Hi. I tried to calculate the time constant on this circuit that includes elements like diodes and transistors together with the capacitor and since about a week ago i found out that Time constant for a capacitor isnt simply C times the resistor it charges/discharges through(unless the circuit is R and C in series), i have had a little trouble calculating it in circuits like the above one. Did i calculated this one right?

 

The time constant is RC. You can ignore the 10k resistor and you have lost a factor of 10-3 (time const is 4.7 ms) but, yes, that's the quick answer, a little more than 4.7ms.

 

For an exact answer, you can use:
Vc = Vs(1-exp(-t/RC))
8.1 =12(1-exp(-t/RS))
tidy up, take natural logs and get t = 1.12*RC = 5.26mS

 

The charge formula when there is no initial voltage (Vc=0 at t=0) looks like this:

Vc=Vcc*(1-e^(-t/RC))

Solve for time we get this

t = RC•ln(Vcc/(Vcc - Vc))

t = 47kΩ*100μF•ln(12V/(12V - 8.1V)) = 47kΩ*100μF•ln(3.07) = 47kΩ*100μF * 1.12 = 5.2s

 

My thanks!