Power Supply for Projects. Design Note 43

IMAGEFor bread board assembly of Micro controller and Arduino projects ,we need  well regulated  5 V and 3.3 V DC power supply. Here is the simple but efficient regulated power supply that gives four stage power outputs  – 12 V unregulated, 9 V, 5 V and 3.3 V regulated DC .

It is a Transformer based power supply using a 12-0-12 volt 200 mA Step-down transformer. The low volt AC is rectified by diodes D1 and D2. The high value smoothing capacitor C1 removes ripples and gives a clean 12 volt DC .This 12 volt DC is unregulated which can be used to drive back end components like Relay, Buzzer, Lamps etc. Regulator IC 7809 (IC 1) takes the 12 volt unregulated DC and gives 9 volt regulated DC. IC 7805 (IC2) takes 9 V DC from IC1 and gives 5 V regulated DC. Capacitor C2 removes noise from the power supply. Green LED with the current limiting resistor R1 is the power on indicator. Diodes D3 , D4 and D5 are used to reduce the 5 V DC to 3 to 3.8 V DC. Silicon diodes usually have a forward voltage drop between 0.4 V to 0.7 V. This may vary based on the make of the diode. So three  IN 4007 diodes drops 1.2 – 2.1  Volts. Therefore, the output from D5 will be around 3 – 3.8 V.  But the forward voltage drop of the diode will not be accurate in all makes and may vary between 0.4 to 0.7 volts. Anyway a mid rage of 3.5 volt can be expected from D5.


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Assemble the circuit on a common PCB. Transformer is 200 mA only and the regulator ICs gives around 100 mA output current which is sufficient for the project. Use suitable sockets and pins to tap the 12V, 9V, 5V and  3.3V outputs.

One thought on “Power Supply for Projects. Design Note 43

  1. hello sir,
    if I want -12 , -9 , -5 v is it okay to reverse biasing D1 and D2??
    thank you

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