DC loads.....

Discussion in 'Electronics Resources' started by hse, Sep 15, 2010.

  1. hse

    Thread Starter New Member

    Jul 28, 2010
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    Hi

    I have a question that is maybe a little newbie like:)

    I have heard the word DC load and I was wondering if, it is a device used for testing power supply, motors etc. meaning that the must draw a specific amount of current (actually meaning that they need to use some power -> maybe dissapated as heat)....or?
     
  2. TubeIdiot

    New Member

    Sep 24, 2010
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    Hi,


    As far as I know you're right. Usually it is a very simple current source made of some parallelled power transistors on a heatsink.

    Jeroen
     
  3. sceadwian

    New Member

    Jun 1, 2009
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    In your example a simple resistor of a specific value (to draw the desired current) rated to dissipate the power (yes as heat) is all that's needed. Ceramic power resistors are cheap and easy to get even in pretty high wattage ratings, though once you get past a handful of watts heatsinking is a very good idea, even at low wattage heatsinking is often required to keep the temperature down, it'll prolong the resistors life dramatically, heat is the number one killer of electrical devices.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    In the test instruments world, a "DC load" is a computer-controlled instrument that can be set to draw a constant voltage, current, or power from a power source. The usual method for doing this is to use a power transistor (usually a MOSFET) to control the current. The transistor(s) need to be on generous heat sinks with forced air flow, especially in the larger sizes of units.

    Thus, they are fundamentally "adjustable resistors" that let you dump electrical energy in and dissipate it as heat.

    Their utility is that they can change the load presented to the energy source in a precisely defined fashion up to thousands of times per second. They often measure current to the nearest mA and voltage to the nearest mV.
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    Does this website cater to a lot of test instrument persons? You're reading WAY too much into the term DC load someonesdad. For power supply testing a ceramic resistor for the rated power that needs to be tested is pretty simple, no active devices required.

    http://en.wikipedia.org/wiki/Dummy_load Might help, what is called a 'load bank' would be more useful than an active load.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    A programmable load is a standard in test equipment gear. Most of them are basically constant current devices with high wattage tolerances.

    I have several concepts that I'll eventually pick among and build myself.
     
  7. sceadwian

    New Member

    Jun 1, 2009
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    This is fine and dandy Bill, but a DC load can be nothing more than a resistor, for motor power supply and even RF loads they're commonly used.
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. Wendy

    Moderator

    Mar 24, 2008
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    The problem with non adjustability is you can not test power supply over the range. This is important if you want to measure regulation and see how well a power supply or regulator works

    You could use the decade box approach, which works pretty well.
     
  10. someonesdad

    Senior Member

    Jul 7, 2009
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    sceadwian, I responded appropriately to what the term "DC load" means to me. Your "definition" is fine too -- but most people just call it a resistor.

    Hopefully, the OP gets a variety of opinions and definitions -- and gets his question answered. Until he elaborates with more information, we won't know if we've answered his question.
     
  11. sceadwian

    New Member

    Jun 1, 2009
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    My thoughts exactly someonesdad.

    I'd think a decade box would be the absolute worst power supply load I can think of.
    Between various resistance ranges the power handling of the resistor network switched in at that moment could be multiple orders of magnitude over or under the needed power dissipating.

    For a basic example, if you use 1/4 watt resistors in your decade box at 1ohm you'll have 1/4 watt of power dissipation, at 9 ohms you'll have 2.25watts of power dissipation, it's the same with every stage from there so the entire box would have to be rated for 1/4 watt, no more if you try to use more power and flip the wrong dial... POOF.
     
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