Basic Electronics Paper 2, May/June 2015

Question 2

  1. State two properties of a parallel RLC circuit at resonance.
  2. .

In Figure 1, calculate the value of

  1. Impedance;
  2. Total current;
  3. Voltage drop across the capacitor.



            The expected answers were:
 (a)       Two properties:
            (i)         Impedance is very high.                     
            (ii)        Current is minimum.              
            (iii)       XC  =  XL                                                                                                                                                                      
 (b)       (i)        
                                        =  11.18   Ω                                                            
                                     =  1.073A                                                                              
                             =  1.073 x 20  
                             =  21.46V  

            This is one that the examiner believes should have been differently posed; the first part
was on ‘parallel resonance’, whereas the second on series resonance. The parallel  
resonance is being featured for the first time  and important properties derived from the
V-I relationship given below as follows:
; from which we find that at resonance;

  • Capacitive reactance = inductive reactance
  • Impedance is maximum (or admittance is minimum) and is a pure resistance
  • Current is minimum or voltage is maximum.

Not many candidates could give the correct responses.
The second part is well-known series RLC circuit (though not at resonance). One common mistake by the candidates is that quite a few expressed impedance as
  instead of