triac control of simple AC to DC converter

Discussion in 'General Electronics Chat' started by eeabe, Dec 29, 2015.

  1. eeabe

    Thread Starter Member

    Nov 30, 2013
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    9
    I'm working on an automated test that will switch AC power to a product at 115V, 400Hz. The product uses a simple step down transformer followed by a diode bridge and a large capacitor to get DC power, so the current moves in "spurts" only when the voltage is near the peak.

    If I build a "standard" triac drive circuit (such as the circuits in Fairchild's very nice app note AN-3003), I believe the opto and the main triacs might oscillate at some high frequency during the non-peak voltage times due to the main triac turning off right after it turns on due to lack of holding current since there is basically no load for most of the cycle. Anyone built a similar circuit and had it work fine without oscillating? I haven't simulated it yet; I use LTSpice and I haven't decided to dive in and figure out how to build a triac yet...

    I'd like to see the AC voltage applied to the load during basically the entire cycle regardless of current so that I don't have noise issues due to a high voltage oscillation, and so that the test system simulates having power applied by a power supply like it will have in normal use. I'm thinking about driving the triac gate or maybe SCR gates via line voltage and a resistor (not passing through the load) so the triac or SCRs can be held on with gate current, and not require any holding current across the load. This may require more components and will generate more heat, so if there is a simple method, or if I'm worried for no reason, I'd love to learn about it. Any thoughts or experience?

    Thanks.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,793
    I'm curious about the source of your knowledge because something about your descriptive language seems bit off. English is a poor substitute for a schematic and you really should know and understand that.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    I would like to see your circuit diagram....
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,555
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    Is this a DC load, what is the nature of it?
    Max.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,793
    Doesn't the primary of the transformer provide a load which is always connected?
     
    eeabe likes this.
  6. eeabe

    Thread Starter Member

    Nov 30, 2013
    59
    9
    Yes, and since it will be 90 degrees out of phase, I'm thinking that SCRs will be the way to go.

    Thanks for that helpful reminder.
     
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