I have been looking into RC low pass filters and I have a question. Is there a link between the values of the resistor and capacitor or they should be selected in such a way that the desired cutoff frequency is obtained?
I used this calculator:
For a 1 ohm resistor and a 3300uF capacitor the frequency is 48.2 Hz.
For a 1 KOhm resistor and a 3.3uF capacitor the frequency is the same 48.2 Hz.
What I am asking is are the results of the 2 low pass filters the same? Is there a difference to the end result and the way they behave in the grand scheme of things? Is there a better combination of resistor and capacitor for that cutoff frequency? The 48.2 Hz is just an example to try and better understand low pass filters. I don't have a project that needs that frequency.
I am thinking that a higher value capacitor will make a PWM signal smoother when powering a LED, for example. So if I need this I should go with a smaller resistor value and a bigger capacitor. I haven't figured out in what circumstances a bigger resistor and smaller cap will work better.
You need to set the impedance appropriate to the use, not just the cutoff frequency. The value of R is
basically the impedance. An Arduino cannot drive 1 ohm, it can drive 1k or 10k.
If the filter has to drive some circuit then it needs to have a lower impedance than the circuit it drives.
For instance if you need to drive a 10k load, the value of R needs to be a lot less than 10k, perhaps 470 ohms
or so, but if the load is very high impedance (opamp input for instance), then R could be much larger.
jamestk:
I am thinking that a higher value capacitor will make a PWM signal smoother when powering a LED, for example. So if I need this I should go with a smaller resistor value and a bigger capacitor.
To add a bit to MarkT's point, an LED is a highly nonlinear device, but as an order of magnitude estimate of it's nominal impedance one can assume 1.4 V (for a red LED) / 20 ma = 70 Ohms. By Arduino standards that's low impedance, so your intuition that there would be less flicker with a larger capacitor is correct.
That said, the usual approach for LEDs as indicators is to use a PWM frequency that is high enough not to be apparent to the human eye. For non-indicator applications other considerations may drive the circuit design.
For a LED, the approach would be to make the resistor value the one you are going to use in any case to limit the current to the LED, and then use a capacitor somewhat larger than that calculated to match. However you would also want to add a diode in series with the resistor. Do you see why?
Note that the constant-current LED drivers available to drive luminaires (which are series chains of LEDs) all have an electrolytic capacitor across the output to stabilise the switchmode converter as I have explained in another discussion, which is why they cannot be dimmed by PWM.
