Author Topic: Filtering PWM to smooth DC  (Read 23534 times)

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Online IanB

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Re: Filtering PWM to smooth DC
« Reply #25 on: September 22, 2018, 09:57:31 pm »
Attached is a simulation which takes things to the extreme R = 1R, C = 1000µF and a load resistance of 80R to be close to the 170mA. The PWM frequency is a bit higher at 2kHz and a duty cycle of 20%. The voltage across the load settles at 13V, with 830mA current spikes being drawn each pulse.

If you were to replace the resistor with an inductor, what size inductor would produce reasonable performance?
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #26 on: September 22, 2018, 10:57:20 pm »
Attached is a simulation which takes things to the extreme R = 1R, C = 1000µF and a load resistance of 80R to be close to the 170mA. The PWM frequency is a bit higher at 2kHz and a duty cycle of 20%. The voltage across the load settles at 13V, with 830mA current spikes being drawn each pulse.

If you were to replace the resistor with an inductor, what size inductor would produce reasonable performance?
Big, especially if the frequency is 500Hz as per the original poster's circuit, not the 2kHz I used in my simulation. Without calculating anything properly, just plugging figures into the simulation, I get around 47mH, which would need to be rated to 200mA.

Of course a resistor is much more economical, but it depends on what voltage drop and performance at low duty cycles is acceptable.

There is another possibility: filter the PWM to pure DC using a rectifier and capacitor and use an emitter follower with a higher value RC circuit to drive the load. I'll post a schematic tomorrow.
« Last Edit: September 22, 2018, 11:00:40 pm by Hero999 »
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #27 on: September 23, 2018, 09:00:37 am »
And here it is. It will drop nearly 2V at full output but that can be minimised by replacing D1 with a MOSFET rectifier and R3 with a current source. Another possibliity is to change it to low drop-out configuration, with a P device for Q1, but it will be difficult to stabilise.
« Last Edit: September 23, 2018, 09:05:19 am by Hero999 »
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #28 on: September 24, 2018, 02:36:48 pm »
Wow, thank you so much Hero999, you are really going above and beyond!

I've spent a few hours playing around with different RC vaues and studying the results. Getting a good feel for how it all fits together now. It seems that for the specific scenario of feeding PWM through an RC LPF you could say that:
  • Capacitor value primarily affects ripple (smaller cap will drop more in voltage during off-portion of PWM cycles)
  • Resistor value primarily affects control range in two ways:
    • Limits the top end of your PWM control (smaller resistor will drop less voltage and thus get closer to the 12V max)
    • Shifts your control curve (smaller resistor will charge the capacitor faster, e.g. 10% pulses could be enough to fully charge the capacitor and if it is big enough to carry through the remaining 90% this would shift your effective control curve to ~0-10% PWM)
Therefore in practice it seems best to do a balancing act choosing your resistor (as low as possible for the upper limit but not so low that it destroys your control curve) and then choose an appropriate capacitor based on how much ripple you can tolerate. Of course all of the above depends on how great the load is. Greater loads will require smaller resistors and larger capacitors to maintain similar results.

A few questions about your RC circuit:
  • Is that LTSpice?
  • How do you arrive at a load resistance of 80R?
    Do you simply use Ohm's law and divide the voltage by the rated current? If so, then judging by my volt/amp measurements this remains fairly constant at lower speeds.
    Can you measure the ESR of a component like this by simply measuring resistance with a multimeter?
  • You mention 830mA current spikes but the graph shows the spike going as high as 14A and eventually settling at 830mA?
And the LC circuit:
  • Would, say, a B82791H2251N020 be a good inductor here? (47mH, max 250mA / 2.4R, 30% tolerance)
  • How would the characteristics differ from an RC circuit? Why is this better?
    If these big inductors have resistance values in the same ballpark as the resistors we used for RC, is there still a point in using them?
As for the rectifier circuit... I think I have some more studying to do. :)
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #29 on: September 24, 2018, 08:40:26 pm »
And the LC circuit:
  • Would, say, a B82791H2251N020 be a good inductor here? (47mH, max 250mA / 2.4R, 30% tolerance)
  • How would the characteristics differ from an RC circuit? Why is this better?
    If these big inductors have resistance values in the same ballpark as the resistors we used for RC, is there still a point in using them?
As for the rectifier circuit... I think I have some more studying to do. :)
No, it's stupid. Also that inductor is large and made for mains filters, has two 47mH coils, that thing likely would experience tremendous losses. Viable would be like 30kHz PWM and inductor in hundreds of uH range. If done decently there will be barely any energy losses and 100-220uF capacitor would be more than enough.
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #30 on: September 24, 2018, 10:02:48 pm »
 
Wow, thank you so much Hero999, you are really going above and beyond!

