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SCR how does it work? — Parallax Forums

SCR how does it work?

mctriviamctrivia Posts: 3,772
edited 2010-10-22 18:05 in General Discussion
According to my text book this schematic will let you adjust the trigger point on a SCR anywhere in the positive half of the sine wave. But I do not believe this to be true. If I am not mistaken the transformer, cap and varister will shift the gate signal out of phase with the main signal by invtan(1/(r*2*PI*f*C)) degrees. So if r is 0 the phase shift would be 90deg and if R is infinity the phase shift would be 0 degrees. The example show a phase shift of approximately 100 degrees though so I must be interpreting this wrong. Anyone know how it actually works?

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Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2010-10-21 08:39
    Without the cap, the max delay would be 90 deg. With the cap, there is an additional possibility of another 90 deg.

    As well, the DC trigger level raises and lowers with the pot so the effect of the shifted wave extends out to 180 deg.

    I've not used this particular layout but the effect is the same as a "ramp and pedestal" with cosine modified trigger from the old RCA or GE thyristor manual.
  • mctriviamctrivia Posts: 3,772
    edited 2010-10-21 09:00
    still dont see how it works. i know a cap in series with a resister cause a phase shift of invtan(1/(r*2*PI*f*C))

    a cap, inductor, and resister is

    Xl=2*PI*f*L
    Xc=1/(2*PI*f*C)
    theta=invtan((Xl-Xc)/R)

    theta can be -90 to 90 but if L and C are fixed it can only varry over 90 deg
  • mctriviamctrivia Posts: 3,772
    edited 2010-10-22 17:16
    I think I figured it out. the resister changes the rate the cap charges. once the cap charges to 0.7V it triggers the SCR gate. by adjusting the RC constant you can set the trigger point anywhere from 0 to 180 degrees.

    Silly me over thinking things.

    Am I correct?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-10-22 17:25
    Yep, yep; that's the $64 answer.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-10-22 17:26
    ... but we know there are lots better ways to do this.
  • mctriviamctrivia Posts: 3,772
    edited 2010-10-22 18:01
    yes but that is besides the point. trying to figure out what the book was talking about.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2010-10-22 18:05
    yes, yes, yes, yes, yes of course

    How about the transformer phasing?
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