Simple circuit Problem

152

03:22:14

Hey guys, I am just having trouble  calculating the time it takes for the transistor to switch off after releasing the switch. 
Assume we power on, and released the switch, using the equation Vc = Vo*e^(-t/RC). I replaced Vc by 0.7 Volts, realizing that the transistor will turn off Vb<0.7, I get t app = 1.5 seconds
But here, the led turns off after 3 seconds.
What am I missing?

published 10 years ago

rich11292000

10 years ago

Time is slower than real life

Lawliett

10 years ago

@rich11292000 come on man for serious, Its a 50 or more percent different answer. Is my calculations right or wrong?

Nidron

10 years ago

Transistor never will cut off because the final value of the c is 0.7, the same than vbe. On the other hand the forward beta is enough big to hold the maximun ic for a few seconds before it starts to fall, this time should be calculated first and added to the exponential equation

yra

10 years ago

When you see the clock symbol next to the play button you can adjust to real time or any other time you need to monitor all actions in the circuit.

Lawliett

10 years ago

@Nidron But how can Vc final be 0.7 when it discharges? And is there an equation to determine the few seconds?

Lawliett

10 years ago

@Yra so does that mean my calculations are correct? I just want to know the value of t.

yra

10 years ago

Your math is close, 4700*.00022= 1.034 time. The program is setup to calc for ideal conditions mostly, the math behind it can be tough for real world, assume a sine wave and 5 cycles to fully charge a cap. Now assume that the cap low limit is .7, the math would look like this: time = 1.034*5*.7= 3.619 time led is on.

yra

10 years ago

The formula is derived from ec (examples; rc step response)

yra

10 years ago

This program is a good way to try out ideas without spending lots of money on part's it's also ideal conditions not real world, and for the price it's a great tool, if you need more you have to spend 10's of thousands on a cad system like Motorola has in R&D dept.

Lawliett

10 years ago

@yra thanks man, I really do appreciate your answers. But only one thing I don't understand, why do the Vc final is 0.7? Shouldn't the Capacitor be discharging till it reaches app 0?

yra

10 years ago

It was explained to me that any voltage below .7 of rated is out of designed nominal or rated voltage range and considered unusable. A motor will suffer heating with low voltage, unusable voltage.

yra

10 years ago

I hope this helps you with your design of circuits.

brambolinie

10 years ago

You have asked me for help, but I have a lot of trouble with transistors aswell. I want to help but I can't I'm really sorry. Good luck!

Lawliett

10 years ago

@yra Thanks for the explanation. It helped me alot. But still, I have to dig on deeper in the transistor understanding. Thanks so much.

Lawliett

10 years ago

@brambolinie Don't worry! At least, now you can look at the comments I received. They are really helpful.

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