I've spent a few hours playing around with different RC vaues and studying the results. Getting a good feel for how it all fits together now. It seems that for the specific scenario of feeding PWM through an RC LPF you could say that:
  • Capacitor value primarily affects ripple (smaller cap will drop more in voltage during off-portion of PWM cycles)
  • Resistor value primarily affects control range in two ways:
    • Limits the top end of your PWM control (smaller resistor will drop less voltage and thus get closer to the 12V max)
    • Shifts your control curve (smaller resistor will charge the capacitor faster, e.g. 10% pulses could be enough to fully charge the capacitor and if it is big enough to carry through the remaining 90% this would shift your effective control curve to ~0-10% PWM)
Therefore in practice it seems best to do a balancing act choosing your resistor (as low as possible for the upper limit but not so low that it destroys your control curve) and then choose an appropriate capacitor based on how much ripple you can tolerate. Of course all of the above depends on how great the load is. Greater loads will require smaller resistors and larger capacitors to maintain similar results.

A few questions about your RC circuit:
  • Is that LTSpice?
  • How do you arrive at a load resistance of 80R?
    Do you simply use Ohm's law and divide the voltage by the rated current? If so, then judging by my volt/amp measurements this remains fairly constant at lower speeds.
    Can you measure the ESR of a component like this by simply measuring resistance with a multimeter?
  • You mention 830mA current spikes but the graph shows the spike going as high as 14A and eventually settling at 830mA?
And the LC circuit:
  • Would, say, a B82791H2251N020 be a good inductor here? (47mH, max 250mA / 2.4R, 30% tolerance)
  • How would the characteristics differ from an RC circuit? Why is this better?
    If these big inductors have resistance values in the same ballpark as the resistors we used for RC, is there still a point in using them?
As for the rectifier circuit... I think I have some more studying to do. :)
Yes, I simulated it in LTSpice.

80R was calculated using the load's current and voltage ratings and applying Ohm's law. I assumed it was a resistive load, which could be incorrect. If the current doesn't reduce much, when the voltage drops, use a constant current load.

The 14A spikes, during start-up don't matter, as they don't last for long. The peak current can be calculated from the supply voltage and resistor value.

Yes, tyhe ESRT of an inductor can be measured with a meter.

As mentioned above, it makes more sense to use a higher frequency and smaller inductor, but if you can't change the frequency, then a large inductor is unavoidable. The losses won't be that high at the low frequency of 500Hz.
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #31 on: September 24, 2018, 10:16:07 pm »
As I said before, I see barely any sense going resistive PWM as well. Just use something like LM1117-ADJ and put smoothed PWM on it's ADJ pin. No current spikes, no large cap needed. As a bonus, output voltage will be independent from the load.
« Last Edit: September 24, 2018, 10:19:34 pm by wraper »
 

Offline David Hess

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Re: Filtering PWM to smooth DC
« Reply #32 on: September 24, 2018, 11:58:11 pm »
The buck regulator structure using a capacitor, inductor, and diode is the best way.  Some old motherboards worked this way to control the speed of non-PWM capable fans.

Essentially it becomes a buck regulator without feedback control so the output voltage becomes the supply voltage * duty cycle.
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #33 on: September 25, 2018, 07:45:07 am »
As I said before, I see barely any sense going resistive PWM as well. Just use something like LM1117-ADJ and put smoothed PWM on it's ADJ pin. No current spikes, no large cap needed. As a bonus, output voltage will be independent from the load.
Assuming there's a permanently live conductor, then that would work perfectly. When I designed the previous circuit, I made the assumption that it was just a two wire connection: one 0V and one PWM.

I wouldn't even bother with the regulator, just use an emitter follower. Remove D1 and C1 from the circuit I previously posted and connect 13.8V directly to Q1's collector.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #34 on: September 25, 2018, 02:09:12 pm »
The peak current can be calculated from the supply voltage and resistor value.
How would you calculate that?

Assuming there's a permanently live conductor, then that would work perfectly. When I designed the previous circuit, I made the assumption that it was just a two wire connection: one 0V and one PWM.
That's correct, at present there's only those two wires going to the fan. I could always lay an additional 12V line from the PSU but the simpler the solution the better.


Now I'd like to try an LC filter but I'm getting completely lost in the massive spectrum of inductors that are available. My local hardware store has 2 radial 47mH inductors (both shielded), one with an RDC of 82 (Q=70) and one with an RDC of 52 (Q=100). Both have an IDC of 8mA.

