I'm having difficult calculating the capacitance when giving resistance, voltage, hertz, and output amps. Does anyone know how to solve this?

The question reads:

A resistor of 200 ohms and a capacitor of unknown value are connected to a 230 v, 50Hz supply resulting in a current of 0.85A flowing

Calculate the value of the capacitor in micro Farads, and show all the working.

Okay, so I came to the conclusion that the input current is 1.15 amps, but it says that the output current is 0.85A, how do I account for this with the equation?


by Henrys
September 28, 2020
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1 Answer

Answer by vanderghast

Without cap, you are right, it is 230 volt / 200 ohm = 1.15 amp.

With a cap, the reactance is not just due to the resistor but also to the cap. The cap "resists" with 1 / (2 pi f C) and when combined with a resistor, both are added using Pythagoras' like theorem for a rigth triangle. So

0.85 amp = 230 volt / ( 200^2 + 1/( 6.28 * 50 hz * C)^2 )^0.5

solved for C gives C = 17. +/- microFarad. The closest standard cap value is 18 microFarad. (Given the usual cap. tolerance, that is the most likely "intended" value).

+1 vote
by vanderghast
September 29, 2020

Technically, I should have use 325 volt p-p to get 230 volt rms but then, I would have not got 0.85 amp as easily spotted (or would have I ? ). Always embarrassing to get the most proper setting, with AC.

by vanderghast
September 29, 2020
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