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Problem Power Supply

Discussion in 'Troubleshooting and Repair' started by efrim785, Jun 29, 2017.

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  1. efrim785


    Jun 29, 2017
    hello folks,
    i found a little power supply and i try to use it but the voltage value does not correspond to what i selected i disassembled, the circuit is really simple, a transformer that have 6 output each one output a specific ac voltage, and a pcb that contain a switch that gonna select what voltage do you want and a full bridge rectifier and a capacitor so i measured (using a multimeter) the voltage for the 3V rail i found that the transformer output is 3V ac but after the full bridge rectifier and the capacitor i measure 7V ... i changed the cap and the bridge and still get the same result, so have someone an idea on what is going on ?
    than you for trying to help.
  2. duke37


    Jan 9, 2011
    This is an ustabilised supply, it will be designed to have a high voltage when unloaded and the voltage will come down to the rated value when loaded at its maximum.

    7V seems very high, 3V AC rectified is 3*sqrt(2) peak = 4.2V and you have to take off the rectifier drop.
    You could use a higher voltage tapping than you need and use a regulator IC to get exactly what you want.
  3. efrim785


    Jun 29, 2017
    thank you for your reply, i thought that something was faulty.
  4. kellys_eye


    Jun 25, 2010
    Making the output stabilised would be a good idea but considering the cost of such ready-made devices these days it would be a fruitless exercise.
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