Why is this happening?

[mstevens]

In technology circles, descriptions and terminology are all about precision: stating exactly what is meant without ambiguity or possibility of misinterpretation. Under these circumstances a steady unvarying voltage is different from a varying voltage such as from a full wave rectifier, even if the net heating effect of either is the same. Physicists and engineers will therefore be precise about which is which whenever this matters. A fluctuating DC voltage will be described as a "DC voltage with an AC component superimposed" or an "AC voltage with a DC offset". Any rectified voltage may be loosely described as "DC", but the precise term DC is reserved for voltages with no frequency components above 0 Hz. (You would likely be very upset if your DC lab power supply had ripple on it, and would reject any argument of "But it is DC! The voltage doesn't reverse direction!")

I believe it is bad for to not teach beginners correctly. I did not make the definitions, I'm just giving them to you. Hmmm... thought experiment directed to you IanB... Say you have a .1 mVpp AC signal with a 12V DC offset is that AC or DC?  Say you have a 1 uVPP AC signal with a 12V DC offset is that AC or DC? ... My point... when does the AC quit becoming AC and is just a perturbation to the DC?

I agree with you that you hear the terms an AC signal with a DC offset, or a DC voltage with some AC ripple; but, considering the strict, accurate definitions of AC and DC, those are misnomers. One should refrain from using them with beginners because they can be potentially confusing.

A rectified (full or half) sinusoid is DC; it may not be and ideal DC source but it is well defined unambiguous DC. I'm typing on a bad keyboard now so I won't do it. But, find a post the definition of DC. Then explain to me the ambiguity of how a full or half wave rectified sinusoid does not adhere to the definition.

Have you noticed that all DC power supplies specify "ripple" and not AC on the DC voltage? They all have as you put it, some AC component on the DC voltage. I don't think you could find any datasheet/specs on ANY DC that would refer to that AC component as AC and not as "ripple."

The fact that were having this discussion shows the need for proper education. Per my statements to rolycat there is no ambiguity in the definitions of AC and DC; physics does not work like that. In Physics, definitions are ALL well defined and unambiguous; they must be or we don't have science.

[Zero999]

Why are you lot bothering to argue about this?

The correct way of dealing with this is to post a drawing. There's no point in arguing about definitions and semantics. The original poster probably got bored pretty quickly and hasn't returned.

[AG6QR]

According to the dictionary definition, AC is defined as alternating current not voltage.

So if you want to be pedantic about the dictionary definition, it's always incorrect to use the term "VAC" or "Volts Alternating Current".  Current is measured in amps, not volts.  What you see displayed on a typical oscilloscope screen is a voltage, not a current, and if it's alternating, then it should be called alternating voltage.  Furthermore, multimeters should not use "AC" and "DC" to denote settings that are designed to measure voltages.

A mains outlet may be providing current flow in only one direction, even while it's providing voltage that alternates.  Would that be called AC or DC?  A battery may be providing a steady voltage, but the current may alternate.  Is that AC or DC?  Or do you need to introduce terms like "AV/DV" for alternating voltage and direct voltage?

This is a rathole from which there is no escape, I'm afraid.  The pedants lost when we started to use the term AC to refer to voltage.

My point is that the terms "AC" and "DC" don't always mean what the dictionary says.  In practice, they usually refer to voltages, not currents, though sometimes voltages and currents both alternate.  And the term "AC" often refers to voltages which are not constant, but which have a periodic waveform, sometimes regardless of whether that waveform crosses the zero line. 

I agree wholeheartedly with C4757p:   If you have a waveform that is not obviously either DC or AC, describe it instead of arguing about terminology.

[dannyf]

i am interested to find out why do i get those resaults,

DMMs have very high input impedance and low capacitance. With no load (resistance or capacitance) after the rectifier, the reading is really undefined - dependent on the meters used.

With much lower input impedance, analog meters will provide better consistency.

High input impedance also makes the whole circuit suspect to static.

[Falcon69]

If I am driving north at 60miles/hour, reducing my speed to 10 miles and hour does not mean I change direction. If stop (0 mi/hr) and then start traveling north again does not mean I change direction.

But, if you were driving west at 1,675 km/hour (the speed of the rotation of earth), and slow down below that, say, 1670km/hour, then technically, you WOULD be changing direction at a speed of 5km/hour. Even though you would not physically know it.