Is the RDC value really the series resistance I can expect in a DC circuit? If so, then values this high render it completely useless here.
Is the IDC value really the max DC current it can handle? If so, then values this low are equally useless here.
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #35 on: September 25, 2018, 02:29:42 pm »
Now I'd like to try an LC filter but I'm getting completely lost in the massive spectrum of inductors that are available. My local hardware store has 2 radial 47mH inductors (both shielded), one with an RDC of 82 (Q=70) and one with an RDC of 52 (Q=100). Both have an IDC of 8mA.
Don't bother with that. It's a completely non viable solution. And with those small inductors even non working. Unless you get into 10+kHz range, forget about LC.
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #36 on: September 25, 2018, 03:38:01 pm »
Now I'd like to try an LC filter but I'm getting completely lost in the massive spectrum of inductors that are available. My local hardware store has 2 radial 47mH inductors (both shielded), one with an RDC of 82 (Q=70) and one with an RDC of 52 (Q=100). Both have an IDC of 8mA.
Don't bother with that. It's a completely non viable solution. And with those small inductors even non working. Unless you get into 10+kHz range, forget about LC.
There's no reason why an LC filter won't work. It will just need an inductor 50 times the size of the equivalent circuit, working at 25kHz. Now this may not be ideal, but it's certainly possible and the current is under 200mA, which helps to make it easier too.

The peak current can be calculated from the supply voltage and resistor value.
How would you calculate that?
Ohm's law. It's no coincidence the peak current is nearly 14A, the supply voltage is nearly 14V and the series resistor is 1Ohm.

Quote
Assuming there's a permanently live conductor, then that would work perfectly. When I designed the previous circuit, I made the assumption that it was just a two wire connection: one 0V and one PWM.
That's correct, at present there's only those two wires going to the fan. I could always lay an additional 12V line from the PSU but the simpler the solution the better.


Now I'd like to try an LC filter but I'm getting completely lost in the massive spectrum of inductors that are available. My local hardware store has 2 radial 47mH inductors (both shielded), one with an RDC of 82 (Q=70) and one with an RDC of 52 (Q=100). Both have an IDC of 8mA.

Is the RDC value really the series resistance I can expect in a DC circuit? If so, then values this high render it completely useless here.
Is the IDC value really the max DC current it can handle? If so, then values this low are equally useless here.
Those inductors are completely unsuitable. The DC current rating is far too low.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #37 on: September 25, 2018, 04:27:54 pm »
Don't bother with that. It's a completely non viable solution. And with those small inductors even non working. Unless you get into 10+kHz range, forget about LC.
There's no reason why an LC filter won't work. It will just need an inductor 50 times the size of the equivalent circuit, working at 25kHz. Now this may not be ideal, but it's certainly possible and the current is under 200mA, which helps to make it easier too.
Looks like you aren't kidding about that size. Maybe I'm looking in the wrong places but after browsing DigiKey and Mouser I still haven't been able to find an inductor that would be even remotely usable for this. Anything with sufficient inductance, current capacity and a low enough DC resistance is massive and often insanely expensive. So far the closest I could find was this thing... I guess this will remain a thought experiment after all.

Does it really need to have that much inductance though? I don't need to smoothen my DC output that much. Even with 2V ripple it would still be good enough to eliminate the PWM noise. Then again I don't think I fully understand the impact of inductance in an LC circuit.

The peak current can be calculated from the supply voltage and resistor value.
How would you calculate that?
Ohm's law. It's no coincidence the peak current is nearly 14A, the supply voltage is nearly 14V and the series resistor is 1Ohm.
So then you divide your resistor voltage drop by the resistor value? ~0.8V / 1Ohm seems like the only way I can arrive at ~830mA.
« Last Edit: September 25, 2018, 04:30:56 pm by Rapsey »
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #38 on: September 25, 2018, 04:29:32 pm »
Now I'd like to try an LC filter but I'm getting completely lost in the massive spectrum of inductors that are available. My local hardware store has 2 radial 47mH inductors (both shielded), one with an RDC of 82 (Q=70) and one with an RDC of 52 (Q=100). Both have an IDC of 8mA.
Don't bother with that. It's a completely non viable solution. And with those small inductors even non working. Unless you get into 10+kHz range, forget about LC.
There's no reason why an LC filter won't work. It will just need an inductor 50 times the size of the equivalent circuit, working at 25kHz. Now this may not be ideal, but it's certainly possible and the current is under 200mA, which helps to make it easier too.
I said it won't fork with particular inductors. And such circuit is just not feasible even if it works (with big inductor and capacitor) due to size and price.
« Last Edit: September 25, 2018, 04:32:07 pm by wraper »
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #39 on: September 25, 2018, 05:41:52 pm »
As I said before, I see barely any sense going resistive PWM as well. Just use something like LM1117-ADJ and put smoothed PWM on it's ADJ pin. No current spikes, no large cap needed. As a bonus, output voltage will be independent from the load.
I've gone over the LM1117's data sheet and I don't think it would work well for this. I only have 12V to work with and at my target current of 170mA the LM1117 has a dropout voltage of 1.1V, so unless I misunderstand linear regulators that means I'll never get much higher than 10.9V out. An RC circuit with 5R drops less than that, and with 2.5R I can get up to 11.5V in practice (at the expense of squishing my control curve into the bottom of the PWM range).

