 4 wire PWM fan controllers on a PCB. 

Thanks to rtr12 for pointing me in the direction of several pre-made pwm control boards available on ebay. For others who may be interested, try a search for "pwm fan speed controller" or something similar, and you will get all kinds of hits. 

Rod