20w inverter ???

Discussion in 'The Projects Forum' started by onlyvinod56, Jun 22, 2012.

  1. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008

    Here iam attaching a schematic.
    Its a 20W inverter(emergency lamp) circuit.
    Its not a copy from any source.

    I know that a 555 acts as free running oscillator in astable mode.
    Keeping the astable multi concept in mind, i have designed the shown circuit.

    When i connected 12v DC supply, the output is around 1000v without load.
    But when the load (15W bulb) is connected it is not glowing.

    12V dc supply is from a regulated power supply having a maximum current range of 0-2A max.

    What is the problem here?
    • 20w.PNG
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  2. #12


    Nov 30, 2010
    That transformer should do less than 100 volts. Are you sure you didn't read the meter wrong? [12 x 450/58 = 93V]

    It won't hold up under load because you have a 1k resistor limiting the drive current to about 10 ma and the 3055 transistor has low gain. It can allow less than 1 amp under those conditions, and that is less than 12 watts of input to the transformer.

    In another thought, running a transformer in one polarity only causes trouble with the magnetism saturating the core. I'm not good at this part so I will expect someone to clarify for me.

    That's what I can see for improvements.

    Edit: A Thanks to cork_ie for spotting something I didn't notice.
  3. cork_ie

    Active Member

    Oct 8, 2011
    Unless I am badly mistaken you are running this at about 7KHZ and 50% duty cycle if potentiometer is in mid position. 2N3055 should be OK at this kind of frequency.
    What kind of transformer are you using?
    Could you try this with a 2.2uF capacitor and see what happens?
    Last edited: Jun 22, 2012
    #12 likes this.
  4. Sensacell


    Jun 19, 2012
    Your design uses the transformer in flyback mode, without a load, the output voltage will be huge as the energy stored in the transformer core has nowhere to go- except maybe toasting your driver transistor. Most inverters of this type use a push-pull symmetrical design that allows unused energy stored in the core to be returned to the 12V power supply- very much like a regular transformer running on ac would. google "12 volt inverter circuits" - you will see many examples of this concept.
    #12 likes this.
  5. bertus


    Apr 5, 2008

    As noted the frequency will probably to high for the transformer.
    Also the base resistor for the 2N3055 is to high.
    Now the transistor will not be switched full.
    A base resistor of 68 Ohms will work better.
    (creating a base current of about 0.2 A).

  6. cork_ie

    Active Member

    Oct 8, 2011