A

Pictured above is a very simple

As the input voltage increases the current through the

*

Here is a hand-worked example which shows how to choose the correct

Since we need 8 Volts we can choose between a 7.5V or an 8.2V

Our load device needs 100mA of current, plus we also need at least 5mA for the zener diode, therefore lets set I

Zener diodes are available in a range of difference power ratings. If a large current flows through a small zener diode it will be destroyed, therefore we calculate the power to be lost in the diode and select a diode rated above that value. Here the zener power rating is equal to the zener voltage multiplied by the maximum current (I

We multiply the full maximum current by the zener voltage since when no current is flowing through the load - e.g. when the device is switched off - all of the current will flow through the zener diode.

The voltage dropped across the resistor is equal to the difference between the source voltage and the zener voltage = 12-8 = 4 Volts, and therefore the resistance according to

If the source voltage is likely to be much over the 12 Volts stated then the voltage dropped across the resisitor will be larger and so a resistor with a larger resistance may be required.

The power dissipated in the resistor is equal to the voltage drop across the resistor multiplied by I

If the source voltage rose to 15.5 Volts then we would have 15.5-8.2 = 7.3 Volts dropped across the 39 Ohm resistor: a current of 187mA. If the source voltage fell to 10.6 Volts then we would have just 10.6-8.2 = 2.4 Volts dropped across the 39 Ohm resisitor: a current of 61mA. Therefore in both cases we easily have enough current passing through the

At maximum voltage we would have 1.37 Watts of power dissipated by the resistor - therefore it is good we chose a 2 Watt rated model. We would also have a potential maximum of 1.5 Watts dissipated by the zener diode - therefore we should also change this to a 2 Watt rated model to be on the safe side.

If you require a certain fixed output voltage - e.g. 5V, 12V, 15V from a given input voltage, there are a wide selection of linear regulator chips available. For example the L7805 for +5V (pictured above) and L7812 for +12V are the most popular. These just need capacitors on the input and output side to smooth the voltages, and are very reliable.

Where an adjustable output voltage is required, the

Where the input voltage is very close to the output voltage, an L7812 or LM317 for example cannot be used as the output from these ICs is always at least a couple of volts less than the input voltage. In these situations an

Article Last Modified: 18:20, 10th Jan 2014

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