A circuit often introduced in beginning electronics is the zener regulator circuit. The zener regulator is a simple circuit for providing a constant DC voltage supply, and also a great way to learn more about how a zener diode works.
The idea with the zener regulator is to take advantage of the breakdown behavior of the zener diode. In the zener diode I-V characteristics curve shown below (note: ignore the numerics, just observe the shape and relationships), when the diode is reverse-biased beyond a negative threshold voltage, the negative zener current flushes out of the gates. The zener diode is doing all it can to hold the output voltage at the output level for the device, even in the face of large input voltage. There key here is that the zener diode is reverse-biased and operating in its breakdown mode.
This lab will have both a SPICE model and a practical circuit demonstration. To keep things simple, the input voltage will be +9V DC, which anyone can obtain with a simple 9V battery. For the zener diode device, I selected a 1N4278, which is a 3.3V zener diode rated at 1W. In order to select the right value for the resistor, I consulted the 1N4278 datasheet for the values Vz and Iz. Vz for the 1N4278 is of course 3.3V, and Iz, the breakdown current threshold, is 76 mA. With these values I could then calculate the acceptable resistor value.
(V – 3.3 V / R) >= Iz
R <= (9.0V – 3.3V) / 76mA
R <= 75 ohms
The resistor needs to be under 75-ohms, plus or minus our tolerances. From my resistor grab bag, I selected a 47-ohm resistor to begin the experiment.
To model this circuit with SPICE, we only need three components: the input source, the resistor, and the zener diode. The SPICE model for MacSpice is shown below.
* Zener Diode Voltage Regulator Vin 1 0 DC 9V R1 1 2 47 D1 0 3 zener .model zener D (BV=3.3) .control dc Vin 0 9 0.5 print v(1) i(Vm) v(2) .endc .end
The SPICE model performs a DC sweep on the 9V input source, starting from 0V and increasing to +9V in 0.5V increments. The output of the DC sweep is shown below.
Circuit: * Zener Diode Voltage Regulator * Zener Diode Voltage Regulator DC transfer characteristic Mon May 16 23:30:20 2011 -------------------------------------------------------------------- Index sweep v(1) v(2) -------------------------------------------------------------------- 0 0.00000e+00 0.00000e+00 -2.51984e-35 1 5.00000e-01 5.00000e-01 5.00000e-01 2 1.00000e+00 1.00000e+00 1.00000e+00 3 1.50000e+00 1.50000e+00 1.50000e+00 4 2.00000e+00 2.00000e+00 2.00000e+00 5 2.50000e+00 2.50000e+00 2.50000e+00 6 3.00000e+00 3.00000e+00 3.00000e+00 7 3.50000e+00 3.50000e+00 3.33298e+00 8 4.00000e+00 4.00000e+00 3.36744e+00 9 4.50000e+00 4.50000e+00 3.38260e+00 10 5.00000e+00 5.00000e+00 3.39209e+00 11 5.50000e+00 5.50000e+00 3.39849e+00 12 6.00000e+00 6.00000e+00 3.40378e+00 13 6.50000e+00 6.50000e+00 3.40829e+00 14 7.00000e+00 7.00000e+00 3.41213e+00 15 7.50000e+00 7.50000e+00 3.41548e+00 16 8.00000e+00 8.00000e+00 3.41845e+00 17 8.50000e+00 8.50000e+00 3.42112e+00 18 9.00000e+00 9.00000e+00 3.42353e+00
As can be seen from the output, when the input source is reaches a level above +3.3V, the zener diode, which is reverse biased, see this as -3.3V and opens the flood gates for the zener current.
In the picture below, the 47-ohm resistor is the horizontal device, and the diode is the vertical device to the right.
I hooked up my multimeter to the output port of the zener regulator, as expected enough the output voltage was 3.309V. With such a large voltage drop over the small 47-ohm resistor, it heats up very quickly; after all, that difference in energy has to be dissipated somehow, and the resistor dissipates energy via heat.
What if we hooked up a potentiometer to the output of the circuit? I had a 10k-ohm potentiometer in my toolbox, so I hooked it up to the output of the zener regulator.
I started at zero ohms and slowly turned the dial. The zener regulator was able to hold the output voltage until I had turned the dial about three-quarters of the way, or roughly 7.5k-ohm. The output dropped to 0.556V.
And what if I swapped the 47-ohm resistor and the potentiometer?
Unless I had the dial almost completely at the zero position, the zener diode was not able to regulate 3.3V, as shown above. The resistor between the zener diode and the input source must be small enough to meet the threshold Iz from the datasheet.
Both the SPICE simulation and practical demonstration backed up the theory of the zener diode as depicted by the I-V characteristic curve. The regulator circuit provides a steady 3.3V DC while operating under the correct parameters. For practical regulator circuits, however, the zener regulator is quite in efficient and dissipates a lot of energy through heat in the resistor. For simple lab experiments and learning purposes, however, the zener regulator is a convenient and simple way to create a fixed DC voltage supply. Of course it is easier to purchase a few three-terminal linear regulators as well, such as the LM series regulators (3V, 5V, 6,V, 9V, etc.)