Hi

I know that to find R in a RC Time constant is to do the following

eg: 2(seconds) / 0.000470(farads) / 0.693 = 6140ohms

what does 0.693 represent, i thought that it might have been 0.7V?

 

I think it indirectly comes from the constant e:
http://en.wikipedia.org/wiki/E_(mathematical_constant)
Maybe you meant 0.632 which is the point in the RC curve where the voltage equals 63.2% and T=RC?
http://ourworld.compuserve.com/homep..._Bowden/rc.htm

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The way the resistor-capacitor charging equation works out, if you set the time duration to be equal to R*C (the RC time constant which also appears in the equation) then the capacitor will have charged to 63% of the full voltage, or discharge down to 100%-63% = 37% of the full voltage.

The 0.693 number used for 555 timer calculations comes from how the voltage charges back and forth between 1/3V and 2/3V, while the RC time constant is for charging and discharging between 0V and +V.

...I think. I've never actually sat down and did the calculations. THat's just my reasoning. I'd like to know too actually.

 

If you want to find the time constant, you don't need the 0.693

The time constant is just RC

The time constant is the total change divided by the initial rate of change.
For example, 470 µF and 10 khm: resistor, time constant is 4.7 s.

If the capacitor is charged to 5 V, the current is 5 / 10000 = 0.5 mA
0.5 mA causes the capacitor to change at 0.0005 / 0.00047 = 1.06 V / second.
Time constant is 5 / 1.06 = 4.7 s, and it doesn't matter what starting voltage you chose.

(Time constants apply to all sorts of things, not just electricity)

Because the voltage falls, the current falls, so the rate of change of voltage also falls. It's called exponential decay.

The formula is V = Vstart * e^(-t/RC)
where V is the voltage on the capacitor
Vstart it what it starts at
e is the mathematical constant = 2.717......
t is the time in seconds
R is the resitance
C is the capacitance

The way that time constants are often measure or used is with "half-lives", or the time to get to half the start value.

If the time to get to half the start value is Th, then:-

0.5 = e^(-Th/RC)

This gives Th/RC = 0.693 and that is where 0.693 comes from.

 

0.693 is used to calculate how long the capacitor charges to 67% of the supply voltage then discharges to 33% of the supply voltage in a 555 astable oscillator circuit.

Attached Images

555 timing.PNG (16.8 KB, 12 views)

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