Oscilloscope Trigger Controls

A student is trying to measure an AC waveform superimposed on a DC voltage, output by the following circuit:

The problem is, every time the student moves the circuit’s DC bias adjustment knob, the oscilloscope loses its triggering and the waveform begins to wildly scroll across the width of the screen. In order to get the oscilloscope to trigger on the AC signal again, the student must likewise move the trigger level knob on the oscilloscope panel. Inspect the settings on the student’s oscilloscope (shown here) and determine what could be configured differently to achieve consistent triggering so the student won’t have to re-adjust the trigger level every time she re-adjusts the circuit’s DC bias voltage:

A student wants to measure the “ripple” voltage from an AC-DC power supply. This is the small AC voltage superimposed on the DC output of the power supply, that is a natural consequence of AC-to-DC conversion. In a well-designed power supply, this “ripple” voltage is minimal, usually in the range of millivolts peak-to-peak even if the DC voltage is 20 volts or more. Displaying this “ripple” voltage on an oscilloscope can be quite a challenge to the new student.

This particular student already knows about the AC/DC coupling controls on the oscilloscope’s input. Set to the “DC” coupling mode, the ripple is a barely-visible squiggle on an otherwise straight line:

After switching the input channel’s coupling control to “AC”, the student increases the vertical sensitivity (fewer volts per division) to magnify the ripple voltage. The problem is, the ripple waveform is not engaging the oscilloscope’s triggering. Instead, all the student sees is a blur as the waveform quickly scrolls horizontally on the screen:

Explain what setting(s) the student can change on the oscilloscope to properly trigger this waveform so it will “hold still” on the screen.

All electric motors exhibit a large “inrush” current when initially started, due to the complete lack of counter-EMF when the rotor has not yet begun to turn. In some applications it is very important to know how large this transient current is. Shown here is a measurement setup for an oscilloscope to graph the inrush current to a DC motor:

Explain how this circuit configuration enables the oscilloscope to measure motor current, when it plainly is a voltage-measuring instrument.

Also, explain how the oscilloscope may be set up to display only one “sweep” across the screen when the motor is started, and where the vertical and horizontal sensitivity knobs ought to be set to properly read the inrush current.