Think of a zener diode as a smart device that changes its resistance so
that the voltage across it remains unchanged. When forward biased, a
zener diode acts like a normal diode. However, when a zener diode is
reverse biased it acts like a smart resistor. The following is the LGM
model of a zener diode, where a little green man (LGM) monitors the
voltage across the diode. He changes the resistance to whatever it
takes to keep that voltage from changing.
The LGM Model of a Zener Diode
This is a 6 volt zener diode. The little green man's instructions
(posted on the wall) are to watch the volt meter and change the
variable resistor to whatever it takes to keep the voltage at 6 volts.
Notice that the little green man watches the volt meter but doesn't
watch the variable resistor. He changes the resistance until the
voltmeter reads 6 volts. He doesn't care what that resistance is.
Now let's set up a zener diode circuit. There must be a current
limiting resistor in series with the diode. Let's say you place 10
volts directly across a 6 volt zener diode. The diode resistance will
drop to its lowest possible value as the diode tries to bring the
voltage down to 6 volts. The full 10 volts going through this very low
resistance will result in a high current that will instantly burn up
the diode. With a suitable resistor in series with the diode, the little
green man can adjust the variable resistor to a value that will hold
the voltage at 6 volts, without allowing too much current to flow
through diode. We'll see how to choose this resistor later.
The LGM Model of a Zener Diode in Action
Here
there is a 100 ohm resistor in series with the zener diode. When 10
volts is placed across the circuit the little green man cranks the
variable resistor until he sees the meter reading 6 volts. This happens
when the variable resistor is set at 150 ohms.
Recall the series circuit rules. The voltages across the resistors must
add up to 10 volts. The voltage across this zener diode must be 6
volts; that's what zener diodes do. That
leaves 4 volts across the current limiting resistor. The zener diode
has 50% more voltage than the current limiting resistor, so it must
have 50% more resistance, which is 150 ohms.
Now let's increase the applied voltage to 18 volts.
The LGM Model of a Zener Diode in Action
When
the applied voltage increases to 18 volts the little green man has to
crank the variable resistance down to 50 ohms. This keeps the voltage
across the variable resistor at 6 volts.
The voltages must now add up to 18 volts and the voltage across the
zener diode must still be 6 volts. That leaves 12 volts across the
current limiting resistor. To maintain these conditions the little
green man cranks the variable resistor to 50 ohms.
Now let's look at a zener diode from another point of view. Notice that
the zener diode is reverse biased in the demonstration. This is the
normal operation of a zener diode. When forward biased, a zener diode
acts no different from any other diode.
The rating of the above zener diode is 6 volts at 40 milliamps. To
start with, lets apply a voltage that is below the zener diode
voltage. The diode will increase its resistance to the maximum possible
value as it tries to bring that voltage up to 6 volts. It can never get
to 6 volts because the zener diode cannot give you more voltage than it
gets from the source. As illustrated above, all it can get is the same
1 volt supplied by the battery. Like any other
reverse-biased
diode, the zener diode acts like an open circuit under this condition.
There is no current flow and the voltage across the zener diode is
equal to the source voltage. Another way to look at it is, the zener
diode is an open circuit, so there is no current flowing in the
circuit. There is no current flowing through the
current limiting resistor, so there is no voltage drop across it. We
start with 1 volt, lose no voltage across the series resistor and have
1 volt left across the zener diode.
Now
lets increase the applied voltage to match the zener diode voltage. The
zener diode is now in a state where it is just about to start
conducting. It is still acting like an open circuit and the voltage
across the zener diode is still equal to the source voltage. However,
it is at a threshold where it can compensate for any further increase
in applied voltage by decreasing its resistance.
Here, the applied voltage is greater than the zener diode voltage. The
zener diode lowers its resistance until the voltage across it is 6 volts. This being a series circuit, the
voltage across the zener diode and the current limiting resistor must
add up to the applied voltage. Therefore, we have 6 volts across the
zener diode and 1 volt across the current limiting resistor adding up
to 7 volts (this satisfies Kirchhoff's voltage law). If we increase the
applied voltage the voltage across the zener diode will remain at 6
volts, but the voltage across the current limiting resistor will increase, making up the
difference between the zener diode voltage and the applied voltage.
Zener
diodes can be obtained with a wide range of voltage and current
ratings. The zener voltage is specified at a certain current. In the
above example the rating is 6 volts at 40 milliamps. A series resistor
is selected to keep the nominal current near the rated current. Let's
say the nominal applied voltage is 7 volts. This will put 1 volt across
the current limiting resistor. To calculate the resistor we divide the
current into this voltage. That's 1 volt divided by 0.04 amps giving us
a resistor of 25 ohms.
Zener diodes are used where a stable voltage reference is required,
such as in regulated power supplies. Another common use for zener
diodes is voltage clamping (voltage limiting). For example, modern MOS
devices often have built in zener diodes on the device gate. MOS
devices are very sensitive to static electricity. If you touch the gate
lead with a static charge on your body, the zener diodes will prevent
the gate voltage from rising above the zener voltage. This is why
modern MOS devices are much more robust than early devices.
A MOS transistor showing internal zener diodes.
Today, proper antistatic measures are adequate to protects MOS devices,
thanks to the built in zener diodes. In the early days, merely thinking
about static electricity seemed to be enough to destroy MOS
transistors. Zener diode clippers are used in many places where you
need to limit the voltage as you connect from one circuit to another.
Clipping is done either to protect the circuit (as in MOS devices), to
remove unwanted noise (as in FM radio receivers), to remove unnecessary
parts of a signal, etc.
Reality check
In the real world zener diodes aren't as perfect as shown above. If you increase the
current through a zener diode the voltage across it will also increase,
just as with any other component. However, with a
zener diode, large changes in current result in tiny changes in
voltage. As described above, a zener diode changes its impedance in
response to current changes, but it does not compensate perfectly. To get
the voltage as steady as possible, you need to keep current variations to a minimum.
The test current specified for a zener diode is usually 1/2 to 1/4 of
the maximum current. Even so, zener diodes will tend to get warm when
run at the test current. As such, air currents blowing across the diode
can cool it enough that the voltage will drop significantly. Random air
currents can cause the voltage to go all over the place. I usually run
zener diodes at 1/2 to 1/4 of the test current. This greatly reduces
this heating and cooling effect. The voltage will be lower than at the
test current, but the diode will not heat up enough to cause problems.
Be careful not to let the current drop below the zener threshold
current. Below this current the voltage will drop well below the
specified voltage and will be extremely sensitive to current changes.
The zener threshold current is not specified in datasheets. You may
have to find your optimal current by trial and error.
