- Hello, I'm Dr. Barlow and I'm an instructor
in physics here at Santa Fe College and I wanna talk to you
today about making basic measurements in DC circuits.
We'd found in Physics II labs here that students
are challenged by making these measurements correctly;
so we thought we would create this video so they would
have a chance in their free time
to review this important topic.
I'm going to talk only about three very important
measurements today and that is making measurements
of voltage, current, and resistance in a simple DC circuit.
In the DC circuit that we're going to consider today
is very simple.
It'll have a battery up here, and three resistors
in series in this circuit.
I have four important points denoted
in this circuit: points one, two, three and four.
Please take note of them.
I'm gonna to refer to them later
when I'm making measurements.
It should ne noted that this circuit
and all those that we used in the labs here are of low power
and are not dangerous.
So now I wanted to talk briefly about the equipment
we're going to use and you're going to use
to make these measurements.
We have depicted here a digital multimeter and as the name
implies it's capable of making a variety of measurements
in DC and even AC circuits.
For instance, it can measure current, resistance, voltage,
and sometimes they can also measure capacitance.
But it's important that you have the meter set up correctly,
for the measurement you're going to make
or you will not get correct results.
So you'll see at the top there is a dial.
The dial must be set for the quantity you want to measure.
For instance, current would be over here,
resistance here, and voltage down here.
Then we must also select whether we want
an AC or DC quantity many times.
And the we must also select the range
appropriately in the dial up here.
Now down below you'll see the ports
where we're able to insert the probes that we will use
to interface with the circuit.
We will always have one in the common port.
But where we put the other probe depends
upon the quantity that we are measuring.
For instance, resistance or voltage, I would place the probe
in the left port, for current I would place it over here;
and then be thoughtful about the range
and I would be to make the measurement.
Here's another meter we're going to use,
sometimes in the lab, I wanted to mention.
This is an analog current meter and we also have
to have it set up correctly to make the measurement.
Notice there's a switch here that allows you
to select between AC or DC current.
Usually AC is denoted by a squiggly line on these meters.
So I have it there in the DC setting.
Again we'll have a probe always in the common
or the black port, but where we put the other probe
in this meter selects the range
which we're going to measure.
For instance, if I put the other probe here,
it says that meter will measure
between zero and 0.05 or 50 milliamps.
Place the probe here and I will have a range
of zero to 0.5 amps here; here zero to five amps.
So we have to have this meter set up correctly
as well to make the measurement.
So, I've mentioned probes so let me just show you
a little bit and talk a little bit
about the kind of probe we're going to use
in the meter today.
Of course it's just a wire but the ends
are of a particular type.
This is the end that always interfaces with the meter.
It's just a straight post and we call
this type of end, a banana end.
On the other end, I'm gonna have
an alligator clip-like thing here and it will allow me
to hook it to various points in the circuit
and it will stay affixed strongly
and it's a very good end to use.
We refer to this probe as a banana alligator
and I'm gonna to use these today.
So let's get on now to a discussion
of the measurements that we're going to make.
I'm going to first demonstrate making a measurement
of DC voltage in a simple DC circuit.
So we have to have the meter set up correctly,
and then we have to use it appropriately in the circuit.
We wanna always place it in parallel
with the element of which we want to measure voltage for.
For instance, here I'm measuring
the voltage drop across a resistor.
I do not disassemble the circuit to the measure voltage
and I must have power to the circuit.
It's a simple measurement to make so I'm going to go
over here now and make this measurement at a couple
of those positions in the circuit that I showed you earlier.
So, I'm going to select the correct quantity
on the dial, DC voltage.
I have the probes in the correct ports,
and I will first measure the voltage from points one to two
in the circuit that I had up earlier
and notice I get about one volt.
I'll now measure the voltage from points two to three.
One volt again.
I will measure the voltage from points three to four.
Again, one volt.
And I'll conclude by measuring the voltage
from points one to four in the circuit
and I get about three volts.
Interestingly, that's the sum of the three previous voltages
I measured and also the voltage of power source here,
the two resistors.
Notice I had the meter set in the correct position
and I placed the leads in parallel with the measurement
I wanted to make, I did not disassemble the circuit
and I left power on to the circuit.
So let's discuss briefly common mistakes
that students make when measuring voltage in a DC circuit.
They'll have the meter in AC rather than DC mode.
