Regulated Power Supply

Discussion in 'General Electronics Chat' started by dukeman, Apr 25, 2016.

  1. dukeman

    Thread Starter Member

    Feb 7, 2009
    13
    0
    Hi,
    I would like to test a regulated power supply's output voltage and its ripple either with a dmm or a scope. The questions that I have are: do I need to put a load in order to make those measurements? If yes, what kind of resistive load should I use? Is dummy load one of them?


    Thanks for your insight in advance.
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Yes, the power supply should be loaded when being tested. Any kind of resistor will work for a load just so long as they have a high enough power rating.

    What is the power supply rated for??
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I build an electronic load for my test equipment box.

    Electronic Load

    It is more than practical to kludge a one time use transistor version.
     
  4. recklessrog

    Member

    May 23, 2013
    338
    102
    P1010037.JPG
    One of the most used pieces of equipment I own Is this 25 Ohm 500 watt variable resistor (rheostat) that is about 70 years old and was originally a motor speed controller.
    It makes a perfect dummy load or current limiter or whatever else it fills in for when testing a new design or the performance of a psu etc.
     
    Last edited: Apr 25, 2016
  5. dukeman

    Thread Starter Member

    Feb 7, 2009
    13
    0
    First, thanks for all the replies. One final question: what is a dummy load and when to use it?

    Thanks for your insight in advance.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    This quote from wikipedia is about as good as any: "A dummy load is a device used to simulate an electrical load, usually for testing purposes."
     
  7. recklessrog

    Member

    May 23, 2013
    338
    102
    In very simple terms, a "dummy" load, is something that you connect to your power supply to simulate the the actual device you wish to use the power supply for. With DC supplies, usually for all intents and purposes, a resistive load is adequate for most hobby use. Use Ohms law to determine a resistor value that will draw the current equivalent to your intended maximumum depending on the output voltage of the power supply. (Simplistically, 1 volt across 1 ohm will draw 1 amp producing 1 watt to be dissipated as heat) This is why it is called a dummy load, because all it does is convert the energy supplied into heat so make sure your resistors have an adequate wattage rating. Large wirewound resistors can be arranged in series or parallel (or a combination of both) There are some that can be bolted to a heatsink for cooling.
    While your power supply is hooked up to the dummy load, you will be able to monitor it's main performance characteristics such as how well it regulates the voltage with varying amounts of load, measure the ripple, noise etc. and if you have current limiting built into the circuit, you can check that it works as expected.
    The thing in my picture is simply a long length of resistance wire wound over a vitreous coated aluminium tube with movable bronze fingers to adjust the resistance. It survives all sorts of abuse as I have had it nearly red hot at times. (warms the workshop up too)
     
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