### Author Topic: RC Time Constant  (Read 1441 times)

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#### John0922

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##### RC Time Constant
« on: May 21, 2015, 12:45:18 am »
Hi- I'm new to electronics, so please forgive what might be a very basic question. I came across a circuit that I built that allows a 10 micro-farad capacitor to charge to 5 volts through a 1K resistor with the press of a button (fed with dc), then it slowly discharges through a 6 meg resistor. It takes about a minute to discharge and works fine.

I thought if I fed the same circuit with a 5 volt square wave from a function generator at about 1 hz it would charge the capacitor and the capacitor would not discharge at all, because it was constantly being "topped off." Instead, it does discharge on the downstroke of the square wave. I thought this might be caused by the current changing direction, so I put in a DC offset to make it never go below zero volts, but got the same result.

From this I gather that the downstroke of the square wave is different from simply disconnecting a DC source. Could someone explain?

John

#### c4757p

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##### Re: RC Time Constant
« Reply #1 on: May 21, 2015, 12:51:50 am »
From this I gather that the downstroke of the square wave is different from simply disconnecting a DC source. Could someone explain?

Yes, very much so. What you have to keep in mind is that voltage is a relative measurement - even if your DC offset keeps the square wave above "zero", it still goes below the voltage that the capacitor was at at the top of the square wave, so it's still negative with respect to that node.

In other words, current can absolutely flow into a positive source, it just has to come from a more positive source.
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#### rs20

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##### Re: RC Time Constant
« Reply #2 on: May 21, 2015, 01:09:21 am »
I agree with c4757p. With a resistor, current flows from the high voltage side towards the low voltage side; and the "V" in the resistor formula (V = IR) refers to the relative voltage (i.e., the difference) across the resistor terminals.

If you want a source that alternates between 5V and "disconnected" state (also called "open", "high impedance", "Hi-Z", etc), there are many ways to achieve this -- have your function generator drive a transistor so that it switches on and off, and connect your resistor to 5V through the switch; use a diode to prevent the capacitor from discharging backwards through the resistor, etc.

#### John0922

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##### Re: RC Time Constant
« Reply #3 on: May 21, 2015, 02:42:39 am »
Thanks so much for the replies. I learned a lot!

John

#### IanB

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##### Re: RC Time Constant
« Reply #4 on: May 21, 2015, 03:36:40 am »
Also note that if you just "disconnected" your DC source from the 1 k resistor on the down stroke of the square wave, then the voltage on the resistor would not go down at all, and you would not in fact have a square wave. To make it a square wave, the function generator has to actively "pull" the voltage down to zero. The output from a signal generator may be considered as a "forcing function", because it actively forces the output voltage to follow a pattern.
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#### John0922

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##### Re: RC Time Constant
« Reply #5 on: May 21, 2015, 05:09:42 am »
Thanks Ian-- yes I noticed this when I was measuring the input and output when I disconnected from DC. They both reflected the charged state of the capacitor.

Smf