2/4: More with the Digital Oscilloscopes and AC Voltage Divider, Experimentally...and Transistors!

in which Julia investigates the oscilloscope, function generator, and MOSFET

First we completed Hayes' & Horowitz's Lab 1-5.  We connected the function generator directly to the oscilloscope and learned to trigger the scope, following H&H's sage advice:
"Watch the edge that triggers the scope, rather than trigger on one event and watch another.  If you watch the trigger event, you will find that you can sweep the scope fast without losing the display off the right side of the screen."
(Actually, first we learned to press the "output" button on the function generator to actually produce a wave.  Interestingly, even when "off" the function generator still generates small pulses on the nanosecond scale that differ based on its mode--for example, the nanosecond "noise" of a turned-off sine wave program is more sinusoidal than the "noise" of a turned-off square wave program.)
We explored the "vertical gain" knob, which controls the y-axis scale (volts/division), and the horizontal sweep speed, which controls the x-axis (time).  We observed a square wave, and although the voltage appeared to rise instantaneously to a constant value on the 50-ms scale, adjusting the sweep speed (zooming in to the nanosecond scale) revealed an extremely short risetime.  We used a time cursor to approximate the risetime at 30 ns.
Connecting the SYNC cable from the function generator to the scope as well added a square wave that lines up with output wave (i.e., the peak of the sine wave output occurred at the middle of the square wave), as shown below:

The function generator's sine wave output (yellow) aligned with its sync output (blue).

Because the sync's wave has predictable, clearly defined edges, it is often easier to find an appropriate trigger edge by following the sync output instead of the function generator's output (such as a sine wave).

The function generator outputing a "pulse" (yellow) along with the sync (blue).

We set the scope to "AC" and added an offset to the function generator wave, which made the output not line up with sync. (So keep the scope on DC!)
Finally we measured the frequency of several waves (to good agreement with the programmed frequency on the function generator); we measured the period both manually (by using the time cursors) and automatically (by pressing the "measure" key).

We then continued with H&H's Lab 1-6:

After some initial confusion based on how to actually measure a resistor in circuit (if you try to measure just by placing the scope leads on either end of the first resistor in series, it's an inaccurate reading that might damage the scope), we concluded that this setup halves the amplitude of the function generator's wave.  This is not a particularly surprising conclusion--the voltage divider divided the voltage!

A diagram of our very simple function generator circuit.

A diagram of our circuit.

Finally we turned our attention to transistors--specifically, a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET).  Basically an electronically-controlled switch, a transistor will allow current to flow (or not flow) depending on the voltage applied to it (its "gate voltage").  This is typically small (1.5 V in our case), and its enormously high resistance (hundreds of millions of ohms!) means a MOSFET requires very little applied current--the LogoChip can easily supply 1.5 V. (Another useful analogy is of a voltage-controlled resistor; at high voltages, the MOSFET effectively has no resistance, and at low voltages effectively infinite resistance.) A MOSFET design typically consists of a p-type semiconductor with two channels of n-type semiconductor surrounding a top layer of silicon dioxide, capped with metal oxide.

Essential MOSFET rules (from Electronics pdf):
         1) For an n-channel MOSFET (shown below), if Vgs<=0 V (i.e., gate is less positive than source), the channel is non-conducting (A and B below).
         2) For an n-channel MOSFET, if Vgs >= 1.5 V (i.e., gate is more than 1.5 V more positive than source), the channel is conducting (D below).

A MOSFET diagram showing the effects of various gate voltages. As a switch, a MOSFET is used in accumulation (B, "off") or inversion (D, "on"). (modified from Ismeil-Beigi lab group at Yale).

We built a motor-powering circuit using a LogoChip to control an IRL510 MOSFET to control current flowing from batteries to the motor. 

Pin diagram for an IRL510 MOSFET showing Gate (G), Drain (D), and Source (S), from Electronics pdf.

The LogoChip + MOSFET + motor circuit--we later changed to +9 V by adding a second battery pack. (image from the "Electronics" document of Phys 310)

MOSFET circuit with one battery pack (+4.5 V) powering the motor.

This circuit sort of works--setting the LogoChip's "output1" pin high makes the MOSFET conduct and the motor turn (setting low stops the MOSFET's conduction and turns the motor off).  But the motor doesn't run particularly well (it doesn't take much torque to manually stall the motor), because it's designed to run on 9 V, not 4.5.  What to do?  Add another battery back!

MOSFET circuit with two battery packs (+9V) powering the motor.

Now the motor runs well--it takes much more torque to manually stall a motor powered with 9 V.
In this circuit, the LogoChip doesn't provide the voltage supplied to the motor, but can still control whether current flows. (So we don't fry the LogoChip (it can only handle about 5 V) but we use it to turn the motor on or off, with enough current to make the motor run well.)

Next Lab: RC circuits!