Determining the fuse for a transformer

Discussion in 'General Electronics Chat' started by Molisi73, May 22, 2014.

  1. Molisi73

    Thread Starter New Member

    May 22, 2014
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    0
    Hi all. I bought this used step-up & -down CT-100 transformer without a fuse (see attached photo). I used a piece of metal to check that the transform does work. But I need to find out exactly the right type of fuse I need to get for it. Does anyone know this type of transformer, or how can I determine the right fuse for it? Any help will be very much appreciated.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Looks like 10安培 to me :p
    Max.
     
  3. Molisi73

    Thread Starter New Member

    May 22, 2014
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    Thanks Max. Is that 10 amps your best guess?
     
  4. #12

    Expert

    Nov 30, 2010
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    It looks pretty large, but fuses are one of the things I'm not so good at. Look at the fuse holder. Is it designed for a 3AG? A cartridge fuse? Can you find anything that tells whether it has a wattage rating? (What? You can't read Chinese?)

    All I can say from here is, "Slow-blow". This kind of machine has a start surge.
     
  5. wmodavis

    Well-Known Member

    Oct 23, 2010
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    150
    Look inside the case and see if there are any clues.
     
  6. Molisi73

    Thread Starter New Member

    May 22, 2014
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    I’ve added a photo of the “input” side of the transformer. The ruler seen gives you an idea as to the size of the fuse-parts and the transformer. There was no fuse left inside the fuse-“box” (hole/holder); however I found one of the end-caps still stuck inside. The cap (about 5mm high) unfortunately doesn’t show any markings on it. The fuse “box” (hole) is about 2cm deep and 5mm wide. That’s as much information I can find.

    Thanks again for all the comments and responses.

     
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  7. #12

    Expert

    Nov 30, 2010
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    Read that busted off fuse end to see what kind of fuse it is. The marking might not be there, but try anyway. It's either a 3AG or an AGC or a short fuse declared in millimeters.
     
  8. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Did you remove the metal housing cover and look inside?
    Any markings on the transformer to indicate ratings?
    Look for clues. How big is the transformer - particularly the iron core?
    The other end of the fuse may still be in the fuse holder.
     
  9. Ragwire

    Member

    Dec 9, 2013
    36
    1
    What's the VA stated on the tag? Not saying this is necessarily good for your unit, and you should always use the manufacture's specifications, but the old NEC watered down to a rule of thumb is the volt amperes divided by the voltage, times 1.25. Generally use a slow blow type. e.g., 1000VA on the tag divided by 120V primary winding supply = 8.33A. 8.33 A x 1.25 = 10.41 Amps. Allowance is then made to use the next standard size up, if the calculation yeilds and in-between number like this. If I recall correctly, the stated standard sizes are 1, 3, 5, , 8, 10, 12, and 15 amps. I would still not exceed 10 for a time delay fuse. Note this example is for fusing the primary, not the secondary.

    Try to find manufacturer information, of course.
     
  10. Molisi73

    Thread Starter New Member

    May 22, 2014
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    As a first timer here, I’m very impressed with the support everyone has put in, small or big, useful or funny, to my request. I am gathering all the responses and see if I can figure out the right fuse for the transformer. I wish I was able to get any reading off the fuse-cap, which I couldn’t, and I did take it to an electronic store who couldn’t figure out the right fuse for me. I’ll look deeper into any other detail I may have missed and I have a good feeling that I have a way better lead now than 8 hours ago in figuring out the right fuse for my transformer. Once again, thank you all sincerely, and continue to keep up the great support network here. You’re all awesome!
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,570
    2,381
    Linear transformer supplies are very rugged devices, especially of that apparent size.
    You can always size on the low side, and if you get nuisance blowing, up the value till it hangs in.
    In any event, I think a 10amp is going to do the trick and cover most eventualities, including in rush etc.
    Max.
     
    Last edited: May 23, 2014
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The fuse size is a standard (these days) 20x5mm glass fuse. The smaller of the two common glass fuse sizes.

    The CT100 model number likely indicates it is a 100W transformer, that is just a guess but does match the physical size of the unit. It's barely big enough for a "100W" transformer. "Chinese 100W" means don't run it at 100W for very long! ;)

    I'd run a 2.5A slow blow fuse, or a 3.15A. Both will give you plenty of protection if it is shorted out.
     
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