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DC to AC

  1. Dec 26, 2003 #1
    I am going to build a small portable 300V capacitor charger.
    I have a 9<->230V transformer and as a powersource i will use a 9v battery.

    The thing i do not know is how to convert DC-AC that is needed. I found a circuit where they used a 2N3055 transistor and a resistor but i do not remember exactly how it was done.

    After the transformer i will use a bridge rectifier to convert it back to DC again.

  2. jcsd
  3. Dec 26, 2003 #2
    What if you feed the DC through an Oscillator tank circuit? The output would then be AC at the oscillating frequency setting of the tank, yes?

  4. Dec 26, 2003 #3
    Well, how do i build a oscillator tank circuit?
    Or can it be salvaged from some electronics?
  5. Dec 26, 2003 #4


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    You can cop out, go to Target and buy a 12V inverter.

    Depending on how 'nice' you want the AC to be, a really simple solution is to use a timer chip and transistors to simply switch the current back and forth in a square wave.
  6. Dec 26, 2003 #5
    A tank circuit is nothing more than a capacitor and an induction coil (an induction coil is simply a coil of wire). By changing the capacitance of the capacitor (if it is a varible capacitor), you change the frequency at which the current will oscillate.

    The induction coil itself can be used as the second leg of a tranformer (which is nothing more than two induction coils side by side), in order to induce the current into the tank circuit.
  7. Dec 26, 2003 #6
    Cont... Sorry, clicked the wronge button...

    Or the induction coil can be used to induce the power from the tank circuit by being the primary leg of a transformer. For your case, you would atach the dc power leads to the tank circuit (one lead on one side, the other lead on the other side), and then induce the alternating current from the induction coil.

    Think Safe!
  8. Dec 26, 2003 #7
    A tank circuit will not convert a direct current to AC. If you connected a tank circuit to a DC source, the inductor, if no resistor was in series with it, would short the DC source. If you did have a resistor to limit the current through the coil, then you would simply have a direct current flowing through the coil. No current would flow through the capacitive branch when the capacitor is fully charged. Effectively, with direct current, an inductor has 0 reactance and a capacitor has infinite reactance.

    Inductive Reactance
    [tex]X_l=2\pi fl[/tex]

    Capacitive Reactance
    [tex]X_c=\frac{1}{2 \pi fc}[/tex]

    Both of these are measured in ohms.

    When dealing with RCL circuits, the total impedance is given by:


    At the resonant frequency,

    the total reactance is 0 and this means in a series RCL circuit that the total impedance is at it's lowest.

    A parallel RCL circuit operating at the resonant frequency will have maximum impedance.

    The frequency of a DC signal is 0 Hz so reactance doesn't come into play. Now if you're talking about pulsating DC, that's a different matter all together.
    Last edited: Dec 26, 2003
  9. Dec 26, 2003 #8
    Yes, thank you. Because the pulsating dc would in effect expand and collapse the electromagnetic field that is being generated onto the coil whereas a constant dc source would only create it, not collapse it, and it is the creating and collapsing of the electromagnetic field combined that produces the ac. Therefore, the next step (or prior step), would be to generate pulsating dc from constant dc. It would be at this point where the pulsating dc polarity would be reversed with each pulsation, right? Could this pulsating dc be accomplished by using flip flops?
  10. Dec 26, 2003 #9


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    Pulses can be generated with any number of simple feedback circuits. The problem is that with high-frequency pulses, the coil will probably act as a low-pass filter, and smooth them out instead of treating them as pulses.
  11. Dec 26, 2003 #10
    This may be of interest to you.

  12. Dec 27, 2003 #11
    Thanks for the link. I have attached a file were i have changed the circuit a bit. Do you think this is possible?

    If the resistance on the resistor is right then the transistor do not get enough ampere on the base to stay on when current goes trough the primary. Transistor turns off.

    When the primary do not conduct because the transistor if off there is enough ampere going to the base to turn the transistor on.

    This would result in some kind of a square wave i think.

    Attached Files:

  13. Dec 28, 2003 #12
    Wein Bridge Oscillator

    From practical experience, the best sine wave oscillator (aside from monolithic packages) is the Wein Bridge Oscillator.

    You can use any type of gain device (transistor amp, op amp, etc.) but you need a feedback network of the "lead-lag" type.

    The reason why i like this oscillator is that the gain required is predictable (around 3), because the feedback network's attenuation factor is always aroudn 1/3.

    Research Wein Bridge Oscillators, they are fundamentally easy to understand and are a good starting point before you get into more advanced oscillators.

    For Square waves, best bet is to go with a 555 Timer.
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