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Improve smps

Discussion in 'Troubleshooting and Repair' started by axel2016, Dec 14, 2016.

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  1. axel2016


    Dec 14, 2016
    I have got one og those cheap Chinese SMPS from eBay. A 24v 3a. The annoying thing with it is that the transformer is squeeking like a rat having its tail trodden on. My attempt to reduce that noise was to put a larger capacitor on the line in. The original one is a 47u 450v. So I put in a 470u. That blews op the Fuse And three of the four rectifier diodes.
    So my question is, why they blew up? Cant they cope with a larger capacitor?

    Last edited by a moderator: Dec 14, 2016
  2. duke37


    Jan 9, 2011
    Poor rat:)

    The SMPS rectifies the mains to produce DC for the switch mode to work. On switch on, there is a very large current until the capacitor is charged up. Sometimes a low value resistor is included to reduce this pulse but this wastes power and produces heat. Increasing the capacitor by ten times will increase the surge current and has obviously blown the fuse and diodes. Increasing the capacitor will not affect the sound, this is probably coming from the transformer due to magnetostriction. You could try some glue to reduce the vibration.
  3. axel2016


    Dec 14, 2016
    Well that was a waste of time, diodes and fuses. It makes no less noise touching the transformer so I doubt glue will have any effect. Perhaps if it is complete covered in glue.
    But thanks for the explaination. Larger diodes may work, right? But since it dosnt effect the noise, there is no reason for trying with larger diodes.
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