- [Instructor] Unlike normal diodes which
are used to block current in the reverse direction,
Zener diodes are intentionally used
to conduct reverse current in the breakdown mode.
They're a special type of diode that's designed
to have a very precise breakdown voltage
called the Zener breakdown or Zener voltage.
When the reverse voltage across a Zener diode
is increased to that Zener voltage,
Zener diode will enter the breakdown mode and
allow current to flow through it in the reverse direction.
So the reverse voltage drop across a Zener diode
will never exceed that constant breakdown voltage.
Since the Zener diodes are used in such a unique way,
they get their own unique symbol
which looks like a standard diode symbol
but the cathode lines are slightly crooked at the ends.
Knowing the Zener voltage of a
specific diode is critical to using it.
So the breakdown voltage value is usually printed
next to the symbol on a schematic diagram.
The value 5V1 printed next to this Zener diode
means it has a Zener breakdown voltage of 5.1 volts.
Be careful not to confuse the symbol for a Zener diode
with the symbol for a Schottky diode
which has curly ends on the cathode bar.
Although the two symbols look somewhat similar,
they're very different types of diodes
that are used for very different purposes.
The fact that Zener diodes breakdown
to allow reverse current at a very specific voltage
means they can be used to create a
constant reference voltage from a higher,
and potentially changing, voltage source.
In this simple voltage regulator circuit,
as long as the input voltage is higher
then the Zener breakdown of 5.1 volts,
the Zener diode will enter into it's breakdown mode
and allow reverse current to flow through it.
Maintaining a steady 5.1 volts as
the output voltage from the circuit.
To demonstrate that, I've built the
voltage regulator circuit on my Breadboard
and I'm using a variable power supply
to provide the input voltage to the circuit.
Channel one of my oscilloscope is displaying
the input voltage in yellow and
channel two is displaying the
regulated output voltage in blue.
When the input voltage is below
the Zener breakdown point of 5.1 volts,
the diode acts like an open circuit to block current.
So the output voltage is basically the same as the input.
However, when the input voltage increases
above the Zener breakdown point of 5.1 volts,
the diode breaks down to let enough current through it
to maintain a constant 5.1 volt output.
I can vary that input voltage and as long
as it's greater then 5.1 volts,
the Zener diode will hold the output steady.
The resistor plays an important role in this circuit
because it limits the amount of reverse current
that will flow through the diode
when it's in the breakdown mode
to prevent the diode from conducting
too much reverse current and burning itself up.
However, that resistor creates a few
limitations when using this circuit.
Since the output voltage across the Zener diode
remains constant at 5.1 volts,
any extra input voltage will have
to be dropped across that resistor.
For example, if the input voltage was 6 volts,
0.9 would be dropped across the resistor
because the output voltage is held constant at 5.1 volts.
If the limiting resistor was say, 100 ohms,
according to ohms law, a 0.9 volt drop
across a 100 ohm resistor means 9 milliamps
will flow through the resistor.
So this simple regulator circuit
can provide up to 9 milliamps of current
at 5.1 volts to whatever load it attached to the output.
Any of that 9 milliamps that doesn't go to the load
will pass through the Zener diode to ground instead.
Extra current drains through the Zener diode
to maintain the 5.1 volt output to the load.
Now if this load tries to draw more then the
9 milliamps of current at 5.1 volts,
due to ohms law that will require a
larger voltage drop across the resistor
because all of that current going to the load
has to first pass through the resistor.
Increasing the voltage across the resistor
to provide that additional current
would in turn, decrease the voltage across the Zener diode.
And with less then 5.1 volts across the Zener diode,
it will no longer be in the breakdown region.
The diode will become reverse biased
and act like an open circuit to block current.
So at this point, the load is basically just connected
to a 6 volt supply with 100 ohm resistor in the way
and the voltage being supplied to the load to be
something less then the desired 5.1 volts.
As that example demonstrates, Zener diodes
do not work well for regulating the voltage to loads
that draw large amounts of current.
I could modify the circuit from earlier
to provide more current to the load
by lower the resistance of the limiting resistor.
But that would require my limiting resistor to dissipate
more power, which leads to another problem to consider.
Since the resistors roll in this circuit
is to drop the input voltage to the Zener voltage,
which is does by dissapating energy as heat,
this circuit does not work well to regulate down
input voltages that are significantly
higher then the Zener voltage.
For example, if the input voltage was 24 volts,
that would create an 18.9 volt drop
across the 100 ohm resistor and according to ohms law,
that produces 189 milliamps of current.
That much voltage and current would require
the resistor to dissipate several watts of power.
And that would quickly burn up one of the standard,
little quarter watt resistors
I usually keep in my parts kit.
Now all of these limitations about
not drawing too much current then
don't try to bringing down too high of an input voltage
may sound incredibly restrictive.
But the point is, Zener diodes are not intended
to regulate the voltage that supplies power to a device.
There are other components and
circuits that exist to do that.
The voltage produced by a Zener diode
should just be used as a reference voltage
for other circuit elements that need it.
For example, when working with
micro-controllers and other digital devices,
it's common to need to produce a 3.3 reference voltage
when you only have a 5 volt power supply.
And this is a great use case for a Zener diode
cause all you need is a 3.3 volt Zener diode and a resistor.
As long as you keep those limitations in mind
and don't try to draw too much current
and don't try to bring down too high of a voltage
this Zener diode based regulator circuit
should work just fine.
Another type of diode that's very
closely related to the Zener diode is a
Transient Voltage Suppression diode or TVS diode.
The purpose of a TVS diode is to protect a circuit
from voltage spikes, or electrostatic discharge,
by diverting that energy to ground.
A TVS diode is like a Zener diode except
that it doesn't conduct in either direction
until its breakdown voltage is reached.
For that reason you can think of a TVS diode
as two Zener diodes facing each other
with their cathodes connected together,
which is what a common symbol for TVS diodes looks like.
If the voltage in either direction
exceeds the breakdown voltage, the TVS diode
will allow reverse current to flow through it.
You often see this TVS symbol show up
on schematics near a device's power supply.
For example, on the schematic for
revision 2.1 of the Raspberry Pie Microcontroler Board,
there's a 5 volt TVS located near
the board's 5 volt USB power supply.
That TVS is there to protect the board
from any unwanted power surges on the
USB cable providing power to it.
It's not being used to regulate down
a higher input voltage to provide power to the device,
it's just there to protect against
unexpected voltage spikes.
Although the schematic symbol for the TVS
looks like two Zener diodes stuck together,
on the actual Raspberry Pie board
the TVS diode is really a special purpose TVS chip.
Which is how the TVS is usually implemented in real devices.