Hello again. We're going to be finishing up our lab
demos on resistors used as sensors. Today we're going to be talking about a
Wheatstone Bridge. And again, this kind of is, the capstone
of this whole section, this whole module. You've already seen a schematic of a
Wheatstone Bridge, and you've seen how to analyze it.
And what we've found just to remind you is that you set up the Wheatstone Bridge
to balance and you balance R2, which is right here.
And R3 show that this voltage across here is 0.
And you can often do that by trying to put a, ammeter across here, between, a
and b. And make sure that there's no current
flow here. And once you do that, once your
Wheatstone Bridge is balanced. You have this relationship here between
the, the resistors. Now when it's used as a, in a sensor, we
often times take Rx, and we make that the variable resistor.
So, Rx is a variable resistance, and then we measure the voltage between a, between
points a, and b. Now why do we use a Wheatstone Bridge?
Why don't we just measure this directly? Well it's because of relative values.
Sometimes this resistance varies very very small.
It and so, the voltage difference might be very, very small.
So, for example, we can measure an absolute terms what v sub b is.
And v sub b maybe it's two volts. But maybe the difference between this a
and b is on the millivolt range. So, if we measure v sub b, it would be
two volts, and if we are measuring everything relative, using a, like the
voltage divider law directly along one leg of this like we did with the
potentiometer. Then we'd be measuring maybe 2.0,0,15
volts. And it's hard to get a reading like that.
It's hard to scale, a meter like that. So, instead we get rid of, the basis, we
get rid of the, we zero it out so we're only measuring this.
So, once we put in the Wheatstone Bridge configuration we're only measuring the
difference. So, we can scale up our meter, so that
we're measuring things on the millivolt range.
So, let's start our lab demo on the Wheatstone Bridge.
Lets look at a very common application of a Wheatstone Bridge as a sensor.
Look at this, is a flexible beam. It's kind of thin and long.
So it, it can be bent, see how long it is.
Very thin, and it, it can be bent. It's a flexible beam.
I've mounted two strain gauges on each side.
Two on the top and two on the bottom. Strain gauges are variable resistors.
They vary based on the length of this strain gauge.
As the strain gauge elongates, then the resistance goes up.
And if it shortens, the resistance goes down.
So, this is mounted, you see, these wires here?
This is mounted and, and wired as a Wheatstone Bridge, with R1 and R4 on the
top. Now let's look at the schematic of this.
It's just a, a sketch of, a cutout of the beam, with R1 and R4 strain gauges on the
top, R2 and R3 on the bottom. You can see that some of these wires are
connected to ground, and some of them to a power supply.
This is a 5 volt power supply. And the other two are connected back and
forth so the, the leads from the strain gauges are connected together.
With pick-off points A and B. If I put that, that's back into the, the
standard configuration of a Wheatstone Bridge.
I've got R1 and R2 mounted on the top. Are rather R1 and R4 mount down the top,
R2 and R3 on the bottom and I've got these pick off points.
So, I'm going to be measuring the potential at point A and the potential at
point B. Now if this bends backward, so R1 and R4
elongate, and R2 and R3 shorten. So, R1 and R4 elongate, R2 and R3
shorten. Then R1 goes up, R4 goes up.
And the other two go down. So, remember how Wheatstone Bridge works.
It's based in a voltage divider law. So, if this decreases and this increases,
the potential at A will go down. Similarly, if this increases and R2
decreases, the potential in B goes up. That means the voltage difference V sub A
minus V sub B becomes negative. So, if I bend this backward, the
potential, V sub A minus V sub B, becomes negative.
Now, if I bend this up, and these two, R2 and R3 elongate, so they become larger,
larger resistors. These two become larger resistors.
While R1 and R4 decrease in resistance. That means a potential A goes up, and a
potential B goes down. So that V sub B minus V sub A, becomes
more positive. So bending it, bending it back so these
elongate. Means that the potential becomes
negative, then the difference between VA and VB becomes negative.
And then if I bend it the other direction, so that these elongate then
that voltage difference becomes positive. Let's look back at our physical device
here. So there's our one R4, and you can see
the little wires back and forth, connecting the Wheatstone Bridge.
Let me show how this bends, so right now I'll be bending it, so that R1 and R4
elongate. If you look at the resistance, or the
voltage on the digital multi meter, you can see it becomes negative, like we
predicted. And the more I bend it, the more it
becomes negative. And that reading is in millivolts.
Now let me bend it in the other direction.
And notice as we predicted it becomes positive.
So, bending in this direction I get positive voltage.
The more I bend it the higher the voltage.
And if I go back to, the balance condition, it's close to 0 millivolts.
You may ask why do we bother with four strain gauges?
We looked at a Wheatstone Bridge before. We analyzed it with one flexible, one
reed the variable resister. Why do we now use four?
Well the reason isn't anything to do with electrical properties.
The reason we use four variable resisters here we mount two strain guages on top
and two on the bottom. Is because of thermo properties, we
wanted to gate the thermo expansion of this beam.
As it gets warm the beam will elongate, well if the beam elongates, so will those
strain gauges, and that will change the resistance.
But the thing is if these resistors, the top and the bottom, both elongate by the
same amount, the ratios don't change. So, it still stays in a balanced
Wheatstone configuration. And that way, we get rid of the, we, we
negate all effects of thermal expansion, and the only effects that we will be
measuring with this is the bending effects, and that's what we were trying
to measure. Looking again, as I bend it this
direction, it becomes positive voltage, bending it this direction becomes
negative voltage. It's a very common application of a
Wheatstone Bridge. And Wheatstone bridges with strain gauges
are often times used in not only flexible beams, but also in bridges looking at
loads on, on bridges. In summary, a Wheatstone Bridge is used
to detect small changes in resistance. And we used it in a strain gauge, but it
can be used in other examples as well with other types of sensors.
Now, in the, with the particular strain gauge configuration, strain gauge
application, we used four strain gauges in a Wheatstone configuration, and that
was to remove the thermal effect. In the next lesson, we will be doing a
wrap up for module 2. Nathan and I would like to encourage you
to go back to the form, now that we're wrapping up this whole module.
Make sure that you ask any questions you need, make sure that you finish all the
homework problems, because those homework problems will prepare you for the quiz on
this section.