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Thread: Resistive load directly at the transformer output

  1. #1

    Question Resistive load directly at the transformer output

    Hello guys,

    please let me thinking... Can I connect almost pure resistive load directly at the transformer secondary coil? I mean to take part of the energy away to supply some heating element in order to save on the rectifier because the SMPS is used to supply electronics and heating element, so part of the energy is changed to the heat indeed and it is not neccesary to feed the load from perfectly stabilized voltage output.

    I have tested this on one LLC resonant power supply kit I have. The issue is that about 3W taken away from the transformer output caused about 30W increase of primary input power.

    Kit: http://www.onsemi.com/pub/Collateral...a_cp14u306.pdf

    vystup_pred_usmernovacem.jpg

    Load was connected between red/blue and green point. What is wrong? Please give me a kick, I am ready to study anything but now I do not want what is wrong.
    Last edited by daymoon; 02-05-2018 at 11:09 AM.

  2. #2
    .... Silvio's Avatar
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    Hello daymoon,

    Regarding your problem, It may be that there is a change in power factor at the input of the smps. If you are measuring voltage X current then you can be erratic. This measurement only holds true if the power factor at the input is 1. SMPS tend to have a capacitive reactance in the mains input and this changes power factor. Measurements should be done with a watt meter at the mains input.

    Loading the smps before or after the output rectifier should not make any difference as long as the load is purely resistive.

    Regards Silvio

  3. #3
    Thank you, Silvio.

    I think, you are right. I have to admit that this reason was out of scope of my mind.

    I will remeasure it asap.

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