Zener Diodes
A Zener diode is a special purpose diode that is designed to operate in the reverse breakdown region of
the diode’s characteristic curve. Regular diodes will be destroyed if they are used in the reverse
breakdown region.
Zener Diode Symbols
The schematic and physical symbols are shown. Note that the symbol is similar but somewhat different
than that of a regular diode.
Characteristic Curve of a Zener Diode
Several representations of the characteristic curve for a Zener diode are shown.
The Zener diode is off when the applied voltage is less than the Zener voltage rating. When the applied
voltage is greater than the Zener voltage rating the Zener diode turns on and current flows.
Zener Diode Specifications
Zener (Breakdown Voltage) Voltage - VZ
VZ is the most common specification. Zener diode voltages range from 3.3 V to 100 V or more. As seen
in the graph, the Zener voltage does increase a small amount as the diode current increases. This
reflects the fact that the line in the graph is not perfectly vertical.
The Zener diode used in our lab is the 1N 4734A with
a Zener voltage of VZ = 5.6 V
Power Dissipation – PD
This is the rating for the maximum amount of power the Zener diode can dissipate. Values range from
0.3 W to 50 W.
For a 1N 4734A Zener diode the Power
Rating is 1 Watt.
Power Derating Factor - PDF
The maximum power that the Zener can safely dissipate decreases with temperature. The Power
Derating Factor specifies this decrease in maximum power rating.
For a 1N 4734A Zener diode the Power Derating
Factor is 6.67 mW/ °C above 50 °C.
Example
1. What is the maximum power rating of a 1N 4734A at 100 °C?
The derated maximum power is PD, derated
PD, derated = PD,max - PDF x ΔT = 1 W - 6.67 mW/ °C x 50 °C = 1 W – 333 mW = 0.667 W
Zener Diode Currents
There are a number of Zener currents specified.
Minimum Zener Current - IZ,Min
The first is referred to as IZ,Min or IZ,knee. Locate this current on the diagram above. If the diode current
is less than IZ,Min then the diode is off.
Maximum Zener Current - IZ,Max
If the actual Zener current exceeds IZ,Max then the diode will overheat and be destroyed. Locate the
IZ,Max current on the diagram above. If the diode current is less than IZ,Max but greater than IZ,Min then
the diode is operating in the “on” region of the diode curve.
Zener Test Current – IZ or IZT
The test current IZ is often used as a reference point to specify other Zener diode parameters.
Maximum Zener Impedance (Resistance) - ZZ
The value of ZZ is specified at the Zener test current. This would represent the reciprocal of the slope of
the graph of the characteristic curve at the test current point.
Maximum Zener Impedance (Resistance) at the Knee Point - ZZK
The value of ZZK is specified at the Zener minimum current.
Leakage Current - IR
When the Zener diode is off the assumption is that the diode current is zero. In fact the diode current is
a small value called Leakage current. The current is temperature sensitive.
Go to the course web site and examine the
Zener diode data sheets
Exercise
Consult the Data Sheet for the 1N 4734A Zener diode and record the following values.
Zener Voltage – VZ
(typical)
_______________
Maximum Power Dissipation - PD
_______________
Calculate the Maximum Zener Current - IZ,Max
_______________
Power Derating factor - PDF
_______________
Knee or Minimum Zener Current - IZ,Min
_______________
Zener Test Current - IZT
_______________
Leakage Current - IR
_______________
Maximum Zener Resistance - ZZ
________________
Maximum Zener Resistance at Knee - ZZK
________________
Example
2. A 1N 4734A Zener diode has a Zener voltage VZ of 5.6 V at a test current of 45 mA. What is the
value of the Zener voltage VZ at a Zener current of 75 mA?
The change in Zener current ΔIZ = 75 mA - 45 mA = 30 mA
The maximum Zener Resistance
ZZ = 5 Ω
The change in Zener voltage is ΔVZ = ZZ x ΔIZ = 5 Ω x 30 mA = 150 mV
The new VZ = 5.6 V + 150 mV = 5.75 V
Voltage Regulator Circuits
The block diagram below shows an AC input voltage connected to a Rectifier circuit that converts the AC
input into a D voltage. The filter capacitor “smooths” the DC voltage waveform. The Rectified and
Filtered voltage is connected to a regulator circuit.
What is Voltage Regulation?
A regulated voltage source is one where the output voltage does not change no matter how much
current is drawn from the source. Conversely for an unregulated voltage source the output voltage
decreases as the current drawn from the source increases.
Regulated Voltage Source
Unregulated Voltage Source
Vout
Vout
Iout
Iout
Percent Regulation Factor
As was done with the concept of percent Ripple, the percent voltage regulation is a good way to
describe the performance of a voltage regulator circuit. The percent voltage regulation is given as:
Percent Regulation = (VLoad, NL - VLoad, FL)/ VLoad, FL x 100%
where
VLoad, NL is the output voltage with no load
VLoad, FL is the output voltage at full load
Example
3. Assume that a power supply produces 12 volts when the load current is zero (load resistance is
infinite). If the output voltage drops to 10 volts when full load current flows, then the percent of
regulation is
Percent Regulation = (VLoad, NL - VLoad, FL)/ VLoad, FL x 100% = (12 V – 10 V)/10 V x 100% = 20%
Zener Diode Voltage Regulator Circuit
In the circuit below the output load voltage VL is kept constant (at a value of VZ). For the regulator
circuit to work correctly the input voltage Vin must be greater than VZ.
The output voltage VL is of course equal to VZ
VZ = VL
The source current IS from Ohms Law is given as IS = (Vin - VZ)/ RS
The load current IL also from Ohms Law is given as
IL = VL/RL = VZ/RL
And finally the Zener current IZ is found from KCL
IZ = IS - IL
Example
4. For the circuit diagram above with a 1N 4734A Zener diode and VIN = 18 V, VZ = 5.6 V, RS = 1
kΩ RL = 1 kΩ.
