Capacitor Values??

[silvarblade]

Can any one explain to me how you decide the capacitor values for a ckt. example a voltage divider for a transistor.. i know that the parallel combination of R1 and R2 is 1/10 of HfeRe but how do you choose Capacitor values?

 

[Hero999]

It depends on the application.

Sometimes the formula is given on the datasheet, i.e. 1.1RC for a monostable 555 timer.

You might just use a standard value, i.e. 100nF for a decoupling capacitor.

Often the capacitor's impedance is important because to want to make sure the capacitor can pass low enough frequency signal.

The formular for impedance is:
[latex]X_C = \frac{1}{2 \pi FC}[/latex]

 

[I²R]

The formular for impedance is:
[latex]X_C = \frac{1}{2 \pi FC}[/latex]

Pssst. That's the formula for capacitive reactance. Impedance is the complex sum of capacitive reactance, inductive reactance and DC resistance. But, you knew that.

 

[Hero999]

Yeah I know, it is the formula for the impedance of a capacitor.

 

[I²R]

Impedance and reactance are not the same thing. Sorry, but that is, once again, the formula for capacitive reactance, not the impedance of a capacitor. For an ideal capacitor, you can get away with stating that the value given is also its impedance, but real-world caps also have inductive (particularly large electrolytics) and resistive components which, while generally small, are nonzero.

 

[silvarblade]

i know that a capacitor has reactance. so is there no specific formula like ohms law for current,voltage,resistance which can be used to set the capacitor values in any circuit??

 

[Hero999]

Impedance and reactance are not the same thing.

They are as close is damn it if the circuit is predominantly capacitive.

Sorry, but that is, once again, the formula for capacitive reactance, not the impedance of a capacitor. For an ideal capacitor, you can get away with stating that the value given is also its impedance, but real-world caps also have inductive (particularly large electrolytics) and resistive components which, while generally small, are nonzero.

Sure, if you're looking at an electrolytic capacitor at 100MHz but for the purpose of this discussion we're not and no one would use such a capacitor at VHF.

 

[dukichh5536]

i know this is an off topic question, but plz answer me. Can someone tell me, what does capacitor really do in a circuit?

 

[Hayato]

Deppends on the application.

Can be used for filtering/bypassing, blocking DC, RC timing constants...