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Transformer question

Discussion in 'General Electronics Discussion' started by bigone5500, Nov 12, 2014.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I have an Altronix SMP5 power supply which can output 24vdc at 4A. I have a Radio Shack transformer which suppplies 25.2vac at 2A. I'm thinking that the power supply will be ok but only good for 2A instead of the 4A rated. I should be ok right?
     
    Last edited: Nov 13, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Not really,

    The SMPS has an output that is regulated DC, the transformer has AC output.

    Once rectified and filtered the output voltage from the transformer will be much higher (probably around 36V) and this can vary with line voltage (so maybe ±10%)

    If unregulated the DC voltage from the transformer will vary with load.

    The ripple frequency from the SMPS will be much higher and the ripple voltage much lower than that from the transformer (if rectified and filtered)

    The maximum DC current you should draw from the transformer is around 1.414A (sounds like the square root of 2 doesn't it? -- it's actually the AC current rating (2A) divided by sqrt(2)).

    To get 24VDC from the transformer you would require a regulator, if we assume you use a linear regulator then it will give you 24V at 1.4A, and dissipate about 10W to 15W of heat. This is more heat than the SMPS is likely to dissipate even when running at full capacity (4A)

    So, probably you won't be OK.
     
  3. bigone5500

    bigone5500

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    Apr 9, 2014
    The SMP5 is a power supply/charger for SLA type batteries. It can be used as a UPS of sorts. I will attach a photo in a bit. I have it connected and am getting 24VDC from it. I am just concerned that the board is rated for 4 amps but the transformer can't supply it.

    tmp_30222-20141112_180410-276828662.jpg
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Ok now I understand.

    If you rectify and filter the output of the transformer then connect this to your smps you can draw almost 2A. How close depends on the efficiency of the smps. If its 90%, then you can draw 90% of 2A.
     
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  5. bigone5500

    bigone5500

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    Apr 9, 2014
    Thank you sir. And like my power supply, I was about 90% sure.
     
  6. Colin Mitchell

    Colin Mitchell

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    "then you can draw 90% of 2A." That's still 2A as an AC rating. So the output current is about 1.4A.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    90% of 2A is 1.8A

    edit: I think Colin had failed to notice that the regulator is switchmode and thus 1.4A at 35V is 2A at 24V. (Power in = power out)

    The reduction of this in like with the efficiency simply means that the input power is maintained at the nominal limit of the transformer.
     
  8. Colin Mitchell

    Colin Mitchell

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    Yes. You can get 90% of 48 watts in any combination of volts and amps. I was thinking of a linear regulator.
     
  9. bigone5500

    bigone5500

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    Apr 9, 2014
    Thanks guys. For what I will do with it, 1.4A is plenty.
     
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