# Xc calcs

Discussion in 'Math' started by roadey_carl, Sep 14, 2010.

Not open for further replies.
1. ### roadey_carl Thread Starter Active Member

Jun 5, 2009
116
5
Hi folks.

Im sure this is a easy one for you but Im struggling.
I want to work out what capacitor to use as a series capacitive reactance.
I know 1/ 2.∏.F.C = Xc. what calculation do I do if I have resistance but want to calculate what capacitor to use?

Thanks very much

Carl

2. ### Ghar Active Member

Mar 8, 2010
655
73
I have no idea what you're asking.

What resistance? What are you trying to do?

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
A complex impedance has two parts. The real part is the resistance and is plotted along the horizontal axis. Capacitative reactance is plotted along the neagative imaginary axis aka the jω-axis. The complex impedance is:

R - jXc

Code ( (Unknown Language)):
1.
2. The magnitude of the complex impedance is
3.  ___________
4. √R^2 + Xc^2
5.
6. and the angle is arctan(-Xc/R)
7.
Does that help?

4. ### roadey_carl Thread Starter Active Member

Jun 5, 2009
116
5
well what im trying to do is get a 12v fan working on 230v supply but using a capacitor in series with a bridge rectifirer to do this.

Supply = 230vac
Load = 12v @ 0.11mA.

so... 230-12 = 218v.

218/0.11 = 1982Ω

218x0.11 = 24watts.

so I know I need a resistor thats 2k @ 24 watts to do this.
But how can I do the calculation to give me the answer in capacitance... so I know what capacitor to use to do this?

5. ### bertus Administrator

Apr 5, 2008
15,806
2,389
Hello,

Sorry, we do not allow circuits directly connected to the mains.
Please use a small powersupply for the fan.

Bertus