- Rising Voltage Graph -
e_{c}= E_{T}[1-e^{-(T/t)}]
The rise time for a resistor, capacitor combination is shown in the
graphic above.
1 Time constant [TC] equal R x C. Two TC's equals 2 x[RC],
and so on.
The Time constant is the time it would take for the potential
difference across the capacitor to increase to the same level as the
applied voltage.
The capacitance voltage rises at an exponential rate.
Keep in mind, the voltage over the capacitor falls at the same rate.
The Capacitor Falling voltage is
shown on it's own page.
Additional Data which can be derived from the graph:
0.2 time constant equals 80% amplitude.
0.7 time constant equals 50% amplitude.
Also keep in mind that normal IC logic uses 10% and 90% as nominal Pulse rise time values.
That would be 10% of final value and 90% of the final
voltage value. So in some cases you may need to worry about the 90 to
100% rise time.
Definition of values for the equation above:
T = Time [in seconds]
t = Time Constant
e_{c} = Voltage over the capacitor at any instant
E_{T} = Voltage applied to the circuit
R = Resistance in Ohms
C = Capacitance in Farads
RC Circuit |
The time it takes for a capacitor to increase to it's final value is
determined by the capacitor value and the resistor value. |
Design Hint; Although the rising edge of a pulse will always take the shape of an exponential curve, it may not appear as such.
Depending on the rate of increase and the time scale that it's being viewed at, the wave may appear as something else.
Using a slow time scale or having a fast signal rise time may make the pulse rise time appear almost vertical.
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