I want to control its speed based on the same kind of termistor that is already used within the beamer itself (I recovered the respective termistor from a defective beamer that I am using for spare parts: it is a 50kOhm resistor while cold and the resistance falls towards zero when the termistor is heated).
To keep the ventilator in a "quiet" range I want to power it with between 3V and 6V (i.e. 3V while the beamer is cold and up to 6V while the beamer gets hotter).
What's a "beamer" ? (BMW ?)
Unless I have misunderstood you , the explanation for what you describe is simple. The pot on the buck converter that sets the output voltage is meant to be set at a SPECIFIC INPUT voltage WHICH IS NOT TO BE CHANGED. Every time you change the input voltage , you have to recalibrate the converter for the new input voltage. You cannot simply set it at one input voltage and then change the input voltage and expect it to maintain some relationship of input to output voltage. Did you READJUST (recalibrate) the converter output voltage AFTER you CHANGED the INPUT voltage ?
I don't understand the correlation between a thermister and the output voltage trim pot on the buck converter which has no thermister. Are you saying you are "HACKING" the converter to change the output voltage ? (by adding a thermister ?)
IF this is the case, then I SERIOUSLY recommend you CHANGE your post title in your ORIGINAL post to:"Hacking Buck Converter for temp dependent output"
yes, that's what I explained in my original post: I remove the trim pot and replace it with a thermister (and whatever additional resistors or other components might be needed..) - so that the thermister will will perform dynamic trimming at runtime..done.. I had chosen the original title since my problem would also be solved if a circuit exists that allows to construct "a resistor R1" such that R1= 50kOhm - resistance of thermistor
A better idea, and since we are in the Aruduino forum, would be to use an Arduino to measure the fan speed or temperature and provide the exact programmed speed profile you are after. A temperature sensor would probably be all that you need, besides a driver of some kind.
Is there some circuit that allows me to "invert" the thermistor's resistance? Or is there a way to patch the buck converter accordingly (mb cut the pin1 - pin3 connection)? What are the currents that a thermistor would typically survive - is it likely to be damaged by the current drawn by the ventilator?
Can I suggest you stick with the original (NTC) thermistor. BMW obviously designed the fan to be adequate to cool the engine sufficienly under all conditions - and if you restrict the fan voltage it may overheat..regardsAllan.
beamer = home video projector
No, you cannot "invert" its temperature characteristic
I think you missed something.Quotebeamer = home video projector
Thanks for the various replies. I meanwhile found out that the buck converter is based on this: http://www.monolithicpower.com/Products/Product-Detail/101/Step-down(Buck)/SwitchingRegulators/DC-DCPowerConversion/Converters/MP1584and I located the resistor that is used for the other half of the voltage divider. I just replaced it with my thermistor and with a bit of tuning on the original trim pot side now everything works as desired.To answer the question why I am not using an Arduino/PWM approach: If I had started from scratch I would probably have bought some PWM enabled fan and controlled it using some small Arduino. But since I already had some old fan lying around I did not want to waste money for an additional fan nor wait for its delivery. Directly using the buck converter seems to be a simple but good enough solution. Also the patched buck converter is compact enough to glue it into the fan's frame - which makes for a tidy design.
Hi again,I am happy that method worked for you. It's really an old method but hey if it works, why not use it right?
If you dont get the right profile if it is too extreme then you can try adding a parallel resistor and change the upper series resistor to compensate. The parallel resistor decreases the resistance swing of the thermistor so there is less resistance change over the full temperature range. There is some relatively simple mathematics to it i am not sure if you are interested or not, but the simplest part is to calculate the parallel resistance for both temperature extremes and see if that matches your desired end point settings. You dont get much control over the center range without adding a second thermistor, so the end point settings become a sort of compromise between getting the right end point settings and the desired mid range operation. I am also not sure how accurate you really need this to be anyway though :-)