24VAC to 5VDC

Discussion in 'General Electronics Chat' started by jeffjohnvol, Mar 26, 2013.

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  1. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    I want to make my own thermostat with a small micro.

    I'm trying to figure the most efficient way to go from 24VAC to 5VDC to drive a microcontroller.

    I understand the basics of the bridge rectifier with a leveling cap and a regulator like a 7805 or so, but that is a major amount of power to regulate down, even at low current.

    I searched the forum and only found this:
    http://forum.allaboutcircuits.com/showthread.php?t=17963&highlight=24vac+5vdc&page=1

    which was reading a value, not driving a voltage input.

    I saw this, but wasn't sure if it would be a good fit: http://www.mouser.com/new/texasinstruments/nsclm257x/

    Since size is an issue (don't want a huge thing on the wall), I want to avoid a step down transformer, unless there's a common one that is small enough.

    Any suggestions?

    Some research talks of a "buck" regulator, but I don't know enough about them.
     
    Last edited: Mar 26, 2013
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The most efficient way would be with a switched mode regulator, but before you rule out a linear regulator, how much current at 5V do you need?
     
  3. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    probably only enough to run the micro (maybe 100ma) and 3-4 relays (the small blue ones), so maybe 400 mA.

    Thanks.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    If you design your microcontroller system to operate on very little current you can get away with a simple half-wave rectifier circuit and a 5V linear regulator.
    Edit: You posted before me. 100mA is way too much for a microcontroller. MCUs are available that will run with less than 1mA.
     
    Last edited: Mar 26, 2013
  5. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    FWIW, I'm hoping for something small and inexpensive (aren't we all!) with something that doesn't need a lot of heat dissipation.
     
  6. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    Okay, thats good to know. A lot of googling always has people commenting on the delta of the regulator.

    If the rectified voltage averages to 28 volts (unregulated) the 23 volt delta would be at the far end of the limits of a 7805, wouldn't it?
     
  7. timescope

    Member

    Dec 14, 2011
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    In other words, you want to use the 24v ac to provide the 5v dc power supply for the microcontroller. The LM2574HV is a buck regulator and the circuit shown only needs a rectifier and capacitor to convert the 24v ac to dc, connected to the unregulated DC input pin. Use the 5v version.

    Timescope
     
  8. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    Timescope, so you think the 2575 (5 volt version) would be a good option with the way you describe? I can get those pretty inexpensively: http://www.ebay.com/itm/10PCS-IC-NS...859?pt=LH_DefaultDomain_0&hash=item3a748dcb5b

    Thanks.
    Jeff
     
  9. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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  10. timescope

    Member

    Dec 14, 2011
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    Yes, if 500mA will be enough for your application. These regulators are very easy to use if you follow the layout instructions on the datasheet.

    Timescope
     
  11. jeffjohnvol

    Thread Starter Member

    Oct 15, 2008
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    Thanks Timescope and all that replied. this forum is a great resource. I really appreciate the time.

    Jeff
     
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