- Falling Voltage Graph -

**e _{c}= E_{T}[e^{-(T/t)}]**

The fall 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 decrease to zero voltage.

The capacitance voltage rises at an exponential rate.

Keep in mind, the
voltage over the capacitor rises at the same rate. The Capacitor Rising voltage is
shown on it's own page.

Additional Data which can be derived from the graph:

0.2 time constant equals 20% amplitude.

0.7 time constant equals 50% amplitude.

Also keep in mind that normal IC logic uses 10% and 90% as nominal rise
time values.

That would be 10% of final value and 90% of 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

**t** = Time Constant

**e _{c}** = Voltage over the capacitor at any instant

The graph is meant to show the voltage decay in a capacitor over time.

However what it does not show is the effect a complex circuit might have on the decay.

That is a capacitor discharging through an inductive resistor.

The graph assumes a normalized capacitor discharging through a perfect resistor.

The graph also does not account for any leakage in the capacitor.

Leakage would also account for a small drop in voltage and error in the graph, assuming the time constant was long.

Back to the Logic Design Page, the Capacitor Information Page, Passive Filter Definition or Capacitor Manufacturer Listing Page

Logic Slew Rate | Trace Termination | Ground, Power Planes | Passive Design | IC Package Type Definitions |

Home
| |||||||

Distributors | Components | Equipment | Software | Standards | Buses | Design | Reference |

Copyright © 1998 - 2016 All rights reserved Larry Davis