Capacitor Values?? - Electronic Circuits Projects Diagrams Free

silvarblade · 4th March 2008, 04:12 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, 04:31 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, 06:20 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, 09:51 PM

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

I²R · 4th March 2008, 10:39 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, 07:14 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, 11:45 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.

dukichh5536 · 18th April 2009, 05:21 PM

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 · 18th April 2009, 06:33 PM

Deppends on the application.

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

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4th March 2008, 04:12 PM	#1
silvarblade	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 just cant come up with a good one!

4th March 2008, 04:31 PM	#2
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: $X_C = \frac{1}{2 \pi FC}$ __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

4th March 2008, 09:51 PM	#4
Hero999	Yeah I know, it is the formula for the impedance of a capacitor. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

4th March 2008, 10:39 PM	#5
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.

5th March 2008, 07:14 AM	#6
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?? __________________ I just cant come up with a good one!

18th April 2009, 05:21 PM	#8
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?

18th April 2009, 06:33 PM	#9
Hayato	Deppends on the application. Can be used for filtering/bypassing, blocking DC, RC timing constants...