Figure 1. [Circuit diagram of 24V Regulated DC power supply]

The figure shows the circuit diagram of regulated DC power supply. First, there is a transformer. Where the input 230v is applied. The transformer step-down the voltage to 24v AC. The output of the transformer is applied to the full wave bridge rectifier. Where the 24V is converted to 24V DC and output of the rectifier is filtered using capacitor C1.

Now this voltage is given to voltage regulator IC. The regulated output is given to Darlington amplifier. The Darlington amplifier is used to amplify the current and high gain is produced. Two NPN transistors are coupled means the collector of these transistors is shorted. 

The emitter of the first transistor is used as input to another one. So high current is produced but we want still more current to the third transistor is added with the two. So collectors of all three transistors are shorted and the emitter of the first transistor is the input to other two transistors. So the first transistor amplifies the current is not enough so it is amplified by other two transistors.

After that output of the transistor is high current gain so it would be damageable so the limiting resistors are connected. These resistors limit the current and there are two variable resistors which are used to vary the voltage range from 8.5v to 24v. And another variable resistor is used for varying the Mille ampere current.

At the output, the capacitor is connected in parallel. This is used for filtering the output voltage. This filter is used so that we get pure DC output.


  • Power Supply
  • Voltage regulator
  • Battery Charger



  •  Its output voltage is varied from 8.5V to 24V or according to the input supply.
  •  An output voltage is well regulated.
  •  Lower maintenance
  •  Less space required
  •  Easily understood


  •  Heat is produced across transistor because of Darlington pair current gain high and so heat is produced.
  •  Heat should be properly dissipated through the heat sink, if not then it can damage the components in a circuit.


  • 24V Variable DC Power Supply, before starting to introduce this topic we first have to know what power supplies are……
  • POWER SUPPLY: a device for the conversion of available power of one set of characteristics to another set of characteristics to meet specified requirements.
  •  Conventional series regulated linear supplies maintain a constant voltage by dissipating excess power in ohmic losses, the linear regulator can, therefore, tend to be very inefficient.
  • Depending on the type of output voltage, power supplies can be categorized into two types, they are:
1. Ac power supplies
2. Dc power supplies

  • Regulated Power Supply converts the available unregulated ac or dc input voltage to a regulated dc output voltage. However, in case of a regulated power supply with input supply drawn from the ac mains, the input voltage is first rectified and filtered using a capacitor at the rectifier output.

1926:- “electrical condensers” by course mention high-frequency welding and Furness

1959:-transistor oscillation and rectify converter power supply system U. S. patent 3,040,271 is filed.
1970:- high-efficiency power supply reduced from about 1970 to 1995.
1972:-HP-35 hew left- Packard’s first pocket calculator is introduced with transistor switching power supply for Light Emitting Diodes, clocks, timing, ROM, and register.


Input Rating:-
Input voltage = 230v AC
               AC = 230 ± 7.69%
Max AC input Vin     =  230 + 7.69%
                              = 230 + 30
                              = 260 V AC
Min AC input Vin      = 230 - 7.69%                      
                              = 230 - 30
                              = 200V AC

Output Ratings:-

We required 24V, 3A DC as an output voltage.Output voltage = 7.5-24V Variable DC Output current dependent on load or up to 3A



Normally, there are two types of transformer.
1. Step-up transformer
2. Step-down transformer

The transformer is a block of POWER SUPPLY which used for the step-down the voltage. We use the transformer to step-down voltage 230V AC into 24V AC, 3A The output of the transformer is given to the input terminal of the full wave bridge rectifier.


IF the Power supply has the AC input, then the first stage is to convert the input to DC. This called rectification.
The rectifier circuit can be configured as a voltage doublers by the addition of a switch operated either manually or automatically. This is a feature of large supply to permit operation from normally 120V to 240V supplies

The rectifier produces the unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulse around the AC voltage peaks. This pulses having significant light frequency energy which reduces the power factor.
As a straight rectifier when operating on the high voltage (~240V AC) range. If an input range switch is not used, then a full wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of DC voltage that will be produced by the rectifier stage.

Voltage Regulator:-

IC LM 723


  •  150 mA Output Current without External Pass Transistor.
  •  Output Currents in Excess of 10A Possible by Adding External Transistors.
  •  Input Voltage 40V Max.
  •  Output Voltage Adjustable from 2V to 37V.
  •  Can be Used as Either a Linear or a Switching Regulator.

Equivalent Circuit of IC LM723:-

The LM723/LM723C is a voltage regulator designed primarily for series regulator applications. By itself, it will supply output currents up to 150 mA; but external transistors can be added to provide any desired load current. The circuit features extremely low standby current drain, and provision is made
for either linear or foldback current limiting. The LM723/LM723C is also useful in a wide range of other applications such as a shunt regulator, a current regulator or a temperature controller. 
The LM723C is identical to the LM723 except that the LM723C has its performance ensured over a 0°C to +70°C temperature range, instead of −55°C to +125°C. This IC is available in 14pin and 10pin according to the manufacturers.

Darlington pair:-

Darlington pair is a compound structure consisting of two bipolar transistors connected in such a way that the current amplified by the first transistor is amplified further by the second one.

This configuration gives a much higher common/emitter current gain than each transistor taken separately. The current gain of the Darlington pair is given by ß = ß1 ß2 Thus a Darlington pair provides high current gain. However, there is no increase in the voltage gain. Also, the input resistance increases and the output resistance decreases.
From these project, we can control the DC voltage by using ICLM723 (voltage regulator IC) and two variable resistors. The output voltage is well regulated. The output voltage is smooth DC by using capacitor filter. From this project, we can understand the working of power Transistors and Work of voltage regulator IC.

After making this project we can understand how to define transistors and how to limit the current. And we also understand the working of Darlington amplifier.