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Zener Diode Based Voltage Regulators

Simple Zener Voltage Regulator

This circuit regulates an imput voltage to a smoother, slightly lower voltage. It is susceptible to changes in demand current, and to a large degree is also sensitive to input voltage changes. However, for many applications, it provides a simple, cheap means to provide a reasonably steady voltage supply.


Calculation of R1 is relatively simple. It is essentially required to drop the normal input voltage, to the expected output voltage at the normal output current plus about 5mA:

R1 = \frac{V_{in} - V_{out}}{I_{out} + 5mA}

The additional 5mA is required to ensure the zener diode is energised sufficiently to regulate the output voltage accurately.

Simple Zener/Transistor Voltage Regulator

This transistor based regulator can be made to work with any input and output voltages, and requires very simple components. The exact types are often not important, so long as the general characteristics are correct. Whilst the circuit will smooth out the worst of any input variations, the output voltage is defined by the breakdown voltage of a zener diode. In practice, this is not as precise as may be expected, and may vary with current, temperature etc. However, the use of a transistor reduced the direct reliance on the zener diode makes this simple circuit remarkably useful.

A common circuit configuration is shown below.


The principle of operation is very simple. The output voltage is defined as being the breakdown voltage of the zener diode minus the voltage drop between the base and emitter (0.7V). The zener diode is energised by the current through R1, which must provide enough current to also energise the transistor. This arrangement can be hard to accomplish if very high current outputs are required. However, for output currents of up to a couple of hundred milliamps. At these levels, the resistor R1 can be calculated as follows:

R = \frac{V_{cc} - V_{zener}}{I_{base}}

As a general rule, the current required would usually be around 10-20 milliamps, although some experimentation may be required.

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