4-pin PC Fan.

[ComputerCurious21]

I've heard that you can achieve AC power from DC brushless pc fan. Using the white a blue wires?
I tried it and the pc fan slowed down and didn't produce ac. why?

 

[andy_Man]

Once power goes through the PSU it is DC. Can you explain further what you are trying to do? AC is alternating because of the way it's generated with a generator (Magnet and copper). As far as I know, DC is a more useful version of power.

 

[Eximo]

Not quite sure what it is you are trying to do there. Color of wires is going to vary by system. (Mine are all black as an example)

4-pin fans operate on PWM, Pulse Width Modulation. They are creating DC pulses to simulate an AC sine wave to spin the fan. This is not equivalent to the incoming AC power to the system. Maximum it could produce RMS at +12 to -12 is about 17 volts AC. If it is just 0-12V then we are talking only about 8 volts AC.

[strike](For the life of me I can't find out how they do it on modern PCs, too much information comes up on buying the darn things then running them. I Would have to dig through manufacturer's white papers, and I wouldn't even know where to start looking up the standard the 4-pin PWM uses.)[/strike]

Correction: I was thinking of how larger more industrial sized equipment operates on PWM type connections. That is not the case with PC PWM Fans. There should be no way to get an AC signal out of the motherboard headers. Essentially they are using a control loop similar to the way servos are operated. The PWM signal is for communication purposes, not power. 12V DC is still applied directly to the fan, what changes the speed of the fan is the input of the PWM signal, which operates a MOSFET (Transistor) to act as a variable voltage source to the fans coils.

 

[ComputerCurious21]

Once power goes through the PSU it is DC. Can you explain further what you are trying to do? AC is alternating because of the way it's generated with a generator (Magnet and copper). As far as I know, DC is a more useful version of power.

I'm trying to power a small transformer with a brushless dc fan from a desktop. I seen a video on YouTube and i was wondering if this can work, using the blue and white pins that come from a desktop dc fan.

 

[ComputerCurious21]

Not quite sure what it is you are trying to do there. Color of wires is going to vary by system. (Mine are all black as an example)

4-pin fans operate on PWM, Pulse Width Modulation. They are creating DC pulses to simulate an AC sine wave to spin the fan. This is not equivalent to the incoming AC power to the system. Maximum it could produce RMS at +12 to -12 is about 17 volts AC. If it is just 0-12V then we are talking only about 8 volts AC.

[strike](For the life of me I can't find out how they do it on modern PCs, too much information comes up on buying the darn things then running them. I Would have to dig through manufacturer's white papers, and I wouldn't even know where to start looking up the standard the 4-pin PWM uses.)[/strike]

Correction: I was thinking of how larger more industrial sized equipment operates on PWM type connections. That is not the case with PC PWM Fans. There should be no way to get an AC signal out of the motherboard headers. Essentially they are using a control loop similar to the way servos are operated. The PWM signal is for communication purposes, not power. 12V DC is still applied directly to the fan, what changes the speed of the fan is the input of the PWM signal, which operates a MOSFET (Transistor) to act as a variable voltage source to the fans coils.

I understand. But can you explain this video?
https://www.youtube.com/watch?v=2NzprCxmy7A

 

[Eximo]

Well a transformer is just that. Basically 10:1, or in this case 1:10, so that turns 12V in 120V. However, that is not outputting AC current, just 120V DC at low amperage. A string of LEDs or bulbs designed to run on 120V AC (Measured again in RMS, so that is 169+ to 169-, the RMS values we use is basically what you get when you measure the effective DC current of an AC sine wave.) wouldn't draw that much current, but in this situation batteries are used. For short durations you can draw a heck of a lot of power. That it isn't plugged in for very long is quite telling. Those little wires would probably not be able to sustain it.

AC systems can be run on DC if commutation isn't a factor. Get some quite nice effects with different types of bulbs. Most commercially available LED slight strips run at an effective 30Hz because of their diode nature. Running them at a 100% duty cycle on DC makes for a much more stable, and brighter, light output.

 

[ComputerCurious21]

Well a transformer is just that. Basically 10:1, or in this case 1:10, so that turns 12V in 120V. However, that is not outputting AC current, just 120V DC at low amperage. A string of LEDs or bulbs designed to run on 120V AC (Measured again in RMS, so that is 169+ to 169-, the RMS values we use is basically what you get when you measure the effective DC current of an AC sine wave.) wouldn't draw that much current, but in this situation batteries are used. For short durations you can draw a heck of a lot of power. That it isn't plugged in for very long is quite telling. Those little wires would probably not be able to sustain it.



AC systems can be run on DC if commutation isn't a factor. Get some quite nice effects with different types of bulbs. Most commercially available LED slight strips run at an effective 30Hz because of their diode nature. Running them at a 100% duty cycle on DC makes for a much more stable, and brighter, light output.

I thought Transformers run off AC current only? If its DC it makes a inductance? You are saying 120v DC is coming from a transformer that only runs on AC. If you look at the wires he hooking up to the Transformer you can see that it is the blue and white wires just like my brushless fan, which you said is for PWM. The person in the video said it was a isolated output if i stand corrected.

You also said that you got 17 volts AC from your fan? If so how did you do that.

 

[Eximo]

A transformer is a big inductor in essence. They are not limited to AC input, that is simply one of the more common applications, ie a standard PC power supply, and the best use of them. This particular one is simply two different coils separated by an air gap with an iron frame and is best used converting AC voltages to lower AC voltages. There are more windings on one side then the other, and generally tap off points for producing different voltages. The effect is that if you have 10 windings on one side, and 1 winding on the other side you can 'multiply' or 'divide' by that value. You maintain the amount of power with little loss in the transformer. So a 10V @ 10A input would become 100V @ 1A in this case. Basically this is a common part being used in a manner opposite to the way it would normally be used. Essentially a very crude boost converter.

I'd rather not go into a complete walkthrough of how PWM fans work as I would probably make a few errors along the way. I have a lot more expertise in AC drive motors. But here goes:

The wires coming off the coils of a PWM fan should have a type of pulsing DC on them if I am reading the schematics I have found correctly. They are using a MOSFET, as mentioned before, to drive the coils in the fan. Basically they are controlling the duty cycle of the MOSFET with a PWM input. That duty cycle sets how much power and how often it pulses to effect a rotation speed on the fan.

This signal when measured by an AC meter may actually register something akin to a proper value. However, AC is measured in RMS by sampling the wave form. And DC chopping is the best way to emulate an AC sine wave.

I think you are misreading an earlier explanation and applying it to something else. A true 12V AC signal would have peaks and valleys in the 17 volt DC range, just as a 120V AC signal really consists of peaks and valleys at ~169 volts.

What we are looking at in effect is a simple boost converter taking 12V to ~120V and using that to run a string of lights. And this is being run off of a battery directly through the components and not through any part of the computer. Were you to try this off of one of the fans hooked up to a motherboard it would almost certainly burn out the circuitry over time.