Unknown Inverter

Discussion in 'General Electronics Chat' started by Regected, Dec 16, 2010.

  1. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    Can anyone tell me what type of inverter the attached schematic is? I've run across this similar design several times while working on el wire drivers, and I have not been able to find the same circuit on Google. This just takes a 12 volt input and steps it up to ~140VAC. I wish to understand the operation f this circuit better.

    The transformer's primary winding is 1.36mH, the feedback winding is 21uH and the secondary is 238mH. C2 is a 0805 SMT chip, so the capacitance is unknown. Just guessing by the thickness, I would hazard to say it is less than 0.05mf.
     
    Last edited: Dec 16, 2010
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    It looks like a blocking oscillator to me.
     
  3. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    Sure does look like a variation of a blocking oscillator. Thanks.

    I understand R1 and R2 are biasing the base of the first transistor in the darlington pair, and C1 couples the voltage from the feedback winding to the transistor. Does C2 set the frequency? Or is it just there to kick start the oscillations?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    I've got a simulation running, but I don't believe the value of C3. It's way too large.
     
  5. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    You're right. That's a typo. The circuit has a 6800pF, but the silkscreen on the board calls for a 2200pF.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    With C3 omitted, I get hundreds of volts p-p at the output. With 2.2nF and no resistive load, I get about 27V p-p. With 6.8nF, the oscillator breaks into a very low frequency, with damped (decaying) higher frequency ringing on each low frequency cycle.

    What is this circuit for? What is the load? Is the output really DC, or is there a diode (rectifier) in the output circuit?
     
  7. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    This provides ~140V AC to drive luminescent wire at 3kHz. The load is highly resistive, only drawing 10-20mA as it is right now.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    I believe that EL wires look capacitive. Wikipedia says it's about 1nF (1000pF) per foot.
     
  9. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    I honestly have not tested the wire for capacitance, so I don't know. I did put an amp meter on it though, and it does flow current. If it is capacitive, it's very leaky.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    What is your end goal here?
     
  11. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    Well, my primary goal is to learn how this circuit operates so I have a better understanding of feedback circuits. A more immediate goal is to increase the frequency to make the el wire brighter.
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    What kind of voltage was applied when you used the amp meter? If you used a DMM set for AC amps, and an AC source, a capacitor will show current. The amount depends upon frequency and capacitance, as well as applied voltage. Remember the capacitive reactance formula?
     
  13. Regected

    Thread Starter Active Member

    May 24, 2009
    30
    0
    el wire is capacitive. It tests to be ~30nF for 10 feet with a DC resistance of 20Mohm. Capacitance tested on an HP 4192A impedance analyzer and DC resistance tested with a Fluke 115 DMM.
     
    Last edited: Dec 20, 2010
Loading...