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Wall wart resistance — Parallax Forums

Wall wart resistance

firewaterfirewater Posts: 25
edited 2009-01-03 05:45 in General Discussion
I have a small wall wart rated at 2.7V and 240mA.
I measured the voltage output·with my DMM "Amprobe 34XR-A" and got 2.71V.
Then I measured the resistance "unpluged" and got 218 Kohms.
Using the ohms law calculator here: http://www.onlineconversion.com/ohms_law.htm
I entered the rated volts·and milliamps and got 0.0112 Kohms
But when I entered the actual volt and·resistance values from the DMM I get: 0.0124 milliamps which is no where near the rated 240 milliamps. Whats going on here?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-02 19:10
    Since the output voltage without (I assume) a load is so close to the rated voltage, you most likely have a regulated wall transformer. This means that any resistance measurements you take will include some active electronics, such as a switching regulator. That being the case, such a measurement will be meainingless.

    -Phil
  • firewaterfirewater Posts: 25
    edited 2009-01-02 20:24
    Correct, there is NO load on the wall wart. So for an accurate amperage output measurement, I need to measure the current directly, Correct?
  • Mike GreenMike Green Posts: 23,101
    edited 2009-01-02 20:32
    What are you trying to measure? Are you trying to verify the rating of the wall wart?

    There are many different output current ratings. Which do you want?

    1) Maximum current output before shutdown or damage occurs (short circuit current)

    2) Maximum current at rated voltage (2.7V)

    3) Maximum current at some other voltage.

    In the case of #2 or #3, you have to provide a load resistance and decrease that while measuring the voltage across the load (and the current through it).

    In the case of #1, you have to do the same thing while decreasing the load resistance towards zero and watch for overheating or a voltage/current curve that suggests that the supply is shutting itself down to prevent internal damage from overcurrent or overheating.
  • FranklinFranklin Posts: 4,747
    edited 2009-01-03 03:58
    In most cases the ratings on a power supply will be the voltage it is designed to supply and the maximum amperage it will supply at that voltage. If your device needs less amperage that's all it will get.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-01-03 05:45
    You can't measure the "resistance" of a battery for instance to try and work out it's capacity. If the load is resistive then you can measure the load but then again many loads are complex loads that depend upon the voltage and conditions etc. Moreover, YOU CAN'T MEASURE the "resistance" of a power-supply for many reasons.

    If you want to check that the power supply can still deliver 2.7V at 240ma then you should load it with the equivalent resistance and then measure the voltage. R=V/I -> 2.7/0.240 = 11.25 ohms. The resistor needs to be able to handle 0.65W so I would use two 22R 1/2W resistors in parallel to approximate your load.

    But why are you trying to test your wall wart?

    *Peter*

    Post Edited By Moderator (Chris Savage (Parallax)) : 1/5/2009 4:06:24 PM GMT
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