Troubleshooting variac

Discussion in 'General Electronics Chat' started by magnet18, May 9, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Alright, I got those PSU's a while back,
    http://forum.allaboutcircuits.com/showthread.php?t=54007
    I got one working like a charm, but the second one only works when the variac is turned past 70% or so, I'm pretty sure it's the variac, since outside of that (once it's turned past that spot) it works perfectly, and it definitely has power, it's not a slow climb to normal, it's either good or bad.

    I cleaned the variac with an eraser, didn't seem to make much difference, looking at the contacts it looks like the silver coating has been worn off and it's down to the copper wire, and the brush (not sure exactly what it's called) is pressed against it fairly loosely.

    I can provide more close-ups if you want.

    Any ideas?
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi magnet18,

    Do a ohmmeter check.
    1. Unit unplugged.
    2. Connect ohmmeter to output of variac.
    3. Start at 0% and go up to max.
    4. Should read low ohms all the way.
    5. Watch for high Z jumps as you move slowly.
    Regards,
    Ifixit
     
  3. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Right, I just did that, it reads between 40-40 ohms for the part that doesn't work, when it gets to where it does work it's almost zero...

    I guess if nothing else I could just use it at only the higher settings...
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Just keep in mind that a variac has no isolation from the line. They're handy, but pretty lethal, too.
     
  5. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi magnet18,

    Be sure to unplug the unit.

    The "brush" is made of a carbon composite and wipes over the exposed bare copper part of the wire turns on the variac core. Since it still seems to work above 70%, the problem is likely in the wiper circuit.

    Clean the exposed copper surface, that the wiper slides on, with fine emery cloth until it looks like bare copper. You can check with your ohmmeter too. This will ensure the wiper is making good contact.

    The wiper arm connects to a shaft in the middle. There may be another slider there somewhere to bring the connection out to the front panel. Check that for damage.

    If there is no sliding contact at the shaft, then the other method used is just a flexible insulated wire wrapped around the shaft with enough slack to allow the 360° turn. Check that for damage. The wire may be broken inside the insulation, but makes contact when the tension is just right.

    Good Luck,
    Ifixit
     
  6. #12

    Expert

    Nov 30, 2010
    16,303
    6,812
    You said the brush makes contact fairly loosly. Carbon brushes wear down. Lacking a new contact brush, you could try to increase the spring pressure so the brush will work for a little while longer.
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Thanks, I looked and theres another slider... and it's DIRTY
    Getting at it is going to mean disassembly, which could take awhile. I'm also going to pull a new carbon brush from something and replace it as well.
     
  8. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Right, got it fixed, I disassembled the variac and cleaned all the contacts (primarily a wavy copper ring that was really dirty) using sandpaper, an eraser, and acetone. Now it works lke a charm :D
     
  9. #12

    Expert

    Nov 30, 2010
    16,303
    6,812
    Congratulations on cheating the dumpster out of a rather expensive snack.
     
  10. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    Dumpsters hate me :D
     
Loading...