Danger: Electronics Calculator at Work

[rackmonkey]

I have this little iOS app called EE Toolkit that’s proven pretty handy. But today, responding to a post on  a cutterhead circuit (https://groupdiy.com/index.php?topic=73531.0), I pulled it out and uncritically looked at the results of one of the circuit calculations and formed a response based on those. But after Squarewave corrected my response I went back and thought about it more. Now it’s got me wanting to check my understanding in calling out an error in the app.

There’s a tab for calculating various RC circuit parameters. For a circuit like the RC portion (only) of the circuit presented in the cutterhead post I pasted a link to above, the calculator gives a cutoff frequency. (The author calls that circuit a “parallel RC circuit”.)

But I believe there is no cutoff frequency for this circuit like there is for parallel RC high pass and low pass filters.  There’s a cutoff frequency for HP and LP filters because there’s a path to ground for the filtered frequencies. But with the RC portion of the circuit in the cutterhead post (for which there’s a generic model of in the calculator), there isn’t such a path. So it’s erroneous to apply the equation for cutoff frequency to that RC circuit, is it not?

Here’s a screenshot of the tab with the circuit values applied and the calculated results shown. I don’t need a lesson in how HP and LP filters work. Just curious whether I’m missing something in thinking that the cutoff frequency equation shown in the calculator doesn’t apply here and shouldn’t be shown in this tab.

 

[radardoug]

The problem is you haven't put the head into the circuit, which is difficult as you have no figures for it.

 

[rackmonkey]

You can’t put the head into the circuit,. You can’t add anything to it. The app just presents static circuits that can’t be configured except for the values of the passive components. It’s just a calculator for a bunch of different common circuits, not a tool you can draw a circuit with and have it calculate things based on that.

Make sense?

I’m not asking about the cutterhead circuit itself. I’m asking whether it’s erroneous for this calculator to apply the equation for cutoff frequency to the standalone RC circuit in the screenshot above, as I believe it is. Since there no path to ground for filtered frequencies, it’s not a filter as such. Correct? Incorrect? Should the author not have applied that formula here?

thanks

 

[john12ax7]

It is telling you at what frequency the magnitude of the impedances is equal,  which is a useful calculation.  But it is incorrect to call it cutoff frequency.

 

[ruffrecords]

It is telling you at what frequency the magnitude of the impedances is equal,  which is a useful calculation.  But it is incorrect to call it cutoff frequency.

It would more commonly called turnover frequency. In the network shown, below the turnover frequency, the impedance is predominantly resistive and above it, it becomes predominantly capacitive.

Cheers

Ian

 

[rackmonkey]

And...the lights all turn on.

Thanks for the replies, gents. Very, very helpful!

 

[ruffrecords]

And...the lights all turn on.

That should be the group motto - "To keep the light of understanding turned on"

Cheers

Ian