[sdiy] Wanted: non mathematical description of the functionof RC-filters

[Tom Wiltshire]

Before I understood RC filters, I'd seen a capacitor charging up through a resistor. A capacitor is like a little battery, and the resistor changes how fast you charge it up. Smaller resistor equals faster charge. Pretty straightforward, especially if you can watch it on a scope or a voltmeter. Do a few experiments, try a few different caps and resistors, get a feel for it. Discharging a capacitor is similar. Short it with a wire and it discharges pretty much instantly. Use a big resistor and it takes a while. Watch that on the meter too.

The next step is feeding frequencies through it. Imagine we feed a sine wave in where we previous used DC. Obviously if the resistor is large, the capacitor won't have had time to charge up fully before the sine wave starts going back down again. So the output tries to follow the sine wave, but the amplitude of the sine wave is reduced. If the resistor is bigger, the capacitor charges less, and the output is quieter. If the frequency goes up, the capacitor doesn't have as much time to charge, so the output is quieter. By now you get the idea.

That was how I got it at first. Highpass took me a bit longer, but once I'd seen the lowpass, it wasn't hard to see it the same way, but with the voltage divider the other way up. But for high pass, you need the voltage divider concept too, which I didn't have initially.

There, that wasn't too bad, was it?

I was probably twelve years old or so, and didn't have (or need) any mathematical understanding of it.

HTH,
Tom