Variable Transformer Question

Discussion in 'General Electronics Chat' started by BassNGuns, Dec 12, 2013.

  1. BassNGuns

    Thread Starter New Member

    Dec 9, 2013
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    Hi,
    I would like to use a 3 amp variable transformer to bring up the power slowly after I repair audio amplifiers.
    I don't plan on having the volume up while I'm bringing up the voltage.
    Would a variable transformer rated at 3 amps be enough?
    Thanks in advance. :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    I don't think that is a viable solution. Audio amplifiers do not necessarily function linearly with supply voltage. You may need a minimum supply voltage to get the bias conditions right.
     
  3. BassNGuns

    Thread Starter New Member

    Dec 9, 2013
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    Right, but this would be an AC transformer that I would be using to bring up the unit making sure that there are no shorts present.
    I would also have a 100w light bulb sitting across an empty fuse terminal.
    After the coast is clear I would hook it up to a regular ac line.
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    That's a great idea.

    I don't plug in any unknown without a variac using V and A meters.

    As long as you're doing it. Put it in a box with the meters.

    For the ultimate use an isolation transformer also.
     
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  5. BassNGuns

    Thread Starter New Member

    Dec 9, 2013
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    How many amps is your variac? Just curious.
     
  6. #12

    Expert

    Nov 30, 2010
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    Repairmen have been doing this for decades because the goal is not about getting bias conditions correct. It's about seeing if the filter caps are going to explode gently or violently, or trying to find a sound or a smell or a curl of smoke instead or repeatedly plugging in the machine, unplugging it, replacing the fuse, and plugging it back in. It's a diagnostic procedure, not a THD test.
     
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  7. MrChips

    Moderator

    Oct 2, 2009
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    For that, I built an electronic fuse. It has a meter that can show the peak current draw. I can also set a max threshold current that breaks the AC supply once the max current limit is exceeded. Saves having to replace fuses.
     
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  8. inwo

    Well-Known Member

    Nov 7, 2013
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    From the fusing I believe it's 15 amps. However my isolation transformer is only 3 amps.

    I have a switch when I need more power to bypass the isolation.

    There are appropriate CTs on the amp meter for 0-3 or 0-30 scale.

    I just put some of my CTs and meters on Ebay that I had left over from when I was building this sort of equipment.
     
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  9. inwo

    Well-Known Member

    Nov 7, 2013
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    Good idea.
    Can't tell you how many time I go to bsmt resetting the 20 amp breaker.:D
     
  10. BassNGuns

    Thread Starter New Member

    Dec 9, 2013
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    Do you think with what I'm doing that 3 amps should be enough?
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
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    I think so. It can be overloaded for short periods without damage.
     
  12. JohnInTX

    Moderator

    Jun 26, 2012
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    Agree! A partner and I used to manufacture these in various ratings in the '70s. I still use my 2A model daily for the reasons you've noted. Its saved my bacon countless times. I also use isolation transformers as req'd.

    Its also most useful for low / high line voltage tests (mine is overwound and goes to 140V).
     
    Last edited: Dec 12, 2013
  13. crutschow

    Expert

    Mar 14, 2008
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    That's at least 300W. The idle power of any normal amp with no load should be less than that (unless perhaps if it's a big tube concert amp).
     
  14. #12

    Expert

    Nov 30, 2010
    16,252
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    I wish I had thought of that.:mad: I designed one into a machine I built and never had a thought about packaging one separately for work bench use.
     
  15. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the variac is an old Ham trick, it helps reform electrolytics, and prevent burnt power transformers if the caps are bad. mostly used to bring back old tube type equipment safely.
    cliff
     
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  16. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Ditto on the Vacuum tubes and re-formed electrolytic caps...

    Regards, DPW [Spent years turning op-amps into heaters.]
     
  17. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    I like to separate the power supply, and inject a limited DC supply from one of my supplies. Good time to also test your circuits power supply while they are separated and can run into a dummy load.
     
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