Welcome back to electronics.
This is Dr. Robinson.
In this lesson, we're going to look at voltage regulators.
In our previous lesson, we introduced diode limiters and
we examined circuits that could be used as limiters.
Our objectives for today's lesson are to introduce voltage regulation.
And to examine diode regulator circuits.
Here I have a definition of an ideal dc voltage regulator.
It's a device that maintains a constant dc output voltage,
regardless of variations in the input voltage or the load.
Lets examine the behavior of a regulator in blocked diagram form.
Here I have the block representing the regulator.
On the left side is the input voltage,
a DC voltage applied to supply the power to the regulator.
Now if this were an ideal voltage regulator, this output voltage here
would not vary with changes in the value of the input voltage.
Now, typically this input voltage is a voltage greater than the output voltage of
the regulator.
So say this were a five volt voltage regulator.
Then it would produce at its output five volts.
A reasonable input voltage to that regulator would be, say, 15 volts.
Now, if we have a circuit that uses, in different parts,
different power supply voltages we could use different regulators or
multiple regulators to supply those voltages.
So from this single input 15 volts supply we could add one regulator that
produces five volts for one portion of the circuit.
And we could have another regulator that produces say an output voltage of 12 volts
for different portions of the circuit.
Now, let's add a load to our regulator.
So here I'm drawing a resistor of value RL for load resistor.
Now this doesn't have to be an actual resistor.
This resistance can represent the input to some circuit of interest to us.
Like say, an amplifier, a cell phone,
something that we need to power with a voltage supply.
As, this resistance value changes for this output voltage to
remain constant we know that by Ohms law the current here must change.
So if, say, I made this resistance smaller and smaller and
smaller, for the output voltage across the regulator to be constant,
this current must continue to increase, to keep the equation V out
equals I L, the load current times R L, true.
Now, in a practical regulator, there's some limit to the amount of
current that can be put out of its output terminal.
So, that at some point as this resistance becomes smaller and
smaller, we will lose the regulation capability.
Now, there are two figures of merit that are typically used to
describe a regulator.
These are the line regulation and the load regulation.
The line regulation is the ratio of the change in
output voltage to a change in input voltage.
Now, remember, an ideal regulator, if we change the input voltage,
the output voltage would not change at all.
So for an ideal regulator, the line regulation would be zero.
Now for load regulation, that indicates how well the regulator can
maintain the output voltages for changes in the value of this load resistance.
So, as I make this load resistance smaller, the current increases.
If that current changes by some amount, ideally for
a perfect regulator, the output voltage would not change at all.
So, the load regulation for an ideal regulator would also be equal to zero.
Lets examine the concept of loading by looking at ideal and real voltage sources.
Here I have an ideal voltage source.
If I put a load across this ideal source.
Of value RL, we know that because of the properties of an ideal voltage source,
no matter what the value of this resistance is,
the voltage across this resistor will always be equal to V in.
However, a real voltage source one that we can build in practice has associated with
it some sort of internal resistance which I have modeled here as a resistor RS.
So this source it's output terminals are between here and here.
Now if we attach a load to this real voltage source.
[NOISE] We can see that we have completed the loop
around the circuit, current can now flow.
And the voltage here at the output of the source does not have to
be equal to the input voltage.
Because we can have a drop across this source resistor.
If RL were an infinite resistance such that no current could flow, or
in other words an open circuit.
Then the voltage we measure across the output terminals of the real voltage
source would be equal to V in, just as it would be for the ideal voltage source.
We can write [NOISE] a relationship between the output voltage and
the input voltage for this real voltage source using voltage division.
The out, is equal to, again by voltage division,
RL over RS plus RL times Vn.
We can see that as the source resistance approaches zero, the output voltage
approaches the input voltage and the real voltage source becomes more ideal.
Now this concept of the output voltage
changing with changes in load resistance is known as loading.
We say that we have loaded the source,
because the output voltage is no longer equal to the open circuit output voltage.
Let's look at how we can take advantage of the characteristics of
a diode to implement a voltage regulator.
Here I've drawn the characteristic I-V curve for a diode.
You can see the expected exponential relationship between the current through
the diode and the voltage across the diode.
Now lets say this diode is introduced into a circuit and
biased such that the current through it places it in this portion of the curve.
This steep portion where, for
large changes in current there's small changes in voltage.
Now we can see for this particular diode I can vary the current through the diode
from say 500 milliamps to one amp, and as that current is varied the voltage
across the diode, changes only by approximately 50 mV.
Now, as the current through the diode becomes less and less, we can see that,
say, if we were operating in this region, then for the same amount of current change
in the diode we can get a much larger change in voltage across the diode.
