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Fan Voltage Regulator LM317

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New Member
Hi all, I'm new here and I'm also in the process of learning electronics.
Sorry if my english isn't the best. :)

Unfortunatly I didn't learn anything about voltage regulators, well, almost nothing, my teacher just showed it to me like "see, this is a voltage regulator, do this and that and see what it does", and that was all I've learnt about them... nothing.

I've been searching for schematics to make a fan voltage controller, I came with a lot of schematics but I would like to slightly moddify some of them.
I also have some other problems, I cannot get 1k pots, this is a horrible country when talking about PC and Electricity/Electronics.

What I would like to do is to use a LM317 to regulate my fan voltage from 7V to 12V (I know it won't reach 12V).
I would also like to have a thermistor to control the voltage, and a swtich to select manual controlled (pot) or temperature controlled (thermistor).
Another switch to turn it on and off.
This will work with the 12V form a PC PSU.

I would also like to ask those that help me, to explain how they calculated everything, which things should I have in mind, etc. That way I'll start learning about this little things.

PS: I have some projects to show, do I have to do it in this part of the forum?


Active Member
The first question I'd ask is about the motor, AC or DC, what voltage, current, etc - and of some importance, will the motor/fan operate as you intend by lowering the voltage.

Some motors just want their rated voltage plus or minus some tolerance and will fail if speed control by lowering the voltage is attempted. if I am not mistaken many DC motors respond favorably to some degree, to varying voltage - many AC motors do not.

While this may seem trivial it is significant in many applications - will the motor develop the torque to turn the fan when given a lower voltage? Some "variable speed" schemes do not address the fact that the torque requirements of the driven item can exceed the ability of the motor at less than 100% setting. In the absence of published data a little bit of experimenting on the bench will probably get you the answers you need - set up the fan to run at the lowest speed you anticipate with resistors, lamps, whatever you have - and let it run that way to see if it overheats. If it does fail then you can rethink the approach.

I do not want to make fun of your difficulty in obtaining components however many of us "suffer" the same fate for different reasons - economics being one of them. Many of the people who post here struggle to try and make what they have on hand work. Many of the "experts" who respond are well aware of that and work hard to help us make do with what we have. You have a lot of company in this regard.


Active Member
My understanding of the LM317 is pretty basic but the regulator works to maintain 1.25 volts between the output and the control pin. As I recall, the application literature shows a 240 ohm resistor between the output and the control pin - so in that arrangement 5 ma is flowing. Normally to set the regulator to some other voltage another resistor is run from the control pin to ground (negative). The ratio of the resistors determines the output voltage.

I think you are telling us that you want to use a thermistor in place of this second resistor. If this is the case you need to be concerned about how much current the thermistor will carry and how the resistance, cold and warm, relates to the fixed resistor and the desired output voltage. It would seem that the resistance of most thermistors is pretty high which suggests that the normal 240 ohm resistor be increased until you get what you want - however at some point there won't be enough current flowing thru the control pin - somehow I am recalling 100 microamps but I may have it mixed up with something else. Thermistor manufacturers might indicate how much current can pass thru a thermistor - and how much can pass without significantly impacting it's ability to measure (heating).

If I get a minute later I'll scan the datasheet to see if I can find the minimum current for the 317. I'd kind of like to know myself.
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