Limit voltage or current?

[PhillDubya]

Which is better for DC fan speed control....

Limiting voltage or limiting current?

To clarify...

If I take a voltage drop off of a resistor, the current is still unlimited because it is a parallel circuit.

If I put a resistor in series with the fan, it limits both?

 

[Sceadwian]

Current if you have to but PWMing it at it's rated current is usually preferred. Changing the duty cycle can control the fan speed very precisely.

 

[tcmtech]

I prefer voltage regulation for DC motors. It lets them draw what they need to stay at the speed they are at. With a feedback loop a very acurate RPM can be maintained. OR like Sceadwain says PWM can be used to. depending on the duty cycle and if its referenced by currentor voltage it can give a good speed control as well.

Typicaly current limiting will give you constant torque and varialbe RPM. Constant voltage tends to give constant RPM and variable torque.

 

[PhillDubya]

Assuming PWM is not an option...

A zener diode is not the answer correct? Because the zener will end up limiting the current, unless I make sure to get one rated higher than the highest current?

Would a zener be better than using a voltage divider for constant voltage stability?

 

[tcmtech]

?
can you post a schematic of what you are saying?

 

[PhillDubya]

I know it is kind of hard to see, sorry about that.

But I am wondering which configuration is the best?

I have a cpu fan from a company called Noctua. I don't want the cpu fan to run at full voltage because of noise. They sent me an adapter which is hooked up like the circuit in the middle under the two diode circuits.

I would have thought it would be better to use a voltage divider?

I am just wondering what the advantages of hooking it up like that is, and also, if the diode circuit I showed (either one) would work?

I wouldn't think that zeners would be the best idea, but I could most definitely be wrong...

Don't worry about the given values of the resistors, I was just using them as a general example, and my question pertains more to the theory of the circuitry.

 

[tcmtech]

If its a simple fan motor just run it though a voltage regulator like an LM317 or LM337. they are adjustable and only have 2 external resistors to set the output voltage from 1.2 to whatever you input voltage is.

Heres the actual specs sheets and schematics.

 

[Sceadwian]

A zener will limit the voltage not the current.
A voltage divider is almost always a bad idea because it limits voltage and current at the same time, so if you use something like A POT to do this it will be VERY non linear.
These are all very wasteful ways of limiting the speed of the motor as the same amount of power is generally being used, it's just being wasted in the zener or the resistor you're using. A simple 555 circuit as an oscilator with variable duty cycle is a great way (and very simple) to drive a DC motor. If the motor's current is low enough you can drive it directly from a 555 without any transistor stages.

 

[PhillDubya]

That answers my questions.

Thank you tcmtech and Sceadwian for your help.

 

[Boncuk]

Hi,

if PWM is not an option how about using an NTC resistor in series with the fan?

The resistor will reduce both voltage and current, but is the easiest way to vary fan speed with increasing temperature.

I assume that's what you want.

Use a basic resistor value of the same resistance as the fan motor. Under normal conditions (temperature 25deg/C) the fan will run at 1/2 of the supply voltage, resulting in very low fan speed, and increasing with increasing temperature.

Boncuk