As far as I know linear regulators are also no more efficient than resistors so the only benefit I can see is that they give you linear control over the output voltage, whereas with RC it's a logarithmic curve where most of the control is between 0 and 30% PWM (depending on R/C values).
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #40 on: September 25, 2018, 06:35:47 pm »
Nobody prohibits using LDO with lower voltage drop. As of 2.5R, it's on border with insane circuit design, my condolences to the smoothing capacitor. No sane engineer would ever place something like this into actual product.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #41 on: September 25, 2018, 06:59:12 pm »
Nobody prohibits using LDO with lower voltage drop. As of 2.5R, it's on border with insane circuit design, my condolences to the smoothing capacitor. No sane engineer would ever place something like this into actual product.
That's what I thought. I suppose I should not expect too many years out of that capacitor before it blows up like a balloon? On the bright side, it's "only" cycling at 500Hz.

Also, I just made a rather worrying observation... The control box of my printer (which houses all the electronics) can be powered in two ways: either from the printer's own power supply or via 5V USB.

I wanted to do a test with a different power supply so I powered off the printer (unplugged both mains and USB) and then hooked up a 12V wall wart to my testing breadboard. Of course being the newb that I am I forgot that the printer's fan connector was still connected to the breadboard power rails as well. Guess what, the damn thing powers on. Apparently putting 12V on those fan leads somehow goes back through the FET to supply power to the internals. Guess that answers that, there's no diode in there.

Now I'm even more concerned about having a capacitor discharge into those fan leads during the off-portion of the PWM cycles...
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #42 on: September 25, 2018, 08:57:04 pm »
Don't bother with that. It's a completely non viable solution. And with those small inductors even non working. Unless you get into 10+kHz range, forget about LC.
There's no reason why an LC filter won't work. It will just need an inductor 50 times the size of the equivalent circuit, working at 25kHz. Now this may not be ideal, but it's certainly possible and the current is under 200mA, which helps to make it easier too.
Looks like you aren't kidding about that size. Maybe I'm looking in the wrong places but after browsing DigiKey and Mouser I still haven't been able to find an inductor that would be even remotely usable for this. Anything with sufficient inductance, current capacity and a low enough DC resistance is massive and often insanely expensive. So far the closest I could find was this thing... I guess this will remain a thought experiment after all.

Does it really need to have that much inductance though? I don't need to smoothen my DC output that much. Even with 2V ripple it would still be good enough to eliminate the PWM noise. Then again I don't think I fully understand the impact of inductance in an LC circuit.
Yes, to get smoothing at low frequencies, a big fat inductor is required.

Quote
The peak current can be calculated from the supply voltage and resistor value.
How would you calculate that?
Ohm's law. It's no coincidence the peak current is nearly 14A, the supply voltage is nearly 14V and the series resistor is 1Ohm.
So then you divide your resistor voltage drop by the resistor value? ~0.8V / 1Ohm seems like the only way I can arrive at ~830mA.
Look at the voltage, when the 14A peak is drawn.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #43 on: September 28, 2018, 01:36:02 pm »
By the way, at 100% PWM that LC filter you provided shows a voltage spike before stabilizing. Is that something I should be concerned about? I'm not sure if ~20ms is long enough for such a spike to be dangerous.




EDIT 1: Changing the placement of the diode seems to cut off the spike as it should:






EDIT 2: Because I do not need the smoothing to be perfect I tried an LC filter with a smaller inductor. This actually seems to work quite well. The control curve is much better than an RC filter while also getting closer to 12V. The question remains: is this a sane thing to do?

I'm using a 1N5819 here because I have a few of those lying around. For the inductor values I used a 5900-222-RC (axial, 2.2mH, 1.7RDC, 500mA).



