Why does the rise time of a parallel RC circuit increase when the freq. is increased?

Are you sure it really takes longer and it's not just that the rise time takes up a greater fraction of the square wave period? Since the period is shorter at higher frequencies, that's what you would expect to see.

Time constant = R*C, but only for a series circuit and constant DC voltage.

I'm a little confused when you talk about increasing the frequency.

If you apply AC, the output should never flatten.

If you're talking about a pulsed DC voltage or rectified AC, that makes more sense, but remember the DC voltage is only applied part of the time, so the capacitor would take longer to charge. Still, as long as the duty cycle is the same, increasing frequency shouldn't matter.

Parallel Rc Circuit

you are seeing some other effect. Perhaps the output impedance or the rise time of the generator is changing.

Or you are overloading the generator. Generators don't respond well to capacitive loads.

Or perhaps you have the frequency turned up so high that the output waveform doesn't have time to reach it's steady state value.

Because rise time is not dependent on rep rate.

My guess is that you are overloading the generator.

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