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PC Fan Controller design verification

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konradIC13

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Hello

Im designing a PC fan controller that will let me set speed of up to 5 fans in my PC case, it will be powered with 12V from PSU.
I decided that each fan will have its own voltage regulator and with potentioneter ill be setting voltage going to fan from max to min at which they will stop running.

This is the initial schematic
5522443800_1445808564.png


From left to right. First there is MOLEX connector with 12V on it, then main fuse (its 4A because ill have 5x fan each max 500mA + some space for other stuff) and next main switch. There i have RC circuit to create a softstart (12kΩ and 47μF should give about 4 seconds before full 12V is reached) in case all 5 fans are set to max speed when the power is triggered.

Next the 12V is going to fan speed control section. On schematic there is only one but it will be cloned 5 times, for each fan controlled. The voltage regulator is LM1117 (same as LM317T but its LDO). A fixed resistor of 110Ω and potentiometer of 1kΩ should give me full range of regulation from min 1.25V to the max of about 11V (12V - Dropout voltage).

I dont have yet fans im gonna use, but initial tests on other 12V case fan have shown that minimal voltage at which they start moving on their own is 4V and current draw is at 40mA and at max 12V its 110mA. In this case power dissipated on voltage regulator is from 0.15W to 0.35W.
I think that the TO220 case + radiator should be fine? Also i added capacitors on in and out of regulator as per datasheed, tho im wondering if i shouldn't move the C2 capacitor "outside" (right now i have 5x C2 capacitor, each of separate fan and im thinking if i shouldnt make just one capacitor for all of them). And lastly there is separate fan fuse and a flyback diode across the fan.

And here comes the troubling part.

I want to have a LED light signalising if the fan is turned off or its currently running. Separate LED for each fan.
The 4V is approximately the minimal voltage at which fan starts running. Because of that I was thinking to make a voltage divider.
In first circuit the 2N7000 mosfet is triggering LED. Its Vgs Treshold voltage is from 0.8V to 3V, so with a voltage divider R7 and R8 i should have that 0.8V when the input voltage is 4V and while it will be not enough to trigger mosfet the light will be off. Increasing input voltage will light it up as the fan starts spinning.

After while of looking at schematic i figured, should i use the 2M7000 at all? I made this schematic:
5264326600_1445816619.png


The voltage divider made of R4 and X5 potentiometer (precise for finetuning) will be triggering LED. With R4 with fixed 460Ω and adjustable potentioneter X2 2kΩ i cane make it that when around 4V is going into voltage divider the output voltage is at around 2V which is not enough to trigger LED and when voltage increases to say 4.5V it will light up. The R5 limits current on LED to about 5mA so the power dissipated trough R4 resistor is at around 5mW.

Please check my schematic. Especially if i didnt make any mistakes, forgot about some parts. Or maybe if i complicated something too much and there is simpler way to make it (less parts, safer). Especially a way to make the light go off at voltage lover than X and light up when it goes up to some level.

Thank you in advance for advices.
 
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ronsimpson

Well-Known Member
Most Helpful Member
I do not like the MOSFET circuit.

1) (Blue LED, + resistor + 3.3V Zener diode) all in series. The Zener will substrate out the first 3.3 volts then you have the LED+Resistor. Not the best but an Idea.
2) Same as above but the Zener sits on ground. There is only one Zener and five (LEDs and resistors).

The 3.3 voltage might be wrong. Maybe 2.7V. You can use 1n4148 or 1n7001 or some other diodes to make a "Zener". Each diode is 0.6 volts. So two diodes =1.2V 3=1.8V 4=2.4V This is not real temperature stable but.
 

Pommie

Well-Known Member
Most Helpful Member
Most PC fans have a pwm input which controls their speed. I don't think varying the voltage is going to work.

Edit, I wasn't too clear there. The controller is built into the fan and so needs the full 12V.

Mike.
 

audioguru

Well-Known Member
Most Helpful Member
I agree that the speed of a fan is not controlled properly by varying its voltage. Maybe that is how it was done 100 years ago.
When the DC voltage to a motor is reduced then its torque is reduced then it might not start running when you want it to start slowly.
Pulse-Width-Modulation (PWM) supplies full power pulses for high starting torque and the width of the pulses determines the average voltage which determines the speed.
 

konradIC13

Member
Thank you for replies. I wasnt thinking about PWM untill i found circuits based on 555 IC. It seems really like a much better idea than using 5 loosy linear voltage regulators. It should make design more clean with less total power dissipation on components? Using other schematic online i put together something new

HG4VCc7.png


Using NE555 as PWM generator to drive Q1 mosfet (its low rdson 160mΩ so voltage drop and power dissipation will be small, fast switching for PWM and has low gate charge of 11nC).

Also i removed the soft start from RC circuit because whole current drawn by all fans, about 1 A would flow trough first resistor R1.

Questions.
1. New design verification
2. Is there any way to add a softstart to this circuit so if the PWM will be set to max all fans wont start in full speed at once?
3. How changing whole circuit to PWM will affect turning off the LED when voltage drops below 4V? Before it was when voltage directly dropped below 4V, now it should happen when PWM will drop to about 33% so the effective voltage will be equal to 4V. Will the solution with zener diode still work that way?

