power supply advice

Discussion in 'General Electronics Chat' started by mart_1986, Dec 11, 2009.

  1. mart_1986

    Thread Starter New Member

    Dec 11, 2009
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    hi there

    i want to make a 12v 3amp power supply i have a transformer at 12v 50va what sort of regulator and circuit will i need

    thanks for the advice
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    If your transformer output 12 volt RMS you must use a low drop-out regulator. And also a large filter capacitor say 10000uF or more. But still under full load my quick calculations tells me that your regulator will most probably drop out. You should get a transformer with a slightly higher voltage output.
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    What is the mains frequency there?
     
  4. mart_1986

    Thread Starter New Member

    Dec 11, 2009
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    mains input is 240v uk standard
     
  5. mart_1986

    Thread Starter New Member

    Dec 11, 2009
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    Last edited: Dec 11, 2009
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You will need a so-called "low dropout" regulator. An LM338 is not low dropout. With a current of 3A output, the regulator will drop out about 2.2v to 2.6v, depending upon the temperature of the regulator.

    You will need at least 11,000uF for the filter capacitors (C1, C2) but up to 15,000uF would be better.

    [eta]
    Here are two candidate regulators for you that are stocked by RS Components in the UK:
    3A,12V,LDO Voltage Regulator,LM1085IT-12:
    http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5339381
    The datasheet for the LM1085 is here: http://cache.national.com/ds/LM/LM1085.pdf
    It has a 1.5v dropout at full load; not wonderful, but better than the LM338.

    3.0A LDO Fixed Volt,MIC29300-12WT:
    http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0453568
    The above Micrel regulator has only 350mV dropout at full load, which is very good. Well worth the few extra pence.
     
    Last edited: Dec 11, 2009
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    For your kind of load you do not need a voltage regulator. A unregulated supply will do just fine
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You edited your post while I was adding to mine.

    t06afre suggested that you do not need a regulated supply for fans. While this is largely true, if you are operating five 250mA fans for a total load of 1.25A from a 12v 50VA transformer and bridge rectifier, your light load on the output will result in the fans getting a lot more voltage than you expected.

    They will move a lot of air, but they will be operating above their normal operating voltage. They will be very noisy, and burn up quickly.

    [eta]
    The real question is, will you be wanting to add speed control at a later time? If so, having a regulated 12v available will give you a good starting foundation.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Tell us more about your transformer.

    It has a single primary winding, correct?
    You know that you will need to use a fuse in the supply from the mains power, right? I suggest using a fuse rated for 1A of a slow blow type.

    Do you have a datasheet for your transformer? If so, would you please post it?
    If you don't have a datasheet, can you post a link to the model you bought, or at least give us a manufacturer and model number?
     
  10. mart_1986

    Thread Starter New Member

    Dec 11, 2009
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    this is all i have http://www.rapidonline.com/Electric...s/Chassis-mount-transformers/72692/kw/88-3440
     
  11. mart_1986

    Thread Starter New Member

    Dec 11, 2009
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    so all i need to do is have a bridge rectifer would you smooth it out?
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    With a bridge rectifier the needed smoothing capacitor is 18.7 mF if you assume that the regulator will drop 2V across it. I suggest you to buy a transformer which outputs 16V. This will reduce the value of the smoothing capacitor and the inrush current at power up. You may need an inrush currents limiting resistor at power up as not to blow the bridge rectifier. After the smoothing capacitor has charged up you can bypass this resistor with a switch.
     
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