PWM signal from 555 timer

[fsleeman]

I finally got a power fet (IRL510 is that I could easily get) and realized that what I have is not actually a PWM fan. Use the amplified PWM signal as the input power works but has a slight hum from repetitively low PWM frequency. I had to use a frequency much lower than 25kHz because the short bursts were not enough to overcome the rotational inertia of the fan. Frequencies in the 1kHz seem to work fine.

I was able to use a simple low pass RC filter to make the PWM signal to nice DC voltage when driving an LED, but when I tried to use the filter with MOSFET output it didn't seem to work correctly. I am obviously missing something so does anybody know how I would filter an amplified PWM signal through a transistor to get an amplified DC voltage?

[Zero999]

Don't try to filter the output to power a fan, it will reduce the efficiency and defeat the whole purpose of using PWM.

[fsleeman]

Just to be clear the fan is NOT a PWM controlled fan, its a normal 12V DC fan with a tachometer feedback line. My two options are to rectify the PWM to get a clean DC signal OR pulse the DC fan with the PWM signal. Assuming Hero999 understood me correctly, then the preferred method is to simply amplify the PWM signal to pulse the fan?

[Mechatrommer]

i'm thinking whats the point of filtering PWM? better with DAC in the first place. but reading back @fsleeman post, maybe its the availability issue.

I had to use a frequency much lower than 25kHz because the short bursts were not enough to overcome the rotational inertia of the fan. Frequencies in the 1kHz seem to work fine.

the point is not the short burst is not enough for inertia. a pwm is a pwm regardless of freq, its more regarding duty cycle. a 50% duty cycle of 1KHz PWM is the same effective power/rate as 50% 25KHz PWM. you cannot state that... for higher freq PWM, it is a "short burst". maybe in your case, there is some other factor that prohibit higher freq PWM driving your fan, internal circuitry and filtering maybe.

in order to filter or smooth out the PWM, i think you'll need to put proper capacitance value on the mosfet/transistor output. for higher freq pwm, lower C value is needed, not remember the formulation, gotta open back my thick book to know.

[.o:0|O|0:o.]

Just to be clear the fan is NOT a PWM controlled fan, its a normal 12V DC fan with a tachometer feedback line. My two options are to rectify the PWM to get a clean DC signal OR pulse the DC fan with the PWM signal. Assuming Hero999 understood me correctly, then the preferred method is to simply amplify the PWM signal to pulse the fan?

Hero666 is right, "Don't try to filter the output to power a fan, it will reduce the efficiency and defeat the whole purpose of using PWM."

Since you have chosen PWM to select a particular speed (not the most efficient use of your supply, but you wanted to experiment with the 555...), the purpose of PWM is to deliver controlled amounts of current to your motor. Having from 0V to 12V (supply) provides you with the maximum range of currents available to you (which is the whole point of the MOSFET, in this case).

The problem with inertia is overcome through a gradual increase in the PWM rate before fixing on to a final value (ideally, you would like the current RMS gradient to ramp up initially). The problem with hum is overcome by selecting a frequency low enough to make the energy transfer to the motor as efficient as possible (your DC motor is most efficient at 0 Hz and resonates at certain frequencies depending on its physics) or selecting a frequency above the audible range (or at any frequency that suits the particular physics of your motor).

As Shafri said, you seem to be confusing duty cycle with switching frequency. You can have whatever duty cycle you like at whatever switching frequency suits your motor. It is just a case of switching the MOSFET on for n cycles and off for m cycles (though I am not sure how this is being achieved with the single 555 so far).

My first instinct would have been to use an MCU, especially since you mentioned PWM and tacho-feedback, but if you can do it for less money and complexity with a 555 then I would be interested to know how it turns out!

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[fsleeman]

As Shafri said, you seem to be confusing duty cycle with switching frequency. You can have whatever duty cycle you like at whatever switching frequency suits your motor. It is just a case of switching the MOSFET on for n cycles and off for m cycles (though I am not sure how this is being achieved with the single 555 so far).

I understand the difference between duty cycle and switching frequency. I agree that a 50% duty cycle at 1Hz delivers the same total power as a 50% duty cycle at 1MHz (assuming no rise/fall time), but that has nothing to do with the inertia of the fan. Lets say you need to push a broken-down car. If you push the car in quick short shoves will it move the car as well as long sustained pushes? Obviously the answer depends on a number of factors but static friction is always greater than kinetic friction. If I start the fan with a high switching frequency, nothing happens unless I manually turn the fan. Once the static friction is overcome the fan runs normally. Since I am going to put this into a overheating piece of electronics manually starting the fan is not a viable solution.

The unfiltered PWM signal will be sufficient solution to the problem. I have another fan with the tach output which I might experiment with a micro, its not needed for this project. I just want a fan that runs at a speed lower than 100% I figured I'd use the circuit I already built. Thanks again for everybody's suggestions, it was an educational thread.

[Zero999]

It's not that simple.

Higher frequencies won't work because of the inductance of the motor or in this case, the drive electronics inside the DC brushless motor won't like a power supply switching at 1MHz.

[Mechatrommer]

If I start the fan with a high switching frequency, nothing happens unless I manually turn the fan. Once the static friction is overcome the fan runs normally.

maybe you can ramp at 100% during start for a while, say half a second, just to get the motor moving, and later the PWM reduced back to normal/required operating level. just an opinion, you know your stuff

[fsleeman]

It's not that simple.

Higher frequencies won't work because of the inductance of the motor or in this case, the drive electronics inside the DC brushless motor won't like a power supply switching at 1MHz.

I am not sure if you were disagreeing with me or someone else on the thread, but I agree with your statement. That was the point I was trying to make, sorry if I weren't clear.

[.o:0|O|0:o.]

If the inertia is overcome at 0 Hz directly from the 12V power supply then I don't see why correct selection of PWM rate and start-up duty cycle wouldn't approximate a similar rms current. 1 MHz switching rate is too high. You should keep it below 30 kHz probably (consult the motor datasheet if you can find the part number). Also note that most people cannot hear sound beyond 19 kHz, let alone 22 kHz or 24 kHz. Here is a PIC appnote; a MOSFET is used and there is a discussion about common issues with PWM: http://www.jimfranklin.info/microchipdatasheets/00847a.pdf

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