I was given idea that since i might use NE555 now why dont use its RESET pin?

I would take some opamp voltage comparator like LM393P. On its Input+ i would link the PWM signal from NE555. On its Input- i would put a reference voltage (either from voltage divider to get 4V from 12V or link it from ATX 5V). Then on V+ of comparator i would add 12V and i would link GND and V- to GND. The OUTPUT signal from comparator would be connected to RESET pin of NE555.

When PWM goes to 30% (and effective voltage on FAN is lower than 4V) which would be lower than reference voltage on comparator, then the comparatour OUTPUT would go to low state (gnd/0v) which would set NE555 to reset state and setting its output to 0v thus it will shut down the fan and i could also use it to shut down the LED (now without the voltage divider, tied directly to same voltage powering fan? Will this work?

Like this:
flS9nBh.png

When PWM % lower than 33 effective voltage on fan around 4V which is smaller than reference 4.3V so comparator out is low (0v/gnd) and linked to NE555 reset sets its output to 0 (0% PWM, all low, fans stop).
 
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audioguru

Well-Known Member
Most Helpful Member
The PWM circuit using a 555 can have soft start if you connect a pretty big capacitor (220uF?) from pin 5 to ground.

The blue LED 2 on the lower right is not powered. If it is connected to the PWM that goes to the motor then it will dim perfectly. I do not know why you want the LED to turn off when the motor's average voltage is 4V.
 

konradIC13

Member
The PWM circuit using a 555 can have soft start if you connect a pretty big capacitor (220uF?) from pin 5 to ground.

The blue LED 2 on the lower right is not powered. If it is connected to the PWM that goes to the motor then it will dim perfectly. I do not know why you want the LED to turn off when the motor's average voltage is 4V.

I didnt test it with PWM yet, but when powering a 12V computer fan from adjustable power source, 4V was the voltage at which the fan stopped spinning and stalled. I want that led to signalise it. When fan is not spinning, average voltage is too low then led goes off. When average voltage goes up and is enough to spin the fan the led will light up.
 

Pommie

Well-Known Member
Most Helpful Member
You do not pwm the power to the fan, they have an input that requires a logic level pwm signal that is over 20kHz.

Mike.
 

konradIC13

Member
You do not pwm the power to the fan, they have an input that requires a logic level pwm signal that is over 20kHz.

Mike.
Im using 3-wire fans. They have 12V input, GND and Tacho output. Only 4-wire fans accept logic level PWM.

Also the soft start did not work, manipulating capacitor C2 is affecting PWM frequency. BUT i can use reset pin and RC circuit to keep NE in reset state for a while after turning on (tho it wont technically solve the problem because after the time passes if the fans will be @100% they will start up at 100%). And i plan to use reset pin for something else (read below schematic).

I updated my schematic with a few changes.

gXCSkdg.png


I added a LM393 comparator to the circuit, this is why:

Im tryinng to find a way that will turn off the fan (and its indicator led) completely when the PWM duty cycle will drop below given level. Lets says 35%. At 35% PWM the fan rotation speed will be very low and there will be risk of stalling. I want to eliminate it.

I tried to achieve this with a comparator. The R5 and R6 generate reference voltage of about 4.3V which is like 35% of PWM and its wired to Input-. On Input+ of comparator im adding a signal from NE555 filtered with R3 C4 RC filter (to turn 'digital' square wave into 'real DC' signal so analog NE555 will know how to deal with it). Output from LM393 is pulled up to VCC and then linked to the RESET pin of NE555.

In theory it was supposed to be great. Input+ > Input- LM393P output is high and NE555 generates pulses. When NE555 duty cycle drops to 35% Input+<Input- and LM393P output goes low and thus it resets NE555 (sets it output to low) and when PWM rises again it will turn on NE555.

Unfortunately it does not work this way in simulation. The NE555 is always in reset state and i think there is logical explanation to it (i hope so). Even when potentiometer is set to maximum before circuit is powered up, all outputs in ICs are in low state because of lack of power. Because of that, even if i turn on the switch the NE555 before it is told to generate 100% PWM is still being hold in reset state by signal from LM393P from the previous time moment when power was turned off. Because of that NE555 will never generate PWM and will be held in reset.

Thats how i would explain what is happening. It MIGH be because time domain simulation and in real circuit it might or might not work (dont have the parts yet to test it on breadboard). Could someone please tell me if my way of thinking is right?

Is there any other way how i can turn off the fan's power depending on the signal coming from NE555?
 
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alec_t

Well-Known Member
Most Helpful Member
The NE555 is always in reset state and i think there is logical explanation to it (i hope so).
At power-up, C4 is discharged so the comparator output will be low, causing the 555 to reset. So the PWM output can't get started.
Instead of using the reset input, can't you just put a fixed resistor in series with D1 so that the PWM duty cycle can't be set below a limit value by the pot?
 
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