Sunday, August 21, 2016

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

Electronic Kit School. - Although LM7805 can produce constant voltage regulator in output of power supply, but with some modification circuit we can get more varies output voltage from 3.7 volt until 8.7 volt. 

With using LM7805 IC, in this article we will give you one electronic circuit design that can produce varies voltage output 3.7 volt until 8.7 volt automatic switching. It mean that this circuit will shut off automatically when there are not load in the output power supply. So, it is very important to save the energy using after use this power supply model. Now you can try to use this circuit like shown in Figure 1 below to do this work perfectly. Read also: 12 Volt Ultra Low Drop Power Supply with MOSFET Based Linear Voltage Regulator Circuit.

Figure 1. 3.7V to 8.7V Automatic Switching Power Supply Circuit using LM7805 IC

Component Parts

  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 like in Figure 1 above show you about the varies voltage 3.7V to 8.7V output voltage with working automatic switching power supply using LM7805 IC that can give you very important switching self or automatically shut off power supply when not any load in output. The main component used in this circuit is LM7805 as regulated voltage result that will designed to get varies voltage output 3.7V to 8.7V.

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.

  • Please use heat sink for LM7805 for reducing the temperature.


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