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Easy Q about fuse placement in transormer-based power supply

  1. Sep 5, 2013 #1
    Making an amp for a friend, useing a split supply with center tabbed transformer as ground for the chip, I think I heard the current alternates comeing from = and negative into ground, or from = and ground into negative or somthing, so where do I put the fuse?
     
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  3. Sep 5, 2013 #2

    berkeman

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    What's "="?

    The fuse for a transformer-based power supply should go in the Hot lead into the Primary side of the transformer (the AC Mains side). That is also where the power switch should go. And Earth ground from the AC Mains wall plug should be attached to the metal enclosure of the power supply with a ground lug and nut with a star washer. Do you know why?
     
  4. Sep 5, 2013 #3

    davenn

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    In addition to what Berkeman said ....
    If the amplifier is using a split rail supply say for example .... +40VDC, 0V -40VDC
    then you could also fuse the 2 DC rails between the bridge rectifier and the amplifier circuit
    This is common practice in higher power amps I have worked on

    Dave
     
  5. Sep 6, 2013 #4

    meBigGuy

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    Makes no sense to fuse the centertap since you can short +40 to -40 and have no centertap current.
     
  6. Sep 6, 2013 #5

    davenn

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    what post are you referring to with this comment ?

    Dave
     
  7. Sep 6, 2013 #6
    Ya I have been asking around and yes for this low power circuit a fuse in the primary is what I need, thanks! :)
     
  8. Sep 6, 2013 #7
    But one more thing, I am running small thing like a fan and LEDS and the way my pcb design is right now it will be a little tough getting the regulated + and - to the output terminals for the fan and LEDs, but it is perfiect for useing the center tab and a line from the transformer then send it through a half wave rectifier and filter to the LEDS and Fan. So even though I am using the center tab in most of my circuit can I still use it for power?(understanding it will be half the voltage) , this may be a stupid question but I don't want to mess this up.
     
  9. Sep 6, 2013 #8

    davenn

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    Hey TD good to see you are still having fun and experimenting

    I did a dual supply circuit for some one else in another thread

    attachment.php?attachmentid=61527&stc=1&d=1378510240.jpg

    if you have something similar maybe a different voltage .... there's no problem just using 1/2 the supply, say, the0V rail and the +12V ( or whatever your one is) rail

    Dave
     

    Attached Files:

    Last edited: Sep 6, 2013
  10. Sep 6, 2013 #9
    Lol thanks!, electronics are so fun, I am trying to get into the Massachusetts Institute of Technology. This project is a 40watt amplifier for someone, I am being payed to do this which feels great. I can't test this on my bread board at the moment but is 4700uf good enough filtering for an audio amp running small speakers? once I thought I use 4700uf and with smaller more officiant speakers I could hear it fairily easily.
     
  11. Sep 6, 2013 #10

    davenn

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    the power supply filter capacitors ... say the C1 and C2 in my diagram
    their value if governed mainly by the corrent the PSU is expected to supply the rule of thumb
    value is 1000uF per Amp. There's no harm using larger caps, but if you use ove less than the
    value you should be using, it means there will be more ripple present, which may be heard as hum
    or distortion on the audio output

    Dave
     
  12. Sep 6, 2013 #11
    Ok i think ill take it safe and do 6800uf then. Will the fuse before the primary protect from a crossed circuit on the secondary?
     
  13. Sep 6, 2013 #12

    davenn

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    it will remembering that the primary current flowing and thus the fuse size will be much smaller than the current flowing on the secondary side.
    Some one here much better at maths etc could tell you that if you have say 2A on the secondary, how much current is flowing through the primary ... I should be able to answer that ... but not too with it at the moment :wink:
    For any supply that has significant current draw on the secondary, anything more than an Amp or so, I personally would also fuse the DC line(s) out of the rectifier. Experience has shown me that it will react (blow fuse) a lot faser than the fuse on the primary

    A fuse in the primary really excells for primary side power faults, power spikes, shorted turns in the transformer etc

    Dave
     
  14. Sep 6, 2013 #13
    Ok thanks, I will see if i have room for 2 on the output, they should be rated for my transformers current correct?
     
  15. Sep 7, 2013 #14

    meBigGuy

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    None really. The OP talked about current flowing to/from ground and I just wanted to point out that high currents could flow without ground current.
     
  16. Sep 7, 2013 #15
    That is true and I hadn't noticed it so thank you.
    Something weird just happened, I put a 0 and 12v line through a diode and capacitor, like I said i would for LEDS and the fan, and I am measuring 12v out which is exactly what I need, but when I put a 470uf cap on it came out as around 20v! I can make up for it by using stronger resistors in series with my fan and LEDs, but this makes me wonder if it could happen to my amplifier circuit as well.


    I have my main power supply exactly as shown in the image of Davenns reply above, but after everything is rectifier, filtered, and regulated, the +/grnd/ - splits to my 2 amp circuit boards which I included the 220uf and .1uf caps filtering between positive and ground and negative and ground as recommended in the datasheet.
    Could I have this problem on my amp circuits?
     
  17. Sep 7, 2013 #16

    Averagesupernova

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    The capacitor charges to the peak voltage of the sine wave. It is not weird at all.
     
  18. Sep 7, 2013 #17

    meBigGuy

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    go to the wikipedia page for ripple (electrical). The pictures and equations explain all.
     
  19. Sep 7, 2013 #18

    davenn

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    Hi TD

    just had to do a couple of little but important changes to the diagram
    the new one is below, wont affect operation, but will affect the SAFETY :smile:
    all around the power input area
    1) swap the fuse and switch positions
    2) show incoming earth line
    3) show the earth connection to the transformer core

    attachment.php?attachmentid=61561&stc=1&d=1378610760.jpg


    cheers
    Dave
     

    Attached Files:

  20. Sep 7, 2013 #19

    berkeman

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    @Tesladude -- you haven't answered my question about why it's important to connect Earth ground from the input power cable to your metal enclosure for your power supply. If you are making this power supply for a fee, it's real imporatnat that you get it approved by a safety agency (UL in the United States). Or at least design and build it to the applicable safety standards. You open yourself up to some pretty serious lawsuit issues if your device hurts somebody via shock or catches on fire because it doesn't conform to industry standards.

    So please tell us, why did Dave swap the position of the fuse and the switch in the schematic...?
     
  21. Sep 8, 2013 #20
    ok so I made it, pcb done for power supply done, I have everything on,yesterday it was working, today... not so much...

    my transformer has a 21v peak,that is what I am getting out of the rectifier.

    i measured the voltage from where the big caps are and I am reading 15v dropping.

    I measure from the very input on both regulators, and I get 12v.

    I measure on the out and I get 9v.

    Earlier i plugged it into my amp circuit and was getting a bit of dc with tiny ripple out of my speaker, the entire speaker bulged outward, then all of a sudden it worked wonderfully! Then I leave it alone for like an hour and try again and now I am reading what I showed above.
     
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