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| Filtering PWM to smooth DC |
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| Rapsey:
Hi everyone, My question is more or less the same as this one from 7 months ago, but I didn't want to gravedig. So we have a 5015 blower fan powered by 12V PWM (on the actual power, not just signal). This obviously introduces electrical noise an audible whining noise which I am trying to eliminate. The fan can be voltage regulated to achieve the desired result without any electrical noise audible whining noise. It is rated for an operating voltage of 4.5 - 13.8V and a current of 170mA (= 2.04W). As with the voltage the current tolerance is +-15%. Now the question is how to convert that 12V PWM to (relatively) smooth DC power. I've seen a few solutions: * Just put a capacitor onto it This to me seems like a very poor solution, if you can even call it that. For one thing it messes up the PWM control, i.e. 80% PWM won't be 80% effectively. I'm also concerned about reverse current: during the off-portion of every duty cycle the capacitor will put voltage on the Arduino which is supplying the PWM power. Maybe this is fine but I'd rather ask here than find out the hard way. * Capacitor with resistor in series I've heard that the resistor is added to slow down the charging of the capacitor. Is this accurate? Again, same reverse current concerns as before. * Capacitor with inductor in series Okay, I understand that inductors resist changes in current and capacitors resist changes in voltage. I can see how those two would make a good team here. Again, worried about reverse current and also inductive spikes. Might need to add a diode in parallel with the inductor to allow the current to circulate back if the load is suddenly removed? * Capacitor with inductor in series and a freewheeling (schottky) diode placed in parallel over both (= buck converter) This seems to be best way to do it, but I wonder if it is necessary. I'll probably need to place my solution in-line on the fan wires so the more compact I can make it the better. I am fairly new at this so please point out any mistakes or misinterpretations. Any and all feedback is greatly appreciated! |
| tszaboo:
"This obviously introduces electrical noise which I am trying to eliminate." Why obviously? Why is noise a problem? Fans are brushless motors, they have a commutator, they are noisy, no matter what you do with them. On the other hand they are an inductor, so they resist the change in current. So it is possible that you already have a preatty smooth signal, depending on the PWM frequency. Have you connected an oscilloscope to it. Do you understand electrical noise, an it's effects on the system? |
| Rapsey:
--- Quote from: NANDBlog on September 21, 2018, 02:49:35 pm ---"This obviously introduces electrical noise which I am trying to eliminate." Why obviously? Why is noise a problem? Fans are brushless motors, they have a commutator, they are noisy, no matter what you do with them. On the other hand they are an inductor, so they resist the change in current. So it is possible that you already have a preatty smooth signal, depending on the PWM frequency. Have you connected an oscilloscope to it. Do you understand electrical noise, an it's effects on the system? --- End quote --- Maybe it's not that obvious, I always thought it was. Any motor I have seen that was controlled by PWM on the power has produced this whining noise, from small PC fans to brushless motors in RC planes and heli's. I thought it was a universal side-effect of supplying power in pulses, caused by the PWM frequency itself (unless perhaps when the frequency is outside the audible range for humans). That assumption may very well be wrong. Noise is a "problem" because the 3D printer is located right next to the bedroom and often used for overnight printing. It's not a major issue since it's not that loud, I am simply trying to do what I can to make it as quiet as possible - and learn a thing or two in the process. I do believe you are correct in saying it's a brushless motor. It's categorized with the brushless fans anyway. Someone once told me brushless motors are always 3-phase and thus have 3 wires (this fan only has 2) but I'm pretty sure it's brushless anyway. It's a Sunon MF50151VX. Unfortunately I don't have an oscilloscope (yet) so I can't measure the PWM frequency that way. I can't find any reliable information on it either since it's a firmware setting and I don't have the source code for the firmware that shipped with my printer. I suspect it's a fairly low frequency to reduce load on the Arduino that generates it. I do realize that I need to determine the frequency before I can try anything. Having a scope would also be extremely useful to study the effects of different solutions. It's definitely on my shopping list. And no, I don't know the first thing about electrical noise or its effects on the system. Maybe I used a wrong term for it. All I know is that with PWM it makes a very audible whining noise (even when the fan is not spinning, say at very low PWM %). When I connect the fan to a lab power supply and change the voltage I get the desired speed control without the annoying whining noise. If I can get the same results on the printer itself, why wouldn't I? :) Thanks for the reply! |
| Kleinstein:
The brush-less fans usually have a capacitor inside to buffer there supply. So if there is no series resistor or inductor, there would be significant current spikes that could act back on the PWM circuit. So it is a good idea to have at least some series impedance. The resistor way can work, though it produces some heat and does not allow full speed. The inductor method essentially needs the diode. Inductor and no diode can damage the PWM output, though there might be a diode inside. So one could get away with just a resistor or inductor and diode. It might still help to add more capacitance. |
| MiDi:
--- Quote from: blueskull on September 21, 2018, 05:10:42 pm ---Don't filter it. Reduce PWM frequency to ~160Hz, and magic may happen. --- End quote --- If that's not enough try with a buck-converter ;) |
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