Basic Diode Question on Oscilloscope - AC vs DC coupling

[johnnystarr]

I am using the student edition of Multisim to learn basic electronics.  A simple circuit with a 120vac 60 Hz supply and a 100 ohm resistor load shows the two figures below.  With the Mulitsim scope on DC coupling all looks correct with positive voltage half waves.  But on AC coupling, there is a shift in the peak (+) voltage and now it has a negative voltage component to it. 

Is the negative voltage when AC coupled due to the reverse recovery time?  As AC couple, I have changed the supply frequency from 60Hz down to as low as 2Hz and except for the timebase change, the sine wave voltages look exactly as for 60Hz.  I read that for typical diodes the Trr is measured in ns or us.

Thanks in advance for the help.

John

[johnnystarr]

Ooops forgot to mention that the circuit has a 1N4004 in series with the resistor load.

[ledtester]

Here's an explanation from the electronics stackexchange:

https://electronics.stackexchange.com/a/173758

Basically, in AC coupled mode the zero line represents the "average" of the input signal. See the comments in the first answer for a description of how this "average" is computed.

[NY2KW]

Basically, in AC coupled mode the zero line represents the "average" of the input signal. See the comments in the first answer for a description of how this "average" is computed

That sounds correct but it doesn't jive with some repetitive DC pulse work I did recently with my Keysight 3000x scope.  Maybe it's not that simple an answer.  I used the repetitive pulse training signals at 100Hz and the scope reads it the same either in AC or DC coupled mode, that is all positive, identical signals.  If it was just arbitrarily setting the 'average' signal at zero it should shift the pulse train down half-way but thats not the case

Jerry

[Terry Bites]

AC coupled. If there is no offset on the trace, the scope display is correct.

Note the areas "under the curve" above and below zero on your scope; they are the same. The waveform centres on at the average of the halfwave voltage.
Vav = Vpk/π  eqn. 1

see for the derivation of eqn. 1

[wasedadoc]

Basically, in AC coupled mode the zero line represents the "average" of the input signal. See the comments in the first answer for a description of how this "average" is computed

That sounds correct but it doesn't jive with some repetitive DC pulse work I did recently with my Keysight 3000x scope.  Maybe it's not that simple an answer.  I used the repetitive pulse training signals at 100Hz and the scope reads it the same either in AC or DC coupled mode, that is all positive, identical signals.  If it was just arbitrarily setting the 'average' signal at zero it should shift the pulse train down half-way but thats not the case

Jerry

If the pulse was already AC coupled somewhere before reaching the 'scope, then AC or DC coupling on the scope would make no difference.