In the previous unit you’ve used the oscilloscope and multimeter
to make measurements in ac circuits. As you know from EET 1150,
the multimeter can also make measurements in dc circuits.
The same is true for the oscilloscope. In this unit we’ll look more closely
at how to set the meter or the scope to measure ac or dc.
Work through the e-Lesson and self-test questions below.
Unit 2 Review
This unit will build on material that you studied in Unit
2. So let’s
begin by taking this self-test to review what you learned in that unit.
DC or AC?
The digital multimeter and the oscilloscope can each be used to measure
either DC voltages or AC voltages.
On the multimeter, the AC/DC switch lets you choose between
the two types of measurements.
On the oscilloscope, the input coupling switch lets you choose.
Let’s look more closely at how these two switches work.
Multimeter’s AC/DC Switch
The multimeter’s AC/DC Switch has only two positions: DC and AC.
The photo below shows the location of the AC/DC switch (in the yellow
box) on a Fluke 8050 digital multimeter.
In DC mode, the meter measures only DC voltages or currents,
and it ignores AC voltages or currents.
In AC mode, the meter measures only AC voltages or currents,
and it ignores DC voltages or currents.
At this point you might be saying to yourself, "That’s obvious." Maybe
it is, but when we turn to the oscilloscope, we’ll find that things
are not always as obvious as they might seem. So please don’t skip
over the self-test questions below.
Combined AC/DC Circuits
So far in this course, all the circuits we’ve built and studied have
had either DC voltage sources or AC voltage sources, but not both.
Many real-life circuits contain both AC voltage sources and
DC voltage sources. In such a circuit, a resistor will have both an
AC current and a DC current running through it at the same time.
Also, the resistor will have both an AC voltage drop and a DC voltage
For a circuit that has both AC and DC, it’s especially important
to remember that the multimeter in DC mode shows you only DC
voltages or currents, and that the multimeter in AC mode shows
you only AC voltages or currents. The self-test will give you
practice with some examples.
Oscilloscope for DC
We’ve seen that the multimeter can be used to measure both DC and
AC currents and voltages. It’s also true that the oscilloscope can
be used to display either AC or DC voltages.
In Unit 6 we saw examples of sine waves, triangle waves, and square
waves displayed on the oscilloscope screen. These are all AC voltages.
But what would a DC voltage look like on the scope’s screen?
Answer: When you display a fixed DC voltage on the oscilloscope
screen, you’ll just see a flat line, as in the picture below.
This makes sense if you remember that the oscilloscope displays a
graph of voltage versus time. If voltage has a constant, unchanging
value (such as 5 V), then the graph will be pretty boring—just
a flat horizontal line.
Still, even though we just see a flat line, we can use the scope
to measure the value of a DC voltage by seeing how far this flat line
is above or below an imaginary line on the screen that represents 0
Take the self-test to see how to do it.
Negative DC Voltages on the Oscilloscope
On the oscilloscope, positive voltages will appear above the 0-volt
reference line, and negative voltages will appear below the
0-volt reference line. (As mentioned in the previous self-test, you
can choose the position of this 0-volt reference line. We’ll see below how
to do this.)
So if you display a negative DC voltage on the scope, it will still
show up as a flat line, but this flat line will lie below the 0-volt
reference line, not above it.
Example: Suppose you’ve adjusted the scope so that its centerline
represents 0 volts. If you saw the image shown below, you’d be
looking at a negative voltage, since the trace lies below the 0-volt
Oscilloscope’s Input Coupling Switch
The input coupling switch on an oscilloscope lets you control
the type of voltage (AC or DC) displayed on the screen. An oscilloscope
has one input coupling switch for each of its input channels. For example,
a dual-input scope has two input coupling switches, one for Channel
1 and one for Channel 2.
The photo below shows the location of the two input coupling switches
(in the yellow boxes) on a Tektronix 2213.
Each input coupling switch has three positions: AC, DC, and GROUND.
Continue reading for information on each of these three positions.
Input Coupling Switch Set to AC
When the switch is set to AC, only AC voltage is displayed;
DC voltage is blocked.
Use this position when you wish to measure AC voltage only, ignoring
any DC voltage present in the circuit.
Input Coupling Switch Set to DC
When the input coupling switch is set to DC, both AC and DC voltages
Here’s why I said earlier that things are not always as obvious as
they seem. Many students make the mistake of
thinking that when this switch is set to DC, the oscilloscope will
only display DC voltage.
Use this position when you wish to measure DC voltage, or when you
want to measure a combination of AC and DC voltages.
Input Coupling Switch Set to GROUND
When the input coupling switch is set to GROUND, no input voltage
is displayed; both AC and DC are blocked.
Use this GROUND setting to establish a 0-volt reference line. Here’s
Set the input coupling switch to GROUND.
Use the vertical position control to move the trace to the position
on the screen that you wish to use as your 0-volt reference line.
Switch the input coupling switch back to either AC or DC, depending
on the type of measurement you’re making.
Where Should You Set Your 0-Volt Reference Line?
Remember, you control where the 0-volt line lies on the oscilloscope
screen. By using the input coupling switch and the vertical position
control, you can adjust the scope so that 0 volts is at the bottom
of the screen, in the center of the screen, or anywhere else on the
If you expect to be dealing with both positive and negative voltages,
it’s usually best to use the centerline of the screen as your
0-volt reference. That way, positive voltages will be displayed in
the screen’s upper half, and negative voltages will be displayed in
the screen’s lower half.
If you expect to be dealing with positive voltages only, it’s
probably best to use the screen’s bottom line as your 0-volt
reference, since all positive voltages will be displayed above this
Although it’s not a very common situation, if you expect to be dealing
with negative voltages only, it’s probably best to use the screen’s top
line as your 0-volt reference, since all negative voltages will
be displayed below this line.
Oscilloscope for Combined AC/DC Circuits
Let’s think about what you’ll see on the oscilloscope screen if you
measure a voltage that contains both AC and DC.
For example, suppose we’re measuring a voltage that combines a
12 Vpp sine wave with a 2 V DC voltage. Let’s
assume that the scope’s VOLTS/DIV switch is set to 2 V, and
that the centerline has been set as the 0-volt reference line.
We know that the 12 Vpp sine wave by itself will look like
And we know that the 2 V DC voltage by itself will look like this:
So what will we see if we look at the combination of these two voltages?
Answer: we’ll see the sine wave pushed upward by 2 volts. In
other words, it will look like this:
Notice that the sine wave still has a peak-to-peak voltage of 12 volts,
but instead of going from a low value of -6 volts to a high value
of +6 volts, the sine wave goes from a low value of -4 volts
to a high value of +8 volts. If you think about it, this should
make sense, because it’s what you would expect to get if you took a
normal sine wave and pushed it upward by 2 volts.
Of course, you’ll only see that picture if the input coupling switch
is set to DC. If the switch is set to AC, then only the AC voltage
will be displayed, and if the switch is set to GROUND, then neither
the AC nor the DC voltage will be displayed.
Unit 3 Review
This e-Lesson has covered some important points on using the oscilloscope
to measure DC voltages or a combination of DC and AC voltages.
To finish the e-Lesson, take this self-test to check your understanding
of these topics.
Congratulations! You’ve completed the e-Lesson for this unit.