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Stabilised Power Supply with Current Limiting

Discussion in 'Electronic Projects' started by mechie, Oct 24, 2002.

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  1. Roff

    Roff Well-Known Member

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    Several years ago I bought a Triad isolation transformer. It gets too hot to touch, even with no load. I emailed Triad, saying that I thought it had a shorted turn or something. They wrote back saying that that's the way it's designed. Didn't give me a lot of confidence in Triad. :(
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A cheap transformer gets hot without a load because its primary doesn't have enough turns to make enough inductance. The cheap primary is just a long piece of wire. Sure it gets hot.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Don't you mean gave you none at all?, and never used them again!.
     
  4. dave

    Dave New Member

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  5. Roff

    Roff Well-Known Member

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    I guess I'm master of the understatement. :)
     
  6. Roff

    Roff Well-Known Member

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    Yeah, I thought about that. More inductance means more turns of heavier gauge wire, and that leads to a larger core, both of which raise the cost. Well, I seem to recall selecting it because it was cheap (in the true sense of the word, as it turns out). You get what you pay for. :(
    When I get home, I'll check to see if it's rated for 220V also. If it is, that sucker should be glowing red on 220.
     
  7. Hero999

    Hero999 Banned

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    I don't either, but if you connect a 1A load to a capacitor and rectifier connected to a 1A transfomer it will draw huge 3A pulses with an RMS value of 1.414, which will overload it.

    I've teasted my theory using a scope, a LM317 constant current source, a transformer, capacitor and a small 0.5ohm current sensing resistor (two 1ohms in paralell.
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The relevent word there is 'PULSES', it's NOT 3A continuous, and the transformer is (or any decent one will be?) designed for it.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I think a rectifier charges the main filter cap with current pulses that are about 10 times the load's current. All power supply parts are designed for it: the transformer, rectifier bridge and main filter cap.
     
  10. Hero999

    Hero999 Banned

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    The magnitude of the pulses will depend on the filter capacitor. Somehow I find if difficult to believe that a 10A pulse won't cause the core to saturate, there again the pulses are quite short so I don't know.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I have never measured a cheap transformer that has a cheap soft iron core instead of carbon steel, maybe has a core that isn't even laminated with insulated laminations, and without enough turns on its windings to be efficient. Some guys have and insist that the peak voltage is the same as the RMS voltage due to the transformer compressing the voltage peaks.
    I wouldn't say the cheap transformer saturates its core, I would say that its peak current operates it on the curve of its magnetic hysteresis graph, so it is approaching core saturation.
     
  12. 4RESTER

    4RESTER New Member

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  13. cocostarz

    cocostarz New Member

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    I need help building a power supply for a project at school... 110 ac to 24 v dc at least 2.5 amp.. please can any one help me out with schematics and diagram and a list of all parts i need thanks..
     
  14. mneary

    mneary New Member

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    Is this a project for-credit? What level of contribution is expected of you?
     
  15. bananasiong

    bananasiong New Member

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    Read from the beginning of the post you will find out something..
     
  16. skydemon2008

    skydemon2008 New Member

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    Do you have any better power supply that is suitable for a radio system?
     
  17. Hero999

    Hero999 Banned

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    It depends on what sort of radio you're talking about.
     
  18. zlatkoA

    zlatkoA Banned

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    original answer

    audioguru is 100% in right
     
  19. ranggaperdana_06114

    ranggaperdana_06114 New Member

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    how about the isolated power supply, do you know it and how to make it?
    tanx
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I do not know medical isolation requirements but I bet the insulation must be double-insulated.
     
  21. ranggaperdana_06114

    ranggaperdana_06114 New Member

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    do you have the circuit of it?
     
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