DC Vs PWM - Cooling - Linus Tech Tips

• NikitaSch

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I have 2 options:DC and PWM. I noticed that when fans are on DC mode they are at low rpm like 600 but when on PWM fans are at constant 1800rpm , what is the best mode ? 

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• Kilrah

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The one that correctly works at varying the speed of your particular fan. 

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• curiousmind34

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6 minutes ago, NikitaSch said:

I have 2 options:DC and PWM. I noticed that when fans are on DC mode they are at low rpm like 600 but when on PWM fans are at constant 1800rpm , what is the best mode ? 

If your motherboard/fans supports pwm, use pwm. If your motherboard/fans doesn't, then don't use pwm It's that simple. When DC fans are put into pwm mode they auto ramp to max speed.

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• SupaKomputa

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3 pins fans = DC mode.

4 pins fans = PWM

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• Tan3l6

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In my experience, not knowing if case fans are PWM or DC i used them for a long time with PWM option in BIOS, but after testing in DC mode - the fans are so very much quieter. And thats not from lower rpm, its just compatibility... afaik.

I know what PWN and DC are, in theory. Just didn't give much attention to it.

Pax vobiscum

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• NikitaSch

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14 minutes ago, curiousmind34 said:

If your motherboard/fans supports pwm, use pwm. If your motherboard/fans doesn't, then don't use pwm It's that simple. When DC fans are put into pwm mode they auto ramp to max speed.

Won’t that shorten the lifespan? 

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• SupaKomputa

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6 minutes ago, Tan3l6 said:

In my experience, not knowing if case fans are PWM or DC i used them for a long time with PWM option in BIOS, but after testing in DC mode - the fans are so very much quieter. And thats not from lower rpm, its just compatibility... afaik.

I know what PWN and DC are, in theory. Just didn't give much attention to it.

PWM fans can work in DC mode, not the other way around.

Running in PWM is better since the fan are running at a constant voltage and you can achieve very low rpm control, less than 20% unlike DC fan mode which cannot run at lower than 40%-60%.

DC mode control the speed by lowering the voltage, so the lowest speed depends on the lowest voltage the fan can run (usually around 50%).

If you run 3pins fan on PWM it will receive constant 12v, which is 100% speed, since it cannot process the pwm signals from the fourth cable.

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• NikitaSch

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6 minutes ago, SupaKomputa said:

PWM fans can work in DC mode, not the other way around.

Running in PWM is better since the fan are running at a constant voltage and you can achieve very low rpm control, less than 20% unlike DC fan mode which cannot run at lower than 40%-60%.

DC mode control the speed by lowering the voltage, so the lowest speed depends on the lowest voltage the fan can run (usually around 50%).

If you run 3pins fan on PWM it will receive constant 12v, which is 100% speed, since it cannot process the pwm signals from the fourth cable.

When fans are in pwm mode they ramp up at the max speed and can’t control them even though fans support pwm 

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• SupaKomputa

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Calibrate the fans first before applying the fan curve.

If it doesn't work then the fan pwm might be broken.

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• mariushm

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A PWM capable fan MUST have 4 wires :  voltage (+12v), ground (-) , fan speed sensor,  pwm signal

fan speed sensor = fan sends a signal to the motherboard, to tell it how fast it spins

pwm signal = motherboard sends a signal to the fan telling it how fast to spin

A PWM fan defaults to running at 100% if there's no PWM signal received from the motherboard.

It's also guaranteed to start spinning at the lowest possible setting which - if my memory is correct - is 10% of PWM signal. PWM fans can be adjusted between 10% and 100% of their maximum fan speed

PWM fans have a chip inside the hub which reads the signal coming from motherboard and adjusts the speed accordingly - the fan voltage remains a constant 12v throughout. As an analogy, think of it like you pedaling from time to time to keep the bike speed constant. The chip inside the fan sends some pulses of 12v energy from time to time through the fan to maintain the desired speed. 

In DC mode, the motherboard ignores the PWM wire (even if the fan has it) and simply changes the voltage going to the fan.  From the default of 12v which is 100% speed for the fan. the motherboard may lower the voltage down to even 5v.

Some PWM fans have a limited range, for example they may support only 7v...12v.

Other fans may support a wider range, like 5v ... 12v or 6v..12v but they may have a hard time spinning the blades from a full stop when then voltage is very low. For example, the motherboard may have to be smart enough to send at least 8-10v for around 5-10 seconds, until the fan starts spinning to a minimum rpm, and then it can lower the voltage down to the minimum of 5-6v and have the fan keep spinning at very low speed.

A fan with only 3 wires CAN NOT be controlled using PWM mode, because there's no 4th wire through which the motherboard can tell the fan to adjust fan speed.

With these fans, your only option is the DC mode, where the motherboard can try to adjust speed by changing the voltage going to the fan.

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