8 12V to 5V buck converter using 555 timer IC

converter schematic

This  buck converter is driven by 555 timer IC and converts 12V dc from a battery to 5V dc. The 555 IC  (operated in astable mode ) generates pulses of voltages of 42% duty cycle. This pulses turns the transistor on and off and drives the inductor L (buck converter circuit), and as a result generates 5V dc output. For this configuration,
Ra =20K
Rb =27K
C =1nF
Q = TIP41C or any of the same specs
Cout = 2200uF (higher the better)


  1. I think the diode D is backwards.

  2. thank you sir/maam Anonymous
    for your comment and letting simple-electronics know about the diode orientation problem. The converter schematic above is already modified. If you find additional problem kindly post a comment.

  3. Replies
    1. The inductor is probably between 600 uH and 1mH.
      These are expensive.

      Well the TIP41C can carry 6 amps so the coil wire will have to be sizable. This means thicker wire which means a physically larger coil.

      To use a smaller, cheaper inductor change the components to these values:

      Ra = 2K
      Rb= 2K7
      C= 1nF
      Q= MOSFET FQP33N10 or similar
      L= 60uH

  4. Can I use any values between 600uH and 1mH if I implement the above circuit???? Btw, your Rb is only 2K or 27K? Thank you

  5. Hello, I drew that circuit with the multisim stimulation software but the output voltage is varied but not steady in 5V. So,I can i solved that problem. I already changed some components values but the problem is still there.

  6. its output gives only 10.78V instead of giving 5V
    is there any changes to get 5V..??

  7. Without a load there is nothing to pull the output voltage down. there is nothing sensing the output voltage. This is not a true buck converter. The output voltage will depend on input voltage and load. This circuit is closer to a pwm driver than a buck converter. It may have good current regulation. This would probably make a good led driver but i wouldn't use it to charge usb devices.