# Does/can the magnitude of DC circuit vary along with time?

#### Splashed7184

Joined Jul 15, 2023
5
I am a beginner who is new to electronics, I would especially appreciate if y'all doesn't insert really complex topics, I would have no way of knowing.

I was reading a book and it said something along the lines of "A common use of this would be the output of a rectifier (dc) such as i(t) = ∣5 sin(377t)∣ amps or a sinusoidal current (ac) such as i(t) = 160 sin(377t) amps."

Now, I understand what it says about AC but not about DC, now, the DC magnitude seems to be varying as a function of sin of time but a quick search on Google yields contradictory results, so who is lying to me :')

#### DickCappels

Joined Aug 21, 2008
10,104
AC reverses direction periodically, sometimes randomly while the polarity of DC does not change. Even when DC fluctuates (meaning that it has an AC component) we usually still refer to it as DC since the major component is DC.

Not intending to put too fine a point on it, if the time dependent variation in the voltage is the part of the signal that is of interest, and it has AC and DC components, we may refer to it as AC with a DC offset.

#### SamR

Joined Mar 19, 2019
4,913
Yes DC, after AC rectification, does vary slightly. It's called Ripple. Which is why there are also capacitors in rectified power supplies to smooth the ripple.

#### Splashed7184

Joined Jul 15, 2023
5

#### Splashed7184

Joined Jul 15, 2023
5
Yes DC, after AC rectification, does vary slightly. It's called Ripple. Which is why there are also capacitors in rectified power supplies to smooth the ripple.
Thank you sir!
1. Would you say the google result I attached is wrong (technically) as it mandates that DC has a constant voltage?
Is it accurate to say DC (which is derived from an AC rectifier) roughly has constant magnitude?
2. Can a DC vary way too much? As mentioned here, in the example? i(t) = ∣5 sin(377t)∣
This seems like a DC pulse not a current as at t = 0 and when 377t = 2pi, the magnitude of the current would be 0. It basically isn't that constant, it goes from 0 to 1, is this a valid DC source?

#### ElectricSpidey

Joined Dec 2, 2017
2,636
The Google answer is not wrong, it's just a simplified example of DC from a battery, not a rectified/filtered source or such.

#### Splashed7184

Joined Jul 15, 2023
5
Thank you very much @ElectricSpidey ! I consider this thread solved and closed

Joined Jul 18, 2013
28,050
Many get confused as to the term AC when referring to a AC source that is followed for example by a 1 phase full wave rectifier producing a pulsing DC that transitions from peak DC to 0v.
The term AC should not be used in the rectified output, but DC or Pulsating DC.
AC is defined by a voltage that reverses Voltage Polarity and Current direction every cycle , whereas rectified/pulsating DC does not.

.

#### WBahn

Joined Mar 31, 2012
29,512
The Google answer IS wrong, because it very explicitly states that "the direction and magnitude of the current do not change."

The terms DC and AC are used in so many different contexts and have become, in most cases, nothing more than very loose categorizations. It is largely pointless to look at the words and then claim that they must mean something very specific -- doing that is only going to make the goal of effective communication less likely to be achieved. Instead, you need to focus on what the practical distinction that is meant in various contexts.