Why is DC Voltage called Direct Current ?

I have had this question since a long time, and didn't find any other place to ask this. So thought will just ask here.

The "DC" in DC Voltage/Supplies stands for Direct Current. So why is it combined with Voltage in the name ?

From what I understand it might have something to do with a supply or voltage which has a constant current at one level. Is that the reason ?

• AC Alternates Current direction. DC the current goes in one DIRECTion. – Majenko Mar 31 '15 at 21:58
• Who says "DC voltage power supply?" I've heard "DC power supply," and "<howevermany>-Volt DC supply?", and "constant voltage DC supply?" and many variations on the same; but I don't recall ever hearing the exact phrase that you are asking about. – Solomon Slow Apr 2 '15 at 18:12
• In some languages DC is called Continuous current. – sergiol Apr 5 '15 at 0:16

The Name

With a constant load, DC (direct current) has a constant current and voltage.

This is as opposed to AC (alternating current) which, with a constant load, has an alternating current and voltage. It's essentially a description of the current and voltage's relationship to time. For DC, the frequency is equal to zero, not changing in time. While AC has a frequency equal to not zero, it changes with time.

Relationship between Voltage and Current

The key to the relationship between current and voltage, is the load, or resistance. This comes from Ohm's Law, $V = IR$. As you can see, if $R$, the resistance is constant then voltage ($V$) is equal to the current ($I$) times a constant coefficient. Set $R = 1$ and the relationship is quite obvious.

• And as an adjunct, capacitances and inductances cause $R$ (or more correctly, $Z$) to be a complex number. – Ignacio Vazquez-Abrams Apr 2 '15 at 14:41

"DC Voltage" and "AC Voltage" are gramatically incorrect, but everyone uses those terms.

More gramatically correct terms if we are talking about voltage would be "DV (Direct Voltage)" or "AV (Alternating Voltage)" - but no-one says that.

DC Voltage is a voltage associated with an unchanging current, while AC Voltage is the voltage associated with a current that is continually changing direction.

It is because of the direction of current which doesn't alternates but flow only in one direction in the circuit.