Help with a Dayton DC Speed Control

Discussion in 'General Electronics Chat' started by Tillman33, Jul 30, 2014.

  1. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0
    I have a Dayton DC Speed Control where the potentiometer is not working. It's almost like it's not there. The voltage is at 106 vdc no matter if the pot is fully opened or closed. The pot is good. Has anybody had any experience working on these and know what it may be. Thanks.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the pots do not fail very often, probbaly an scr. has it worked before? there is a field output and an armature output, one is fixed and the other variable.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    Usually a shorted SCR will cause a blown fuse, as this will short one side of the diode/SCR bridge.
    If it operates with the pot removed altogether then it is something turning the bridge on full after the pot.
    Max.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    In one form of speed control AC is bridge rectified to pulsating 120 CPS. Motor & SCR are in series, so a shorted SCR would just give full speed?
     
    inwo likes this.
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    Many of these are SCR/Diode bridge, if the SCR is shorted, no different from a bridge with a shorted diode, results in a blown fuse or similar symptom.
    If it doesn't turn off when the other conducts this is the result.
    Max.
     
  6. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0
    I was thinking it may be the SRC's but wasn't sure. Thanks for the replies!
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    For it to be the SCR's causing continuous 106vdc, one would have to be shorted and the other open circuit.
    Max.
     
  8. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0
    I put in 4 brand new SCR thryristors and I'm still having the same problem.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Does it have tach feedback? If it has the option for tach feedback and you dont need it, it should be jumpered out. Otherwise floating voltages might appear to the drive as a tach signal, and the drive is trying to stop a freewheeling motor.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    If it has 4 SCR's then it is a 4 quadrant drive.
    Are you sure one pair was not straight rectifiers?
    Max.
     
  11. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0

    No it doesn't.
     
  12. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0

    They are the same type SCR's as was originally in there.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    It could be in the firing circuit, some use an Opto for the SCR triggers, if this is shorted then the SCR's are always full on.
    Max.
     
  14. Tillman33

    Thread Starter New Member

    Jul 30, 2014
    6
    0
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    C11, 4N28 is the opto coupler drive to the SCR's in that schematic.
    Max.
     
  16. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    there are two outputs in the dayton dc speed controls, one for the armature, and one for the field. one is variable and one is fixed, both dc.
     
Loading...