Capacitor Values?? - Electronic Circuits Projects Diagrams Free

silvarblade · 4th March 2008, 03:11 PM

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 · 4th March 2008, 03:31 PM

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:
$X_C = \frac{1}{2 \pi FC}$

I²R · 4th March 2008, 05:20 PM

Quote:

Originally Posted by Hero999

The formular for impedance is:
$X_C = \frac{1}{2 \pi FC}$

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 · 4th March 2008, 08:50 PM

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

I²R · 4th March 2008, 09:39 PM

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 · 5th March 2008, 06:14 AM

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 · 5th March 2008, 10:44 AM

Quote:

Originally Posted by I²R

Impedance and reactance are not the same thing.

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

Quote:

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.

4th March 2008, 03:11 PM	(permalink)
silvarblade Experienced Member	Capacitor Values?? 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? __________________ i would explain it to you but it would make your brain explode!

4th March 2008, 03:31 PM	(permalink)
Hero999 Experienced Member	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: $X_C = \frac{1}{2 \pi FC}$ __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

4th March 2008, 08:50 PM	(permalink)
Hero999 Experienced Member	Yeah I know, it is the formula for the impedance of a capacitor. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

5th March 2008, 06:14 AM	(permalink)
silvarblade Experienced Member	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?? __________________ i would explain it to you but it would make your brain explode!

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4th March 2008, 09:39 PM	(permalink)
I²R Experienced Member	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.