The source current is IS = (Vin - VZ)/ RS = (18 V – 5.6 V)/ 1 kΩ = 12.4 mA
The output voltage
VZ = VL = 5.6 V
The load current is
IL = VL/RL = VZ/RL = 5.6 V/1 kΩ = 5.6 mA
The Zener current is
IZ = IS - IL = 12.4 mA - 5.6 mA = 6.8 mA
Effect of Variable Input Voltage
The voltage regulator circuit has limits on its ability to maintain a constant load voltage. This example
looks at the effect of a variable input voltage VIN. Assume the Zener diode is a 1N 4734A .
VZ = 5.6 V
IZ,min = 1 mA,
IZ,max = 179 mA
RL – open circuit IL = 0
At the minimum Zener current of 1 mA, the voltage drop of RS is VRS = 1 mA x 220 Ω = 0.22 V
So the minimum value of VIN is 0.22 V + 5.6 V = 5.82 V
At the maximum Zener current of 179 mA, the voltage drop of RS is VRS = 179 mA x 220 Ω = 39.4 V
So the maximum value of VIN is 39.4 V + 5.6 V = 45.0 V
So if VIN stays within the limits of 5.82 V to 45.0 V then the output load voltage Vout will be constant at
5.6 V
Effect of Variable load Resistance
The other example to investigate is the effect of a variable load resistor on the ability of the regulator
circuit to maintain a constant output voltage. Assume the Zener diode is a 1N 4734A .
VZ = 5.6 V
IZ,min = 1 mA
IZ,max = 179 mA
VIN = 24 V
RS = 470 Ω
Part 1
When IL = 0 A (RL = ∞)
Then the Zener diode current is a maximum value IZ = IS = (24 V - 5.6 V)/470 Ω = 39.1 mA
This value of IZ is well below the IZ,max value of 179 mA so the diode is not in danger of overheating
Part 2
The maximum value of load current IL,max occurs when the Zener diode current is a minimum (1 mA)
IL,max = IS - IZ,min = 39.1 mA - 1 mA = 38.1 mA
So the minimum value of RL is RL,min = 5.6 V/38.1 mA = 147 Ω
So the load resistor RL can be adjusted from 147 Ω up to an open circuit (RL = ∞) and the output load
voltage will b a constant 5.6 V.
Zener Diode Model
Models of electronic components are useful to try and predict or explain the electronic behavior of the
component. The Zener diode’s behavior can be modeled as a voltage source in series with a resistor.
For a 1N 4734A diode the value of RZ = 5Ω and VZ = 5.6 V
Ripple Reduction
A filter capacitor is useful to reduce the ripple voltage of a rectified load voltage waveform. But simply
increasing the value of the capacitor is not always the best way to achieve a small ripple voltage. The
Zener diode regulator circuit has ripple reduction properties as the example below will show. The circuit
shows a Zener diode regulator circuit but the Zener diode has been replaced with its equivalent circuit
model.
Sample Calculation
The input voltage VIN to the regulator circuit is assumed to be +10 V DC with a 0.5 V P-P Input ripple
voltage.
The percent ripple at the input is 0.5 V P-P/10 V x 100% = 5%
This circuit has 3 voltage sources – the + 10 V DC input, the 0.5 V P-P Input ripple AC voltage and the
5.6 V Zener voltage.
To analyze the effects of the circuit on the ripple voltage we need to use the Superposition Theorem. To
do this the + 10 V DC input and the 5.6 V Zener voltages are shorted out. The circuit below shows this.
VRipple,Out,P-P
The input voltage VIN can be designated VRipple,In, P-P. As can be seen in the circuit the resistors RZ and
RLoad are in parallel. The equivalent resistance of these 2 resistors is
Requiv = RZ || RLoad = (5 Ω x 100 Ω)/ (5 Ω + 100 Ω) = 4.76 Ω
The 1 kΩ source resistor and this equivalent resistance form a voltage divider.
The output ripple voltage is calculated as
VRipple,Out,P-P = RZ||RLoad /( RZ||RLoad + RS) x VRipple,In,P-P = 4.76 Ω/(4.76 Ω + 50 Ω) x 0.5 V P-P = 44 mV P-P
The percent ripple at the output is
Percent Ripple = 44 mV P-P/5.6 V x 100% = 0.75%
Clearly both the absolute value and percent value of the ripple voltage have been reduced by the Zener
diode regulator circuit.
Integrated Circuit (3 terminal) Voltage Regulators
A voltage regulator circuit can be built by the use of a discrete component like a Zener diode. However
more sophisticated solutions are available with IC based Voltage Regulators.
One of the more popular devices is the LM7805 voltage regulator. This is a fixed +5 V voltage regulator.
In fact the LM7805 voltage regulator is only one member of a family of voltage regulators designated as
LM78XX – also designated as the LM 140 and LM 340 series of devices. The commonly available output
voltages are +5 V, +12 V and + 15 V. There is also a LM79XX series of regulators that produce negative
output voltages.
Typical Use Block Diagram
Device Packaging
Devices like the LM7805 and others come in different packages. The diagram below shows the TO-3 and
the T0-220 packages two of the more common device packages for soldering on “through hole” plated
circuit boards. These diagrams help the user identify the Input, Output and Ground pins. There are also
a wide variety of packages available for surface mount circuit board assembly.
Adjustable Voltage Regulators
The LM 117/317 series of voltage regulators can produce a variable output voltage from 1.2 V to 37 V.
View the voltage regulator data sheets on the course web site