Variable Regulators
Variable DC Regulator
A linear regulator consists of four basic components:
- Series Pass (Control) element
- Sampling circuit (voltage divider)
- Reference voltage
- Error amplifier
Simplified Linear Regulator Schematic
The 723 regulator is a relatively easy to use, full featured device.
National Semiconductor LM723 Regulator
723 Low Power Regulator
One of the projects which the students build at Heritage, is a 0 to 25 volt linear power supply based on the 723 chip.
723 Block Diagram
Internal Equivalent Circuit
The 723 linear regulator contains all of the components required to make a basic variable linear regulator, except for the voltage divider.
This IC also includes additional features such as current limiting and the ability to drive an external series pass transistor for high-current applications.
Basic 723 Application Circuit
Boosting Output Current
Adding a series pass device does not affect the regulated output voltage.
Since the feedback is taken from the output, the output current driver can largely be ignored. If the input voltage is derived from a potentiometer between the reference voltage and ground, the output can be varied between 0 and the reference voltage multiplied by the gain of the circuit.
There is a practical limit to this circuit, which is determined by the maximum voltage rating of the IC (40 volts).
Boosting Output Voltage
Although the 723 will tolerate a maximum input voltage of 40 volts, it can only source a high current regulated output of about 25 volts. This happens because a 40-volt peak, fully rectified sine wave, only has an average dc value of about 25 volts.
In order to overcome this limitation, a pre-regulator can be used to provide a ‘semi-regulated’ higher average dc voltage before it is applied to the 723 regulator. This will prevent the 723 from being exposed to the peak voltage.
Project Power Supply
LM317 Linear Regulator
LM317 Product Folder by National Semiconductor
LM317 by Fairchild
LM317 Product Folder by Texas Instruments