10% PWM: (this would be the lowest setting for practical use)



20% PWM:



30% PWM:



50% PWM:



100% PWM:




EDIT 3: This is probably a better way to do the flyback diode:



Having the diode in this configuration does raise the output voltage at low duty cycles: 10% PWM has gone from 3.3V to 5V.


EDIT 4: Added a resistor to the flyback diode to dampen the effect of the inductor during the off-portion of the PWM cycles. Mostly to improve the control curve, so I can have access to low voltages (3-4V) without resorting to ultra-low PWM duty cycles (<=5%).



Control curve now looks like this:

Code: [Select]
PWM% => Vout
   5 =>  1.3
   6 =>  1.7
   7 =>  2.1
   8 =>  2.6
   9 =>  3.0
  10 =>  3.5
  11 =>  3.9
  12 =>  4.3
  13 =>  4.7
  14 =>  5.1
  15 =>  5.5
  16 =>  5.9
  17 =>  6.2
  18 =>  6.5
  19 =>  6.8
  20 =>  7.0
  25 =>  8.2
  30 =>  9.0
  35 =>  9.6
  40 => 10.0
  50 => 10.6
  60 => 11.0
  80 => 11.3
 100 => 11.8
« Last Edit: September 28, 2018, 06:24:45 pm by Rapsey »
 

Offline Zero999

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Re: Filtering PWM to smooth DC
« Reply #44 on: September 29, 2018, 06:48:56 pm »
Did you model the ESR of the inductors?

Why do you want the inductor over the plain RC circuit? If it's efficiency, then in the first two edits, the diode is dissipating quite a bit of the power. In the fourth and final edit, R1 is dissipating a lot of power. Of course, if you're not bothered about efficiency, then this isn't a problem.

Another approach is to build a PWM circuit, running at a much higher frequency, run off a diode and capacitor rectifier and use much smaller inductors and capacitors.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #45 on: September 30, 2018, 07:18:54 am »
Yes, I took the ESR or the inductor into account. The reason for using an inductor is that compared to a resistor, it seems to do a much better job of slowing down the capacitor charging while having less ESR, thus getting closer to 12V. The resulting ripple is more like a sine and less like a sawtooth.

R1 is indeed dissipating a lot of power in edit 4 but only during the off-portion of the PWM cycle. That way it does not hamper the max voltage at 100% duty cycle but the lower the duty cycle, the more it reduces the average output voltage. This way it's not like 10% duty cycle results in 90% output voltage.

An additional PWM circuit was my next option to explore. I haven't been able to find any PWM IC's that are also PWM-controlled (hardly surprising), they all seem to be voltage-controlled. Which brings me back to my original problem of how to convert the 500Hz PWM to a linear scaling voltage.

Do you have any examples of that diode+capacitor rectifier? I thought these things were for AC-to-DC conversion. Not entirely sure what there is to rectify when the wave function has no negative portion...
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #46 on: September 30, 2018, 10:33:00 am »
An additional PWM circuit was my next option to explore. I haven't been able to find any PWM IC's that are also PWM-controlled (hardly surprising), they all seem to be voltage-controlled. Which brings me back to my original problem of how to convert the 500Hz PWM to a linear scaling voltage.
Resistor + capacitor. What you've seen so far was not linear because pulling only to one side, not up/down and because of the load attached.
 

Offline RapseyTopic starter

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Re: Filtering PWM to smooth DC
« Reply #47 on: September 30, 2018, 02:55:36 pm »
Resistor + capacitor. What you've seen so far was not linear because pulling only to one side, not up/down and because of the load attached.
I don't understand, without a load any DC supplied to the capacitor through a resistor would eventually charge it to 12V, regardless of the PWM duty cycle. What do you mean with pulling to one side vs up/down? It sounds like DC vs AC but that's probably not what you meant?
 

Online wraper

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Re: Filtering PWM to smooth DC
« Reply #48 on: September 30, 2018, 02:57:08 pm »
Resistor + capacitor. What you've seen so far was not linear because pulling only to one side, not up/down and because of the load attached.
I don't understand, without a load any DC supplied to the capacitor through a resistor would eventually charge it to 12V, regardless of the PWM duty cycle. What do you mean with pulling to one side vs up/down? It sounds like DC vs AC but that's probably not what you meant?
It means pulling to GND as well, not leaving open circuit.
 

Offline Gyro

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Re: Filtering PWM to smooth DC
« Reply #49 on: September 30, 2018, 03:44:13 pm »
It sounds like a really fun discussion but are you sure you're not going a little OTT just to stop a fan whining?  :)

Presumably the PWM changes in response to some kind of temperature sensing, so as long as the fan control range is able to keep up, then is there a problem?

Just asking.  :)
Best Regards, Chris
 


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