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12V to 5V 3A converter

Discussion in 'Power Electronics' started by john2k, Nov 2, 2019.

  1. john2k

    john2k

    131
    2
    Jun 13, 2012
    I have the following pictured DC 12v to 5V converter

    [​IMG]

    On the input I want to put a inline blade fuse. The output says 5V 3A. I was going to put a 3A fuse on the input but then I'm thinking is the input and output amp rating not going to be different? Seeing as input is 12V does that mean it will require less than 3A on the input or more? Or is 3A fuse on the input a good value to use?

    Thanks
     
  2. BobK

    BobK

    7,642
    1,662
    Jan 5, 2010
    It will, on average require less than 3A, but the peak current will be over 3A. A buck converter works by drawing power from the source only part of the time. When it is drawing current, the current will be the same as the output current.

    Bob
     
    john2k likes this.
  3. AnalogKid

    AnalogKid

    2,307
    649
    Jun 10, 2015
    Correct, a fuse on the input can have a rating lower than 3 A and still support the full output.

    The max. output power is 15 W. If the converter is 100% efficient, then the input current at 12 V is 1.25 A. But the converter is probably only 75% efficient at full load, so it needs 1.67 A of input current to supply the 3 A of output current. Switching converters can have a high startup, or "inrush", current that can be a problem for some fuse types. A 2 A fuse should work for you. If it blows when ther eis no problem with the output, change to a "slo-blo" type.

    ak
     
    john2k likes this.
  4. john2k

    john2k

    131
    2
    Jun 13, 2012
    Thank you.

    I've plugged the above pictured converter in to 12V car battery and the output 5V i am powering a carputer. For some reason when i completely turn off the device, this converter makes a really annoying high pitch sound. When I turn the device on again then it stops. It's almost like it needs a small amount of power drain otherwise it makes the noise. But if I completely disconnect the device and power nothing with it then it doesn't make any noise at all. Are there any better and reliable 5V converters? I need between 4.8v and 5v but need it to be reliable.
     
  5. BobK

    BobK

    7,642
    1,662
    Jan 5, 2010
    Put a resistor across the output. Start with 1K, if that doesn’t work, try lower, but watch the power consumption.

    The power is V^2 / R. pr 25 / R for 5V

    So a 100 Ohm 1/4 W resistor would be maxed out, so don’t go that low.

    Bob
     
  6. john2k

    john2k

    131
    2
    Jun 13, 2012
    1K ohm? Any particular wattage? What will happen if the device tries to draw 3A with the resistor in place? Will it still be able to draw this? Do I put the resistor on the positive of the output?
     
  7. BobK

    BobK

    7,642
    1,662
    Jan 5, 2010
    Younput the resistor across the two outputs of the converter.

    A 1K will draw only 5mA. The idea is that, perhaps it might stop whining when there is no load.

    Bob
     
    john2k likes this.
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