| Voltage regulators are used to produce a stable power supply voltage (+Vs) from a higher (varying) d.c. voltage. They can also be used to limit the current supplied. |
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How does it operate?
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Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
Some electronic systems require a voltage supply which is quite stable. Voltage regulators have an input voltage which can vary quite widely. They convert this into a stable output voltage.
For example, the widely used 78L05 and 7805 voltage regulators produce an output voltage which is close to 5V (within the range 4.75V to 5.25V). They will accept input voltages that vary anywhere in the range 7.5V to 35V.
The maximum current that the 78L05 can supply is 100mA. The 7805 provides a maximum of 1A.
Voltage regulators include protection to prevent the current drawn from the input voltage source exceeding these limits. This can be useful, for example in preventing excessive current being drawn from rechargeable batteries. This also protects against accidental reverse connection of the input voltage source. |
The circuit diagram for the two voltage regulators is almost identical. The downloadable Livewire circuit is based on the 78L05. To convert this to a 7805, click right on the voltage regulator in Livewire and select Models > 7805.
Other very similar voltage regulators are available that can provide stable voltages of 12V, 15V, -5V, -12V and -15V. These can be selected in the same way in Livewire. |
Possible applications
- Protecting rechargeable batteries from being damaged by too much current being drawn if there is a short circuit across +Vs
- Maintaining a stable +5V for electronic systems that need a steady voltage (e.g. PICAXE microcontrollers require a supply voltage in the range 4.5. – 5.5V) if the voltage of the power source varies widely e.g. a discharging battery or an unregulated power supply.
Making
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78L05
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How part of the PCB for the 78L05 might look
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7805 pins
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How part of the PCB for the 7805 might look
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Testing
First check that the input voltage is in the range 7.5V to 35V. Then make sure that the output voltage (on the green PCB track) is +5V.
Fault finding
If there is a fault, check that:
- The voltage regulator has been connected the right way round
- The voltage on the centre pin is 0V
- The voltage on the input voltage pin is correct
If there is a fault, check the tracks and solder joints.
Alternatives
- In many situations a battery pack can be used to power electronic systems. This is simpler and cheaper but does not provide such a stable voltage.
- A self-contained mains electricity powered regulated power supply can be used to provide a stable voltage. This is easier, but it is much more expensive and cannot be easily incorporated into an electronic product.
- Fuses can be used to limit current, but they need to be replaced if the ‘blow’.
- The LM317L is an adjustable voltage regulator which can supply a stabilised voltage (the voltage is selected with two resistors) over the range 1.2V to 37V. It is capable of supplying 100mA.
Web links
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