Fixed Voltage Regulators

Voltage Regulators are Direct Current devices which take a high voltage (say 12 volts) and output a low voltage (say 5 volts).  The notes below were compiled using a 7805 device in a TO 220 case.  The objective is to provide an overview of the behavior of this devices, the reader is urged to consult the appropriate datasheet for information on a specific device.  Voltage Regulators are available with wide variations in input and output voltages, current ratings, response times and physical packages.  However, it is thought the nature of the behavior described here is common to most devices.  Voltage regulators can have either fixed or variable output, in the case of devices with variable output (not described on this page) the output voltage is set using a voltage divider which requires additional components.  A common application of voltage regulators is to  provide a stable low voltage supply when only a higher voltage is available, an example being to provide a 5V supply for an integrated circuit to monitor or control a device powered by a 12 lead acid battery.  Also to provide a reference voltage, for example, the voltage across a lead acid battery varies according to the nature of the load and the state of charge, voltage regulator can be used to provide a stable reference voltage as long as the battery is capable of holding a voltage above some minimum level.

Input and Output Voltages

The maximum input voltage should be obtained from the appropriate datasheet.  The minimum input voltage required to provide the rated output voltage will be higher than the output voltage, in the case of 7805 used as an example, the input voltage must be at least 6.5V to provide an output of 5.0 V.  The relationship between input and output voltage is shown in the graphs below:

The input voltage can vary, but it must remain above the minimum level required to maintain the constant output.

Input and Output Current

The input current is equal to the output current plus internal losses.  This relationship for the example device is shown in the graph below:

Efficiency and Heat Dissipation

Whilst these devices are cheap (I have paid between 0.75 and 1.25 GBP at the local components shop), they are not energy efficient.  The minimum energy dissipated by the device is:

Q = I_out (V_in - V_out)

and the maximum efficiency is:

η = V_out / V_in

Thus the maximum efficiency of the example device with a 12V supply and 5V output is 42% (measurements suggest that the actual value is around 35%).

When using these devices at anything more than a few mA, it is necessary to consider heat dissipation.  The writer's experience suggests that a low cost heat sink for a TO 220 casing can be made by bolting it to a short length of copper pipe.

Where energy efficiency is important, a more sophisticated (and thus expensive) solution such as a voltage convertor.

Page Updated: 13-May-2008

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