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Understanding my mysterious power supply

Discussion in 'General Electronics Discussion' started by SparkyCal, Apr 8, 2020.

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  1. SparkyCal

    SparkyCal

    362
    14
    Mar 11, 2020
    I am not sure how to even ask this question, but here it goes. I have a digital power supply. It's this one:
    https://www.amazon.ca/gp/product/B07K7DQZZW/ref=ppx_yo_dt_b_asin_image_o06_s00?ie=UTF8&psc=1

    Dialong in the amount of voltage I want, is pretty styaright forward. However, i am not clear on how to use the current reading. I know that there is a way to limit the current by going through a particular procedure, and I know this is to ensure that circuits don't become overloaded.

    But I am not clear what the current LED and dials are for, when/how to use them and why they never show any values. Even when I turn the knobs, they don't do anything. The supply seems set on the voltage LED reading, and I am not clear on how to make use of the current section. Can anyone give me any clues on this?

    Thank-you
     
  2. Audioguru

    Audioguru

    3,130
    697
    Sep 24, 2016
    If the output current exceeds the setting of the current-setting dial then the power supply reduces the output voltage until the current is what you set, according to Ohm's Law. The LED lights as a warning that the current is limited and the voltage is reduced.

    Example: Set the voltage to 30V and the current to 20mA. Then connect a 2V/20mA LED. The output of the power supply will safely light the LED with 2V at 20mA.

    I design circuits that never overload anything and my circuits are never overloaded so I have never needed and never used the current setting on a power supply.
     
    Cannonball likes this.
  3. SparkyCal

    SparkyCal

    362
    14
    Mar 11, 2020
    Good to know! Thank-you
     
  4. Cannonball

    Cannonball

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    53
    May 6, 2017
    The load determines the current
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    11,266
    2,577
    Nov 17, 2011
    Right.
    The power supply, however, has a built-in current limiting circuit to protect the power supply and/or the load from overcurrent in case of a failure (e.g. a short circuit).
    How to use the current limiter:
    1. Disconnect the load.
    2. Set the desired output voltage using the voltage coarse an vernier knobs.
    3. Turn down the current limit (usually by turning the current coarse and vernier knobs all the way to the left/counterclockwise).
    4. Apply a short circuit to the output - the output voltage will fall to approx. 0 V, ignore this.
    5. Set the desired current limit using the current coarse and vernier knobs. Always allow for some headroom above the expected max. current (e.g. 5 % to 10 %). Note that the voltage will always be approx. 0 V, that's totally fine so far.
    6. Disconnect the short circuit. The output voltage will spring back to the value set in step 2, current will drop to 0 A.
    7. Connect your load. Under normal operation the output voltage should stay at the set value, the output current should stay well below the limit set in step 5
    In case of a failure (short circuit, overload), the power supply will go into current limiting mode, limiting the current to the max. value set in step 5 and reducing the output voltage as required.
     
    Cannonball likes this.
  6. AnalogKid

    AnalogKid

    2,481
    714
    Jun 10, 2015
    One addition to HK's excellent instructions:

    When applying a short across the power supply output to set the current limit, make sure to use a wire size that can handle 5 A continuously. At the lower currents almost anything will work, but at 5 A a skinny little 24 gauge hobbyist clip lead will overheat. 18 ga will get warm, but not enough to be a problem.

    ak
     
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