RC Delay Calculator A basic javascript calculator for RC time constants and voltages
RC constants

A simple resistor and capacitor can be used to control the amount of time that it takes for an output signal to reach a specific voltage. This is particularly useful when you reset a device, to ensure, for example, that a pin does not reach logic high (0.7*VCC for CMOS) or pass logic low (0.3*VCC for CMOS) before x microseconds, giving the rest of the system time to perform some other tasks before-hand.

This simple calculator will tell you how many milliseconds are required to reach the specific output voltage (Vc), based on the supply voltage (V), the resistor value (R) and the capacitor value (C). The resistor value is assumed to be in K ohms, the capacitor in microfarads, and the calculated delay will be in milliseconds.

The formula used to calculate the delay is:

t = -log((V-Vc)/V)R*C

Note: To calculate Vc at a specific time, the formula can be modified to:

Vc = V-(V*exp(-t/(R*C)))
The calculator

Enter the first four values, and click "Calculate" to determine the delay to reach Vc:

V - Supply Voltage:
Vc - Output Voltage:
R - Resistor (KOhms):
C - Capacitor (uF):

Calculated Delay (ms):

The following image shows the results of a 2.0K resistor and a 0.1uF capacitor using a 3.3V supply.

The blue line is the HW reset, and the yellow line is the output of the RC pair. You can see that it takes about 230us (.23ms) to reach 2.3V, which is rougly logic high on a CMOS device using 3.3V logic (0.7VCC). This matches the calculator above if you set the following values:

V = 3.3
Vc = 2.3
R = 2
C = .1

April 27, 2012 11:17