Why does graphical representation of a DC source always a straight line?

enter image description here enter image description here

  • 4
    \$\begingroup\$ Because the frequency is 0 and the value is unchanging over time. \$\endgroup\$
    – dext0rb
    Commented Nov 15, 2013 at 18:16
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    \$\begingroup\$ Are you asking about the graph of a value over time, or the I-V curve of the device? \$\endgroup\$
    – The Photon
    Commented Nov 15, 2013 at 18:31
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    \$\begingroup\$ You need to define your terms better. What "graphical representation"? What is being plotted as a function of what? Pins remaining by number of insertions? AQL by volume and whether customer pays quickly or slowly? We do engineering here where such hand waving has no place. \$\endgroup\$ Commented Nov 15, 2013 at 19:54
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    \$\begingroup\$ This question appears to be off-topic because it demonstrates an utter lack of basic research or thought. \$\endgroup\$ Commented Nov 16, 2013 at 17:47
  • \$\begingroup\$ related: What is AC actually? \$\endgroup\$ Commented Oct 1, 2017 at 3:16

3 Answers 3


Because its value doesn't change over time, that is, it can be written as a constant function. Its graph is, as you'd expect... a straight line.

enter image description here


[This started as a comment, then it ran out of space.]

The drawing in the O.P. probably comes from the Wikipedia article for DC. The drawing looks watered down [oversimplified]. For starters, the axes aren't labeled. Also, what is "variable" gray trace supposed to mean? Aren't green and and blue traces variable too? They aren't constant for sure.

More importantly, there are 2 definitions of DC, and the drawing doesn't reflect this.

  1. By one definition "DC" means that the signal is constant. This is an abstraction, because there's always some variability and noise in real signals. Nevertheless, this is a useful abstraction.
  2. By the other definition, "DC" means that the value doesn't cross into the negative. By this definition, both blue and red traces are DC. Likewise, rectified AC qualifies as DC even if it's not filtered. Sometimes, non-negative varying DC is referred to as "dirty DC".

In the engineering practice, it's usually apparent from the context, which definition is implied. For example: "DC bias" implies the 1st definition, "DC sensor output" implies the 2nd definition.


Actually, every DC voltage is nothing but an AC voltage with zero frequency.

\$ V_{DC} = V_{PEAK} \times \sin(2\pi\omega t + \theta) \big|_{\omega=0} = V_{PEAK} \times \sin(\theta); \\ \quad \text{where} \; V_{PEAK} \text{ and } \theta \text{ are fixed parameters.} \$

Its frequency is zero, so it is not fixed, not changing in time. In other words, it is a fixed value function. That's why its representation is a straight line.


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