Electronics > Beginners
Filtering PWM to smooth DC
wraper:
--- Quote from: Rapsey on September 25, 2018, 02:09:12 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.
--- End quote ---
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.
Zero999:
--- Quote from: wraper on September 25, 2018, 02:29:42 pm ---
--- Quote from: Rapsey on September 25, 2018, 02:09:12 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.
--- End quote ---
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.
--- End quote ---
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.
--- Quote from: Rapsey on September 25, 2018, 02:09:12 pm ---
--- Quote from: Hero999 on September 24, 2018, 10:02:48 pm ---The peak current can be calculated from the supply voltage and resistor value.
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How would you calculate that?
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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.
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--- Quote from: Hero999 on September 25, 2018, 07:45:07 am ---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.
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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.
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Those inductors are completely unsuitable. The DC current rating is far too low.
Rapsey:
--- Quote from: Hero999 on September 25, 2018, 03:38:01 pm ---
--- Quote from: wraper on September 25, 2018, 02:29:42 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.
--- End quote ---
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.
--- End quote ---
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.
--- Quote from: Hero999 on September 25, 2018, 03:38:01 pm ---
--- Quote from: Rapsey on September 25, 2018, 02:09:12 pm ---
--- Quote from: Hero999 on September 24, 2018, 10:02:48 pm ---The peak current can be calculated from the supply voltage and resistor value.
--- End quote ---
How would you calculate that?
--- End quote ---
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.
--- End quote ---
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.
wraper:
--- Quote from: Hero999 on September 25, 2018, 03:38:01 pm ---
--- Quote from: wraper on September 25, 2018, 02:29:42 pm ---
--- Quote from: Rapsey on September 25, 2018, 02:09:12 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.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
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.
Rapsey:
--- Quote from: wraper 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.
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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).
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