They'll select perhaps an incorrect range of the meter.
Sometimes, they'll be a faulty fuse in these multimeters
and that will cause it to measure
an incorrect, measure voltage incorrectly.
The battery in a multimeter could be low.
There's a small battery in these multimeters
that give them power.
It could be low and it will not measure voltage correctly.
Probes in the wrong ports on the meter.
This is often a mistake: students will have
the probes to measure current rather than voltage.
They'll incorrectly place the probes in the wrong positions
in the circuit or they'll have a circuit
with no power to it, it's a dead circuit.
So next we'll talk about making measurements
of current in DC circuits and this is the measurement
that students struggle with the most.
To measure current, we have to place the device,
the measuring device in series with the circuit.
In other words, we must open up the circuit
and let the current run through the meter.
So we still have power to the circuit
but there's a little more work than measuring voltage,
We have to open up the circuit
and let it run through the meter.
So I will now go over and demonstrate
with both of these meters how to make that measurement
in this DC circuit.
So I will first use the digital multimeter.
I will select DC current in the correct range.
I will move the red probe over
to the port for measuring current.
And I will, at point one in the circuit,
I will open up the circuit and let it run through the meter.
And the meter registers about 10 milliamps.
I will do it now at another point in the circuit.
Let's open up the circuit at point two
and measure the current there.
Again. I'm going to let it run through the meter.
AnD again we get 10 milliamp and likewise we would
also get this at points three and four in the circuit
since current is common in a series circuit.
I will now make the measurement with the analog meter.
So I have the analog meter set up.
It's in DC mode and I have chosen the correct range,
an appropriate range for this measurement.
And I'm measuring the current through point two.
And we see the analog meter registering about 10 miiliamps,
relative to the range I've chosen.
So let's discuss common mistakes students
will make when measuring current in a DC circuit.
Often, they'll select an incorrect range on the meter.
We have some of the issues that might cause
problems that we discussed previously.
Usually they'll have the probes
in the wrong area of the circuit.
They will not open up the circuit
and let the current run through the meter.
They also might have no power to the circuit
or a dead power source in the circuit,
which would also cause issues with measuring current.
So finally we would like to discuss today,
measuring resistance in a DC circuit
or measuring resistance of elements
that might go into a DC circuit.
So to measure resistance correctly
we have to the meter set up right
but we must isolate the element from the circuit
that it would go in to measure resistance correctly.
So you see here I'm measuring the resistance
of one resistor and I have the probes on either end.
So I will now go over and I will demonstrate measurements
of resistance with some of the elements that we have here.
So I'm going to do this with the digital multimeter.
I will put the probe in the resistance port
and I will choose resistance on the meter.
And I'm going to measure the resistance
for one of the resistors here.
I must remove it from the circuit
and place my meter on the other side of the resistor;
and I get about 100 ohms.
Now suppose I wanted to measure
all three of the resistors in series.
A common mistake students will make is leaving
the three resistors in series in the circuit
and making the measurement.
When I do that, I'm measuring the resistance
of the resistors in series and everything else
that's in parallel with them in the circuit.
To make the measurement correctly, I would have to isolate
the three resistors from the circuit,
which I've done by removing those two leads,
and only then can I make the correct measurement.
And we see it's in the neighborhood of 300 ohms.
So let's review some of the common mistakes
that students will make when measuring resistance
in a simple DC pursuit.
They'll have an incorrect range, selected many
of the issues we mentioned before.
They'll have the probed in the wrong ports of the meters.
They'll incorrectly place the probes across the item
they're wanting to measure.
But the most common mistake is they will leave the resistor
that they're wanting to measure
the resistance of in a circuit with other resistors
and a power source,
and you will not get the true resistance value
for that resistor.
You must isolate that element from the circuit.
So let's review the things that we've covered today.
Measuring voltage.
We want to place the meter in parallel
with the element that we're measuring the voltage of.
Leave power on to the circuit.
You do not need to disassemble the circuit.
Measuring current.
You must open up the circuit
at the point that you want to measure the current
and let it run through the meter
with power on to the circuit.
Measuring resistance.
You must isolate the resistor
or group of resistors from the circuit
with the power source to measure resistance correctly.
So remember the facts I've given you today,
practice the techniques I've shown,
and you too will be to make effective measurements
in the DC Circuits Lab.