Figure 1: Typical zener diode test circuit.

Transistor-Zener Diode Regulator Circuits

by Lewis Loflin

This will explore the basic operation of zener diodes and their use as voltage regulators. They will be used in conjunction with common bi-polar transistors to boost the output current and can used for real-world voltage regulators by students and hobbyists. They following is for information purposes only and comes with no warranty.

Illustrated above in figure 1 is the typical zener diode voltage regulator. The zener diode is operated in its reverse bias mode (positive on its cathode) and relies on the reverse breakdown voltage occurring at a specified value. It has two main applications:

1. as a reference source, where the voltage across it is compared with another voltage.

2. as a voltage regulator, smoothing out any voltages variations occurring in the supply voltage across the load. Here we will explore number 2.

When being used a voltage regulator, if the voltage across the load tries to rise then the Zener passes more current. The increase in current through the resistor causes an increase in the voltage dropped across the resistor. This increased in voltage across the resistor causes the voltage across the load to remain at a constant value.

In a similar manner, if the voltage across the load tries to fall, then the zener passes less current. The current through the resistor drops and the voltage across the resistor both falls, maintaining the correct voltage across the load RL.

Zenering current or Izt is the minimum current that must flow through the zener to maintain a fixed voltage. Also called the zener diode turn on current (Izt), it must not fall below a certain limit. At that point voltage regulation will cease.

Thus a zener diode makes is valuable in regulating the output voltage against both variations in the input voltage from an unregulated power supply or variations in the load resistance. The current through the zener will change to keep the voltage to within the design limits. Ref. http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html.

Figure 2

Testing a Zener Diode

In figure 2 we have typical zener diode test circuit. The ammeter (Am1) is in series with R1 and D1 and connected to a 0 to 20 volt (yes use 0 to 20 volts) variable DC power supply. For this test I've selected for D1 a 13 volt zener diode, but any value from 6 to 15 volts at 500 mW will do. I'll assume 10 mA for Izt for this example. (Check the individual specification for whatever diode used.)

First one must consider the power dissipation of the zener diode. To get the maximum current through the zener we must divide the zener power rating in watts (.5 watts) by the zener voltage. .5/13 = 38.5 mA. If 10 mA is required just to keep the zener operating, that leaves only an additional 28.8 mA for the load at best.

R1 which must drop about 7 volts (at 20 volts input) to protect the zener by limiting the current. To find R1 we must divide 7 volts by 38.5 mA. This comes out to 180 ohms. The alternative would be to use a higher wattage zener.

As one adjusts the power supply from 0 to 20 volts the voltage measured by voltmeter 2 (Vm2) should rise to 13 volts, Voltmeter 1 (Vm1) should read zero, and the ammeter (Am1) should read near zero.

As the power supply exceeds 13 volts, the ammeter Am1 reading should jump to as we increase the input voltage the voltage reading at Vm2 should be fairly constant, while the voltage reading on Vm1 increases. At 20 volts Am1 should read about 38 mA, Vm2 13 volts, Vm1 about 7 volts.

To summarize the above nothing will happen until one reached the zener breakdown voltage. Beyond that the voltage across the zener remains fairly constant while excess voltage is dropped across the resistor.

Things to consider. This may work out a little different if using computer simulation programs such as SPICE or Multisim versus assembling the real circuit. A forward bias PN junction voltage drop can vary from as much as .4 volts to .7 volts depending on current. The actual voltage on a real zener might vary plus/minus .5 volts. Modern parts have better tolerances than older parts, but in the real world the outcome will vary somewhat. The above is merely an example worked out with real components.

Figure 3

The circuit in figure 2 is OK for very low currents, when larger currents are needed we can add a "pass" transistor. The transistor passes most of the current instead of the zener diode bias resistor. In figure 3 we connect a NPN bi-polar transistor to our basic zener circuit. The current passes from collector (C) to emitter (E) to the load. We will assume R4 equals 180 ohms and D4 is 13 volts. Capacitor C4 is used for additional filtering.

What is the output voltage? In this example we use a 13 volt zener diode. The forward bias voltage drop across any silicon base-emitter junction will be 0.6 volts. Thus the voltage at the Q4 emitter is 13 - 0.6 = about 12.4 volts.

The output current is determined by the DC gain (hfe) of the pass transistor. For NPN transistors Radio Shack stocks TIP31 (TO-220, hfe = 10-50, 40 volts, 40 watts), TIP3055 (TO-220, 100 volts, 15 amps, hfe = 15), and the 2N3055 (TO-03, 100 volts, 15 amps, hfe = 20).

If we choose to use a TIP31 with a gain of 10-50 and assume the same current of 28 mA we calculated earlier, our output current will range from about 280 mA to 1400 mA or 1.4 amps.

What happened to our extra 7 volts? That was wasted as heat in Q4. The amount of heat was a max of 1.4 amps X 7 = 9.8 watts. This is well within range of the 40 watts rating of the transistor. (Use a heat sink!)

Figure 4

The same circuit can be used for a negative output power supply (figure 4) by changing the transistor to a PNP and reversing the polarity of the zener diode and capacitor. But that is not all that has been reversed, we add 0.6 volts so a zener of 11.4 volts will give us about a negative 12 volts on the output. A similar gain PNP transistor will again boost current output based on its DC current gain.

For a PNP we can use a TIP42 (TO-220, hfe = 15-75, 40 volts, 65 watts) or Mj2955 (TO-3, 100 volts, 15 amps, hfe = 20). In both examples we must not exceed the current, power, and voltage ratings of the transistors. Make sure they are properly heat-sinked.

The output power is also limited by the input power, both voltage and current. If the input supply to the regulator circuit is limited to say 1.0 amp, the total power out to a load will be a little less than an amp.

Figure 5

In figure 5 we use a NPN Darlington transistor. They have a typical current gain (hfe) of over 1000.

Figure 6

In figure 5 we use a PNP Darlington transistor. They have a typical current gain (hfe) of over 1000. Remember to reverse the voltage polarity, zener diode, and bypass cap.

Figure 7

In figure 7 we use two transistors connected in a Darlington configuration. If Q5 had a DC gain of 50 and Q6 a DC gain of 20, my total current gain would be 50 X 20 or 1000.

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Figure 8 general transistor pin connections.

Figure 9 transistor sizes.

Figure 10 diode case styles,