DC Motor Speed Controller using UJTs and SCRs

Discussion in 'General Electronics Chat' started by lkgan, Jan 17, 2010.

  1. lkgan

    Thread Starter Member

    Dec 18, 2009
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    Hi everyone,

    The UJT, capacitor, and the resistors make up an oscillator that supplies an ac voltage to the SCR's gate. From my understanding, the SCR wouldn't turn off when the gate pulse become zero. It will only turn off when negative pulse is applied to it. However in this situation, the gate pulse is fed by the Base1 of UJT which is shown in the diagram. The Base1 pulse is only from level 0V to certain positive level voltage (no negative voltage level). My question is, how will the pulse turn off the SCR and eventually control the speed of motor? Thanks in advance...
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    This will not work as the thyristor will not turn off again once triggered.
    Better use a power mosfet with PWM control in that case.

    Greetings,
    Bertus
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    In a DC situation, an SCR will only turn off when current through it drops to some very low level, generally when the voltage between anode and cathode is very nearly at zero. The activity at the gate (which responds to voltages that are positive relative to the cathode) has no further effect once the device is turned on.

    I have seem a kludgy circuit that turned off a big SCR in a DC situation by means of another SCR that, when gated, capacitively coupled a negative pulse onto the conducting SCR's anode and brought the voltage across the device low enough that it (the pulse) dropped it out of conduction.
     
  4. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
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    It looks like the pot controls the duty cycle of the picket fence pulse train of the UJT. If this is correct, then the SCR will see a time varying trigger signal varying the amount of current that can pass thru it, thereby adjusting the DC motor speed. Its kind of a crude speed control, but it will work for a number of applications.

    Cheers, DPW [ Everyone's knowledge is in-complete...Albert Einstein]
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Duane,

    Are you sure about your facts? The SCR once triggered will stay on until the voltage supplied to the motor is removed. This is what Beenthere and Bertus have both stated in their replies.

    hgmjr
     
  6. Duane P Wetick

    Active Member

    Apr 23, 2009
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    If the DC source is a Full or Half Wave rectified, then there will be a 0 current or a minimum current point where the SCR cuts off. Do you agree?

    Regards, DPW
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    I have a slight recollection of having seen some UJT and SCR speed control back about 1970. I never messed with it. It might be found in one of the old circuits collections, like "The Sourcebook of Electronic Circuits".

    There's a copy in the local library - maybe I'll do a research project.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    The poster of the question did not mention that the DC power source to the motor was half-wave or full-wave rectified. The motor power source is labeled 3-6V DC. That leads me to believe that the motor is supplied from a steady DC source.

    If it were an AC power source and the motor an AC motor then the circuit would behave quite differently.

    hgmjr
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    (Sound of hand smacking forehead) Of course it will work with half or full wave rectified and unfiltered DC. The control would be even better if the voltage to the UJT's RC network were synchronized with the DC waveform so the control was predictable.
     
  10. lkgan

    Thread Starter Member

    Dec 18, 2009
    117
    0
    So this circuit would not work right since the gate current do not reach negative level (it does not affect the SCR anyone once it's turn on) and the DC supply to the motor is constant 3V to 6V all the time? I think it will work once the DC supply to the motor is half wave or full wave rectified from ac as mentioned by Duane. What do you guys think?
     
  11. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Once it has been switched on (SCR) by the gate terminal, the SCR remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).
    Source
    http://en.wikipedia.org/wiki/Thyristor
     
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