# DC circuit or AC circuit?

Discussion in 'General Electronics Chat' started by Jas9, Nov 12, 2010.

1. ### Jas9 Thread Starter Member

Sep 30, 2010
36
0
hello friends,
i am bit confused with my circuit to treat it as a dc circuit or ac circuit. Actually i have an equivalent circuit of a car battery (during charging phase) which consists of a resistance(r1) in series with parallel combination of capacitor and resistance(r2). The input is voltage and current from the rectifier but its not a pure dc as it has ripples. So should i consider the circuit as ac or dc?
the reason is i am struggling with the impedance component of the parallel rc circuit. If it is dc then what will be the reactance formula. I know the reactance term will be infinite because of zero frequency but i calculated the frequency from the voltage signal and its varying. So if we use Xc=1/jwc then should this jth term to be considered or not?
My final aim is to calculate r1 and my idea of doing it is calculating the impedance. So my logic is
Ztotal=r1+Zxcr2
i.e. total impedance Ztotal= V/I
r1=series resistance and
Zxcr2=Impedance of parallel r2 and c
r2=r2;
Xc=i/jwc

so what is the right way of calculating r1 if all other terms are given???
thanks

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The common image of a voltage DC is that it cannot vary with time. The thing that separates AC from DC is that AC voltages have excursions that go both positive and negative. A voltage is DC so long as the polarity stays the same - it can still be time-varying.

Think of a half wave rectified DC voltage. You see pulses that go from 0 volts to some positive or negative peak value. It's still DC because the voltage is always on one side of 0 volts.

Even though you may have a pulsing voltage , your battery charger outputs DC. Even so, if it's an alternator, there will be a high frequency component.

3. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
For a fullwave rectifier, the equation for the capacitor C is:

C = Vp / ( 2 * R * f * Vr )

where Vr is the voltage ripple, f is the frequency, R is the load resistance, and Vp is the peak voltage of the input.

As you can see, the larger the capacitance, the smaller the ripple voltage and the closer you will come to a smooth DC value.

You may also want to think about using a voltage regulator. This helps keep the DC voltage constant and further dampens the ripple. It depends on your needs really.

Theory @ http://www.electronics-tutorials.ws/diode/diode_6.html

4. ### timrobbins Active Member

Aug 29, 2009
318
16
Jas9,

Your model has to be 'behavioural' if you want to aim to model DC and AC characteristics. Simplistically, R1 is characterised by the short-circuit current rating of the battery - but R1 changes with time, as it is associated with the chemistry of battery capacity.

The model used to characterise impedance at ripple frequencies is a little more complex. What battery model references have you read up on - there are a lot around - this is a fairly extensively researched topic.