Now it's the steepness of this curve that we can take advantage of
to implement a voltage regulator.
If for example this curve were a perfectly vertical line then we can see that for
any current through the diode, the voltage across the diode will not change.
Here I've drawn a circuit that can be used to implement a diode regulator.
This portion of the circuit is the regulator,
this resistor RL is the load resistor attached to the output of the regulator.
Now, say initially that there's no load resistance.
Or the load resistance is at infinite value.
We can calculate the current that flows around this loop.
We know that, the forward voltage drop across a diode, when it's forward biased,
is approximately 0.7 volts.
So this current, I, can be written as,
I is equal to 12 volts minus 0.7 volts, divided by R1, 100 ohms.
Is equal to 11.3 over 100,
is equal to 113 milliamps.
Now this current is
chosen to bias the diode in the steep portion of its characteristic curve.
Now, let's say we add the load resistor to the regulator.
When the resister is present some of that current that 113 milliamps can
now flow through this branch.
But because the diode is biased on the steep portion of its curve even though
the current through the diode has decreased the voltage across the diode
remains relatively constant.
Now as this resistor gets smaller and smaller and smaller and we take more and
more of the diode current away, so that it can flow through this branch.
We eventually reach a point where the steep part of
the characteristic curve is left and we lose the regulation.
So this circuit can be thought of as a approximately a 0.7 volt regulator.
Where that 0.7 volts is due to one forward voltage drop across this diode.
Now lets look at how that circuit actually behaves in
terms of it's line regulation and it's load regulation.
On this graph I've plotted the output voltage of
the regulator versus the load resistance value.
You can see that for a load resistance value of 500 ohms.
We have an output voltage of approximately 0.75 volts,
one forward diode voltage drop.
And you can see that the output voltage is maintained for values of 400,
300, 200, and 100 ohms of load resistance.
Now, once we decrease below 100 ohms of load resistance, the current through
the load resistor is such that we are pulling more current away from the diode.
And it's leaving that steep portion of that characteristic curve.
Now for this particular curve,
it turns out that a load resistance value of approximately 45 ohms,
results in a decrease in the output voltage by approximately 1%.
So, at this point here, the output voltage has decreased by 1% from
the voltage it is in this region here.
Lets look at how the output voltage varies with changes in input voltage.
In other words, what is its line regulation.
Now nominally, that circuit we're looking at has an input of 12 volts.
This is a plot of output voltage of the regulator versus input voltage to
the regulator.
And this point here the input voltage is 12 volts.
We can see that the output voltage is 0.753 millivolts for that input voltage.
Now say for some reason the input voltage is changed to 15 volts.
Well, if that's done, the output voltage is equal to 0.768 volts.
So there's a very small change in output voltage for
this relatively large change of 3 volts in the input voltage.
And, if we decrease the nominal value of 12 volts to 9 volts for some reason.
We can see that the output voltage is still 0.735 volts,
still a relatively small change in output voltage.
So we can see that this circuit for
that six volt swing the output voltage is relatively stable around 0.753 volts.
Now the voltage regulator that we've been examining has a regulated output voltage
of approximately 14 diode voltage drop of 0.7 volts.
How can we build a diode regulator that has an output voltage greater than
this voltage?
Now here I don't mind at all if you want to pause the video and
think about the answer to this question before I give it to you.
The answer is we add more diodes.
You can see here that I placed in series three diodes,
across the output of the regulator.
Each one introduces a forward diode voltage drop of approximately 0.7 volts.
So, the total output voltage of this regulator would be 0.7 plus 0.7 plus 0.7,
or approximately 2.1 volts regulated output voltage.
Now let's look at how we can use a diode regulator circuit as a component of
a DC power supply, to improve its performance.
You may remember from a previous lesson.
The process involved converting an AC voltage to a DC voltage.
Remember we start with an AC input voltage, possible from our wall outlet.
We apply that to a rectifier.
And let's assume the rectifier is a full wave rectifier.
We then filter this combined AC and
DC voltage to obtain an output voltage that is.
Mostly a DC voltage, but still has an AC component.
Now, we can apply this voltage as the input to a voltage regulator.
[NOISE] And we know that a good regulator will
produce a constant output voltage.
Even when the input voltage has variations.
So the effect of adding this regulator to our power supply is to
smooth the output DC voltage of the regulator.
So we can draw the output voltage of the regulator as ideally a straight line.
The variations in the input voltage have been removed
by the functionality of the regulator, its line regulation.
So in summary, during this lesson we introduced voltage regulation and
we examined a diode regulator circuit.
In the next lesson, we are going to look at the demodulation of AM signals, or
in other words, you can think of this as a good start to building an AM radio.
Thank you and until next time.