Saturday, September 10, 2016

Circuit Schematic 3.7V to 8.7V Automatic Switching Power Supply using LM7805 IC

Eltronicschool. - There are many application for using LM7805 specially to produce regulated power supply. Other application of this IC also can be used to build automatic switching power supply circuit.

In this time, based on LM7805 IC as regulated voltage, we will give you one of electronic circuit schematic of 3.7V to 8.7V Automatic Switching Power Supply using LM7805 IC like in Figure 1. below.

Circuit Schematic

Component Part
  1. Transformer 12-CT-12 with 200mA
  2. Resistors
  3. LM7805 IC
  4. Capacitors
  5. Diodes
  6. LED
  7. NPN and PNP transistor
  8. Relay
  9. Variable resistor

Circuit schematic like in Figure 1 above is 3.7V to 8.7V Automatic Switching Power Supply using LM7805 IC. In this circuit that the output voltage is variable although LM7805 is used to be served as fixed-voltage regulator. The variable output voltage varies from 3.7 volt to 8.7 volt come from the connection of a potentiometer RV1 between common terminal of regulator IC and ground. Thus, the output will increase by 1 volt (taking into account 1.3-volt drop across diodes D5 and D6).

How this power supply can shut off automatically when not any load in output terminals? This conditioan will be achieved with the help of transistors Q1 and Q2, diodes D5 and D6, and capacitor C2. When any load connected in the output terminal, potential drop across diodes D5 and D6 (approximately 1.3V). It is make enough for transistors Q2 and Q1 to conduct. As a result, the relay RL1 gets energy and remains in that state as long as the load remains connected in output terminal. 

Capacitor C2 gets charged to around 7-8 volt potential flow through transistor Q2 at same time. Transistor Q2 is cut off when the load is disconnected. Capacitor C2 still be charged and it starts discharging through base of transistor Q1. After some time  the relay RL1 is not energized, which switches off the mains input to primary of transformer TR1. Now, we can press the SW1 switch to resume the power again. As note that higher the value of capacitor C2 will be the delay in switching off the power supply on disconnection of the load, and it is also apply vice versa.

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