Adjustable power supply 0-30V 2A
Power Supply Basics
As the name suggests, power supplies are suppliers of energy to any circuit. Every electronic circuit needs an adequate power supply at the input to obtain an ideal result at the output. We need to choose the power supply of any device or circuit according to the power requirements of the device. In this experiment, we are making an adjustable power supply, which will provide voltage in the range of 0 – 30V with 2A as maximum current.
Overview
This article describes an adjustable power supply from 0V to 30V. The source provides a regulated DC with voltage in the range of 0 to 30V at the output. The output voltage can be varied by a variable resistance and is capable of delivering a maximum current of 2A.
The power supply is a linearly adjustable regulated type in which the output voltage is constant and can vary mechanically. In this type of power supply, a linear regulatory element in series with the load is connected to the output. Linear element like BJT or FET is used to provide the correct voltage at the output.
In this source, the BJT (bipolar junction transistor) (2N3055) works in its linear mode, in linear mode it acts as a variable resistance. This variable resistance helps provide the appropriate output voltage for any current within the operating range. To attract high loads at the output, a high current transistor is required. This 2N3055 transistor increases the output current of the power supply.
Required components
| Required components | Specifications | Amount |
|---|---|---|
| TR1 Transformer | 18V-0-18V/2A Boost | 1 |
| Diode D1-D4 | SR560 | 4 |
| Zenner diode D5 | 12V, 1W | 1 |
| Zenner diode D6 | 18V, 1W | 1 |
| Transistor Q1, Q3 | BC547 | two |
| Transistor Q2 | 2N3055 | 1 |
| Capacitor C1 | 470uF, 50V | 1 |
| Capacitor C2 | 10uF, 50V | 1 |
| Resistor R1 | 2k, 1W | 1 |
| Resistor R2 | 0.3Ohm, 5W | 1 |
| Potentiometer RV1 | 10k | 1 |
| Fuse | 2A | 1 |
Power supply basics
Every DC power supply needs to follow a few steps to get the proper DC voltage at the output. The diagram below shows these basic steps by which we obtain an AC regulated DC power supply.
BLOCK DIAGRAM
Working
• AC to AC Conversion
The mains voltage (electricity arriving at our house from the government) is approximately 220V, but according to the circuit requirements, only 30V is required at the output terminal. To reduce this 220V to 30V, a step-down transformer is used.
The circuit experiences some drop in the form of resistive loss. Therefore, a high voltage transformer higher than the voltage required for the application (30V) is used and which can supply 2A current at the output. The most suitable step-down transformer that meets our voltage and current requirements is 18V-0-18V/2A. This transformer reduces the main line voltage to 36V as shown in the image below.
• AC to DC Conversion – Rectification
Rectification is the process of converting AC to DC. There are two ways to convert an AC signal to DC. One is through half-wave rectifier and another is using full-wave rectifier. In this circuit, we are using a full wave bridge rectifier to convert 36V AC to 36V DC. As full wave rectifier is more efficient than half wave as it can provide complete use of negative and positive side or part of AC signal. In full wave bridge rectifier configuration, four diodes are connected in such a way that it generates a DC signal at the output as shown in the image below. During full-wave rectification, two diodes are forward biased and another two diodes are reverse biased. We chose SR560 diode because they can allow 2A current through them when forward biased and in reverse biased condition, they can sustain 36V reverse supply. Due to this, we are using SR560 diodes for rectification purposes.
• Smoothing
As the name suggests, it is the process of smoothing or filtering the DC signal using a capacitor. A high value capacitor C1 is connected to the output of the rectifier circuit. As the DC that must be rectified by the rectifier circuit has many AC spikes and unwanted ripples, to reduce these spikes we use a capacitor. This capacitor acts as a filtering capacitor that shunts all the AC through it to ground. At the output, the remaining DC is now smoother and ripple-free.
• Output capacitor
At the output, capacitor C2 is also connected to the circuit. This capacitor helps in quick response to load transients. Whenever the current of the output loads changes, there is an initial shortage of current, which can be met by this output capacitor.
The output current variation can be calculated by –:
With this we can say that the output capacitor will respond to a current change of 10mA for a transient response time of 100us
• Voltage regulation
As is necessary for every circuit that at the output must provide a regulated and constant voltage without any fluctuation or variation. For voltage regulation, a linear regulator is needed in the circuit, the job of this regulator is to maintain a constant voltage at the required level at the output.
In this circuit, the maximum output voltage is 30V, so a 30V zener diode is perfect for output voltage regulation. Here two zener diodes of 12V and 18V are used in series which will give a total of 30V at the output.
(Note: You can also use a 30V zener diode or a different combination of zener diode to get 30V output)
• Voltage adjustment
To adjust the output voltage from 0 to 30V, a variable resistor (RV1) is connected to the output. By varying this resistor we can obtain voltage at the output as needed between 0 and 30V.
• Output current
The zener diode can only supply current in milliamps. Therefore, to obtain high load current at the output, some linear element must be connected in series with the load. This circuit uses an NPN bipolar junction transistor as a linear element. The BC547 transistor is used to provide sufficient base voltage for the 2N3055 BJT. 2N3055 is capable of providing 2A current at the output. For short circuit protection, Q3 and R3 are used in the circuit.
Practical results
Points to remember
• The current rating of the transformer, rectifier bridge and transistor must be greater than or equal to the output current requirement. Then the single circuit can supply sufficient current at the output.
• The rated voltage of a step-down transformer must be greater than the maximum required output voltage. This is due to the fact that the circuit experiences voltage drop due to some resistive loss. Therefore, the input voltage of the transformer must be 2-3 V greater than the maximum output voltage.
• Use a C1 capacitor at the rectifier output, as this capacitor can withstand mains noise.
• Use a capacitor (C2 in this experiment) at the output of the regulator, this capacitor helps to deal with fast transient changes and noise at the output. The value of this capacitor depends on the voltage deviation, current variations and transient response time of the capacitor.
• The capacitor used in the circuit must have a higher nominal voltage than the input voltage. Otherwise, the capacitor will start to leak current due to excess voltage on its plates and will explode.
• The zener must be 1W, otherwise it will start heating and cause damage.
• As the current demand increases at the output, the 2N3055 transistor will begin to heat up. To overcome this problem, a suitable heat sink must be mounted to dissipate excess heat. Otherwise, the transistor will explode.
• As our circuit can consume a current of 2A at the output. A 2A fuse must be connected to the rectifier output. This fuse will prevent circuit current exceeding 2A. For currents above 2A, the fuse will blow and this will cut off the input power to the circuit.
Project source code
Project source code
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