Regulated Power Supply

A regulated power supply converts unregulated AC into a constant DC with the help of a voltage rectifier. Its function is to supply a stable voltage (or less often current), to a circuit or device that must be operated within certain power supply limits. Any deviation from these rating can damage the devices.

Some DC power supplies use AC mains electricity as an energy source. Such power supplies will employ a transformer to convert the input voltage to a lower AC voltage. A rectifier is used to convert the transformer output voltage to a varying DC voltage, which in turn is passed through an electronic filter to convert it to an unregulated DC voltage. The filter removes most, but not all of the AC voltage variations; the remaining voltage variations are known as ripple.

Types:

• Fixed Voltage Power Supply
• Variable Voltage Power Supply

Fixed Voltage Power Supply

The unregulated DC output is fixed to a constant voltage level by using a voltage regulator. Commonly used fixed voltage values are 5,9,12,15 Volts.

Variable Voltage Power Supply

The unregulated DC output can be varied over a range of voltage levels by using a variable voltage regulators.

Simple Alarm

Here is the Simple Alarm circuit using SCR.

Components Required:

1) Breadboard

2) 1N4007 Diode

3) 470 ohm resistor

4) 100 kilo ohm resistor

5) LED (Any Color)

6) C106 SCR

7) Buzzer

8) DIP Switch = 2

9) 9Volt Battery

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• Close the S1 switch and switch S2 is kept open therefore there is no gate current and the SCR remains OFF.
• Hence the LED and alarm connected to the circuit also remains OFF.
• Now close switch S2 also therefore gate current is applied which triggers the SCR to ON state and the gate terminal looses its control and cannot be used to switch OFF the SCR.
• Hence the LED and alarm connected to the circuit switches to ON state.
• To switch OFF the SCR, decrease the gate current below the holding current of the SCR or disconnect the gate current.

 Circuit Diagram:

Figure 1: Simple Alarm Circuit simulation made in Multisim (S1 Closed & S2 Open)

 

Figure 2: Simple Alarm Circuit simulation made in Multisim (S1 & S2 Closed)

Figure 3: Simple Alarm Circuit (S1 & S2 Closed)

SCR (Silicon-Controlled Rectifier)

A Silicon-Controlled Rectifier is a four layer solid state current controlling device. It forms PNPN structure and has three junctions, labeled anode, cathode and gate. Anode terminal is connected to the P-type material, cathode terminal is connected to the N-type material and the gate is connected to the P-type material nearest to the cathode. SCRs are unidirectional devices (i.e. can conduct current only in one direction)

Modes of Operation:

• Forward blocking mode (OFF state)
• Forward conduction mode (ON state)
• Reverse blocking mode (OFF state)

Forward blocking mode

In this mode of operation the anode is given a positive potential while the cathode is given a negative voltage keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from the anode to the cathode until the applied voltage reaches its breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it starts conducting but below breakover voltage it offers very high resistance to the flow of current and is said to be in off state.

Forward conduction mode

SCR can be brought from blocking mode to conduction mode in two ways - either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it starts conducting no more gate voltage is required to maintain it in on state. There is one way to turn it off i.e. Reduce the current flowing through it below a minimum value called holding current.

Figure 1: Silicon-Controlled Rectifier (SCR) Symbol

 

Figure 2: Silicon-Controlled Rectifier (SCR) Schematic Diagram