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Solid State Solution for Fried Autotransformer

Discussion in 'Power Electronics' started by alfa88, Jul 14, 2021.

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  1. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    A tale of woe. It started when I bought a tube tester years ago. It was in pretty poor shape but it powered up. Somehow when I pushed a few switches(I shoulda read the instructions 1st)and my new toy was dead. So I tossed it into the corner and forgot about it. Fast forward. I unwrapped the transformer and to my delight I found an open, repaired, tested, found another open repaired and Ohmed out. Problem is when I realized I was dealing with an autotransformer and that my static test was in error. So I'm thinking that if I could make a multi-tap power supply that will fit I might be back in business. But would running DC through the tube test be legit? BTW I'm talking about a simple emissions tester. Nowhere to be found, though, are what exactly the taps are since the manufacturer(Allied/Knight/Lafayette) saw fit not to include this info. I'm guessing 1, 2, 5, 6, 12, 35, 50 since there are 7 filament settings and the current should be at least 500mA. Opinions? Advice?
     
  2. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    I got some advice from my tube buddy and have 1.2, 3, 5, 6.3, 12.6, 25 & 50 as my targets. I want to keep things as small as possible but it looks like I have to use an external supply with a regulator array in place of the transformer. My understanding of emission testers is that grids and cathodes are tied together and the tube is treated like a diode and in my case ~5mA shows up on the plate. With that in mind I have to make the Voltage floating so I can use LM-338s. Now if I go with 338s, they have a 32V limit so I have to cascade the 50V - 25V stage. Therein lays a possible conflict with the 1st and 2nd stage. I read where a pullup resistor might be needed
     

    Attached Files:

  3. bertus

    bertus Moderator

    1,867
    708
    Nov 8, 2019
    Hello,

    Take a look at your schematic.
    What would happen when the 25 Volts output gets shorted?
    As the LM338 allows a maximum differential of 40 Volts, it will likely blow.
    LM338_max rating.png
    Bertus
     
  4. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    The max rating is for Input to Output differential. In my case ~25V. Max is 40. Am I missing something?
     
  5. bertus

    bertus Moderator

    1,867
    708
    Nov 8, 2019
    Hello,

    I said, what would happen when the 25 Volts gets shorted.
    Then there is a differential of 50 Volts.

    Bertus
     
  6. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    An excerpt from the datasheet:
    Since the regulator is "floating" and sees only
    the input-to-output differential voltage, supplies of several
    hundred volts can be regulated as long as the
    maximum input to input differential is not exceeded.
     
  7. bertus

    bertus Moderator

    1,867
    708
    Nov 8, 2019
    Hello,

    Yes, as long as you do not exceed the maximum input to output differential, there is no problem.
    The problem will arise when the 25 Volts gets shorted.

    Bertus
     
  8. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    It's kind of funny. I was scanning the manual and it seems shorts were always a concern with this tester.
    A fuse at the input of the regulator chain would alleviate any worries.
     

    Attached Files:

    Last edited: Jul 17, 2021
  9. bertus

    bertus Moderator

    1,867
    708
    Nov 8, 2019
    Hello,

    You could use a tracking voltage regulator to have about 15 Volts on the second regulator and 10 Volts on the preregulator.
    In case of a short, the preregulator will see 35 Volts, wich is safe.
    LM338_tracking preregulator 15 volt.png
    Bertus
     
  10. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    Since space so tight I'm forced to use an external 48V PS. To cut down on size I used fixed resistors instead of pots. I'm going to cram an inline fuse in somewhere. I completed the board and have 48.5V, 24V, 12V, 6V, 5.2V, 3V & 1.5V. Pretty much within 10% of filament requirements. Thanks for taking the time to consult.
     
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