Transformers and fuses...

Discussion in 'General Electronics Chat' started by JohnnyD, Sep 7, 2006.

  1. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    Hi,

    I'm currently working on a model railway controller for my dad. I'm using 50VA transformer with a single 230V primary winding and two 15V secondary windings. I'm going to use one winding for the controller circuitry and use the second winding for a seperate AC outlet.

    By my calculations, each winding will produce around 1.6amps @ 15v. So i'm going to use a 1.6amp circuit breaker on each secondary winding to protect that side of things. So my first question is - does that sound OK?

    Where I get slightly confused is on the primary side. If the transformer is 230V and 50VA I think it should draw around 220mA. If i use a 250mA slow blow fuse will that be enough to protect it? I know about the surge that occurs when a transformer is first switched on, and I've been told to use a fuse twice the amp rating of the primary winding. But I thought that was why you use slow blow fuses - to cope with this surge but still protect the transformer properly. So should I use a 250mA, or a 500mA fuse?

    Any advice on this would be great as I don't know too much about transformers at the moment.

    Thanks,

    John.
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Fuses are more complicated than that, you need to look at the time-lag ratings and the I^2t of the fuse vs. in-rush current and waveform, etc.

    However, the empirical method is usually just as useful. Do you anticipate using 1.6A much of the time? First, install a fuse holder so you can easily change the fuse. Try the 1/4A fast-blo, if it is popping too often, change to a 1/4A slo-blo. Starting with these minimum sizes should reasonably protect your device, and by using the holder you can adjust upward accordingly.

    I believe the VA rating usually applies to the secondary, however, it's all a matter of turns ratio and you still get the same numbers (50VA / 30V = 1.67A, etc.), which means if you properly parallel the secondaries you could get 15V @ 3.2A.
     
  3. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    The most I'd anticipate using is around 1A but i'd like the extra headroom in case I wanted to use the controller on a larger gauge layout with larger motors, etc...

    I've fitted the enclosure with one of those 3 pin kettle-type sockets with a fuse holder built-in so it's easy to change them.

    Thanks for the advice, I think I need to look into the stuff you mention at the start of your post.
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    See attached for a couple of things that may help:
     
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